Topic: How many confirmations required for btc received to be safe from double spend? (Read 532 times)

If you want to know the actual answer, use block explorer and statistic website which show mempool history (such as https://jochen-hoenicke.de/queue/).


There's no concern when the agreement of the deal/trade is to wait for 1-6 confirmation before doing anything else or you use escrow/multi-sig platform you could trust.


A people could try that no matter how long it's remain unconfirmed. But as @o_e_l_e_o said, just wait until it the transaction has few confirmation.

Pretty much this jerry. Since you are trading your fiat for their bitcoin, then there are two ways around your fear of incredibly rare double spends. The first is to ask them to send the bitcoin first, and wait until it has 1, 2, 3, or whatever you are comfortable with number of confirmations (and pre-agree with the other party how long you will be waiting) before you send them the fiat. The second is for them to deposit the bitcoin in a non-custodial escrow, such as the 2-of-2 escrow provided by Bisq or the trustless smart contract escrow provided by LocalCryptos, you send the fiat across, and then you both agree to release the bitcoin from the escrow and send it to your wallet. Since it is coming from one of these escrows, then there is no chance for a double spend prior to it being confirmed as reaching your wallet.

Could have been fee. Could have been block time. Could have been a combination.

The "priority" of a fee is constantly changing based on what other transactions are in the mempool at the time.

Not unless the mempool was super full and they had set a fee which was unlikely to confirm for days.

They could try, but they are unlikely to be successful if the original transaction is already viewable across multiple block explorers.

The solution to all this remains the same as the solution I gave you above: Just wait for a few confirmations.

If it's not the transaction fee, it's the block time.
The 10 minutes block time is just an average, it can go up to hours or down to seconds depending on the miners' luck.
Moreover, it can also be affected by miners shutting down/powering up their farms causing a drastic changes in difficulty in the next retarget.

so i traded with someone last time my bank funds for their btc and the first confirmation showed up in less than ten minutes.  Then the second confirmation came in about another ten minutes or so.


i then traded with someone else, the same my bank funds for their btc... it took more than an hour for the first confirmation.  Why is that the case?  Was it because of the different fee they paid? 



However, when i checked mempool both times when they sent, the low and medium priority was one sat kb only?  The high priority was either one sat kb or 5 sat kb with the first trader, but the second trader i did with the other person was 5kb for high priority.  Anyone can explain?



Thus with the second trader it showed 0 confirmations for over an hour.  Would that be a big concern for most of you if that is your first time trading with that person?



in that situation, could that person have double spent since it showed unconfirmed for over an hour?

It is worth noting that these numbers depend on the client type (full node, SPV, etc) and are valid for 99.9% of the times when there is no network disturbance (eg. during forks).
- A SPV client is not capable of monitoring the mempool so all the conditions for 0 confirmation such as fee, parent confirmation, etc. are out of the question.
- A SPV client can't recognize a chain split the way a full node does, depending on the implementation it could be later than normal (eg. single server dependent clients) so the minimum numbers need to be increased.
- In certain times like during a fork when full nodes can reject old blocks there is a higher chance of a chain split and reorg. So the minimum number of confirmation here needs to be also increased.

For some people I guess the other part of the questions is where the BTC is going and what else is happening.
If it's in a wallet that I control then 1 conf for just about anything is good for me. Even if it's for 'real' amounts or property there have been so few orphans / reorgs that I can say it just does not matter.
Lets face it, if I was giving someone $1 million in cash for 42.5 BTC right now I would be more worried about a goon with a gun taking the cash then having someone take the BTC back through a reorg.

-Dave

Simple rules of thumb for me are:

 1 confirm always with anyone for any amount under 500 usd value

and 2 confirms for 500-10000

and 6 confirms for 10000 and up

I do the 1 confirm rule as I have seen a few glitches that seriously tied coins up for days.  They are very rare but 1 confirm is usually under 2 hours so I can wait.

I generally use the following rules when determining how many confirmations I want in a transaction I'm receiving before I'll provide anything of value in exchange:

1. Zero confirmation are ONLY acceptable in 2 situations. First is if I have a trust relationship with the sender (someone that I'd agree to provide something of value today in exchange for a promise of payment in a few days). The second is if I'm confident that I'll be able to extract payment via some other method if the Bitcoin transaction doesn't work out (escrow, lawsuit, collateral, etc). If I'm going to accept a zero confirmation transaction, I'll make sure that it isnt marked RBF, that all of the inputs already have at least 1 confirmation, that there are no competing unconfirmed transactions using the same inputs, and that the transaction included a reasonable transaction fee.

2. One confirmation is acceptable for any amount that I wouldn't feel devastated to lose. While it is possible that the confirmation could be part of a block that will be replaced in a reorg, it is very likely that the transaction will still be in whatever block replaces it. It is highly unlikely that a transaction with 1 confirmation will be replaced. Is it a 0% chance?  No. But very little in life is.  Checks or cash could be counterfeit. PayPal, Venmo, Zelle, and CashApp can be reversed. Any number showing up in your bank account could be faked with collusion from the bank. You'll have to decide what amount would be devastating for you, everyone is in a different situation financially. Therefore, if the loss isn't going to leave me unable to eat or sleep, then 1 confirmation is fine.

3. Three confirmations are acceptable for any amount that would be devastating, but not life-altering. At 3 confirmations we're starting to look at situations where collusion with a mining pool would be needed, and that mining pool would need to be willing to risk losing 20 bitcoins or more for just a small chance at success.  How many people do you know that would be willing to risk a greater than 95%+ chance of losing $400,000 or more for a less than 5% chance of gaining an extra few thousand dollars? That's what you'd have to convince the mining pool operator to do.  Convincing a mining pool to destroy their reputation, and risk their entire cash-cow business just to help you steal a few tens of thousands or even hundreds of thousands of dollars is going to be a hard sell unless the sender of the bitcoins is holding their family for ransom. Therefore, if the loss isn't going to leave me homeless, bankrupt, and destitute, then 3 confirmations are fine.

4. If I feel the amount I'd be exchanging would be life-altering (lottery winnings, inheritance from a rich uncle?), I'd need at least 6 confirmations. Though honestly, for that amount, I'd probably revert back to one of my rules for zero confirmations which is: I'd make sure I was confident that I'd be able to extract payment via some other method if the Bitcoin transaction doesn't work out.

In terms of reversing already confirmed transactions, then there's the 51% attack which you've already discussed and there's the possibility of taking advantage of a natural chain split/stale block as I've discussed. There's also the possibility of an attacker with a significant minority of the hashrate getting lucky and being able to mine a longer chain in secret and therefore overturn one or more confirmations. This is essentially a 51% attack but without the 100% guarantee that it could be achieved. There is also a theoretical attack named after user vector76 which essentially creates and takes advantage of a chain split. Note that an attacker must sacrifice a block reward to pull off this attack.

As far as I am aware, a 51% (or even minority alternative chain) attack has never happened, and a chain split has only ever resulted in very rare cases of a transaction with 1 confirmation being double spent. I am unaware of any transaction with 2 or more confirmations ever being double spent in such scenarios.

A 51% attack would cost hundreds of millions of dollars. Taking advantage of a natural chain split could cost nothing, but would require a tremendous amount of luck.

Since you are so concerned about this, then the best thing to do is simply wait for 3 or more confirmations.

"Double-spend" is loose term, I think you mean reverting an already confirmed transaction and replacing it with another, based form the thread's topic.
Honestly, IDK if there are other ways to do that aside from the mentioned attack, others might know.

Uhm, that's per unit.
You need about 1million units to reasonably do the attack.

51% attack was already discussed multiple times and received great replies in the board that I've mentioned.
I'd suggest you to follow my instructions above to be able to see those topics.

i mean to double spend... so how many ways are there besides the 5 percent attack?


So it cost around 5k usd to do it?  So basically nobody would try to double spend and thus try to scam someone for like 1k right because they lose money that way?  So they need to do it for at least 6k usd?  But what is the success rate for that?  So someone would have to do at least 5 figure usd amounts for it to be worth it?

Do you mean 51% attack?
Basically, you need to have 51% of the network's hashrate or more to have a reasonable chance of success.

Currently, the network's total hashrate is (getmininginfo):
Which is roughly 204.257 Ehash/s, half of that is 102.129 Ehash/s
Which is equal to 928,440 units of Antminer S19 Pro (110TH/s advertised hashrate | $4~5k average price)

With that, I think you can get the picture of the cost; not to mention, the availability of the required mining equipment.

For more info, I'd suggest you to go to this board: Development & Technical Discussion
The use the search box at the upper-right side of the page to search for "51% attack".

but does it cost money for someone to try to double spend? 


such as if it does, how much... because that would mean its only worth it for that person if its a big amount?

The most recent case I can find was at block 733,430. A block at this height contained the transaction dd0ada46f59ee9ac962fae10e2e3d63b8898953b4730c74851f344041988aa54. A different block at the same height contained the conflicting transaction 84ce52b6bde454a4b23f2c1cc9e6e67f0c8a28b91e4e170a717d1581d5d556fc. The second block was built upon first, meaning the first transaction was invalidated and no longer exists. Any node which was looking at the first block would have seen the transaction dd0ada46f59ee9ac962fae10e2e3d63b8898953b4730c74851f344041988aa54 go from 1 confirmation to being rejected as invalid.

The block was over 3 months ago, so this is not exactly a common occurrence. I am not aware of any transactions which were reversed after 2 confirmations outside of serious bugs as mentioned by nc50lc above.

Knowing those things is not a prequirement for you to use Bitcoin safely as you shouldn't consider unconfirmed or transactions with 1 confirmation as finalized. Just wait for multiple confirmations as suggested earlier and that's it. But even if there is an orphaned block and your 1 block deep confirmed transaction reverts to unconfirmed, it's still a perfectly fine and valid transaction that just needs to wait in the mempool to be picked up again. Unless it already was in the other valid block.

It's not easy to go from having 1 confirmation to having that transaction being dropped because of an orphaned block and then double spending that transaction to a different address just in case it was also not included in the valid block. But yeah, knowing if such incidents have happened in the past would be interesting. Double spending has happened and people have been scammed with it, but as nc50lc said, Bitcoin was never 51% attacked. 

[1] I can't find any archive or list with that statistic based from 51% attack since AFAIK there's never any successful 51% attack to the Bitcoin network (CMIIAW).
[2] I can't find any archive or list with that statistic based from orphan blocks either since not every Bitcoin node will receive an orphaned block.
[3] There was an incident when bitcoin had to revert back the blockchain due to a serious bug,
during that time, confirmed transactions after the "bad block" had reverted back to unconfirmed. (more info: Value overflow incident)

Additional info for [2] "orphaned block":
We can assume that if a node accepted an (to-be-)orphaned block and if a transaction is included there but not in the main chain
that particular txn can reverse from '1confirmation' into 'unconfirmed' after that node dropped the orphaned block and accepted the valid block.
But since every node isn't the same (location, internet speed, etc.), it only got 1 confirmation in some nodes (minority of the nodes that received the orphaned block)
and not the majority of the network where it's not confirmed.
If the orphaned block and the valid block both contain that transaction, then the transaction will not reverse even after dropping the orphaned block since it's in the valid block too.
So it's a case-to-case basis.

Plus orphaned blocks are uncommon now.

How many cases has there been where just one confirmation was able to get reversed?  How much does it cost that person to do that?  Or it doesn't?

1 confirmation is good enough for most people who are transacting maybe up to 100 BTC. If you want to put a fiat value to it, maybe up to $1m USD worth of BTC. The second confirmation will come soon enough anyway and by the end of the day you can go to sleep knowing the coins are not going anywhere else.

That's quite possible, yes. If both mempools had a record of that transaction, both could have included them in their own blocks. It would really be interesting to see some statistics on stale blocks regarding how many of the same transactions get included in competing blocks and remain valid and confirmed once one block gets orphaned and how many become unconfirmed and return to the mempool. I am not sure if that is possible though. I am not talking about the overall count of orphaned blocks but the status of their transactions compared to those that got mined in a competing block. 

If there is no double spend, most likely a transaction will get included in both competing blocks so when one block is orphaned then another block would still have your confirmation. If that does not happen then, yes, the transaction will get included in subsequent blocks.

The chances of having a transaction of 3+ confirmations rejected or double spent isn't something I would worry about for normal Bitcoin usage. By normal I mean receiving $100s or even $1000 worth of BTC. If I were to ever receive millions or billions one day, I would wait for at least 6 confirmations. Not sure if that will even happen though.    

1 confirmation can be problematic and it doesn't have to involve a scam or double spend by the other party. The block could be orphaned. You see 1 confirmation, you release the money or goods, and the block your transaction got mined in gets rejected by the network. Then you are back to no confirmations. You weren't scammed and probably your transaction will be mined just fine in one of the upcoming blocks.   

Everyone talking here about minutes between blocks and all but I have one curiosity.  If you have a transaction with one confirmation.  How easy is it really for the sender to scam you from the second or third block on?  Because this seems like something only someone with a large enough incentive could do, so I wonder if it is worth the effort for scamming someone out of $1,000?  Does this happen often at all or do you have to be very unlucky to fall for it?  Except unconfirmed transactions which of course are easy to replace.

I get the paranoia, I am an extremely paranoid one myself.  But I typically consider my transaction confirmed once there are two confirmations, unless the sums exceed thousands.  I sometimes consider a large transaction as 'final' after it gets confirmed twice only.  Pmalek mentioned a few replies above that generally, 6 confirmations are accepted as 'final'.  With 4 or more confirmations, is it not just pretty much impossible to scam someone unless we are talking about a significant, BIG incentive and one unlucky person to be in the wrong place at the wrong time?

-
Regards,
PrivacyG

Why not also ask about 20 and 50 confirmations as well?

You need a magic 8 ball to answer that question. I have seen two blocks being mined in the same minute. I have seen 3-4 blocks be mined in 1-2 minutes apart. And I have also seen that you need to wait for 40 minutes between two blocks. So take a wild guess how long it takes. 

As suchmoon commented, yes it takes 10 minutes on average for bitcoin transaction to be included into a block and would be confirmed if the fee is sufficient enough to make a miner to include the transaction into a block. But, it is 10 minutes on as average but it can be less than 10 minutes or more than 10 minutes. At times, transaction can be confirmed in less than 2 minutes and sometimes it can take up to 30 minutes.


It depends on:

• the mempool, if the mempool is not congested and 1 sat/vbyte transactions are getting confirmed, the fee to be paid is low. If the mempool is congested, the feerate needed will increase and the fee will increase.
• the address used, if segwit address is used as sending and receiving address, fee paid can be 42% reduced for 1 input and 2 outputs transaction if compared to sending from legacy address to legacy address
• the inputs and outputs of the transaction, but because sending to many addresses (high outputs) will only reduce fee if compared to sending one by one in different transaction to the different addresses, so let us consider only input. The more the input the more the transaction weight (vbyte) of the input, the more the fee

Maybe this can be helpful:
Minimizing bitcoin transaction fee

Mempool
For beginners: https://mempool.space/
Advanced users: https://jochen-hoenicke.de/queue/#BTC,24h,weight

Like I have explained above, it can take 30 minutes, it can take less than 2 minutes. If mining hashrates is increased before difficulty adjustment in the next 2016 block height, very possible many transactions will be confirmed in less than 10 minutes. Also if the mining hashrates is reduced like I explained, very possible that transactions will be confirmed after 10 minutes. 

Search engine searches can be full of misconceptions or in a way you will not get absolute information, especially about bitcoin. If he wants to use search engine, it will be good to include bitcointalk to it so he can see articles about it on this forum which is much better or using better reputable sites than random search.

In a large enough sample the average will be around 10 minutes (technically just under 10 minutes, as hashrate is generally going up and blocks get mined a bit faster until difficulty is adjusted).
 

The above assumes a fee sufficient for the transaction to be included in the next block.


Yes.


No.

Do you really need to keep asking these questions that you could just google and get an instant response instead of waiting for someone to reply here?

What the average time it takes for 1 confirmation?  What about 2?  What about 3?  And what about 6? 
 

does it depend on how much sat they use for sending fee?


Would you say about 10 mutes per each confirmation?  could you ever get 6 confirmation in less than 30 minutes or it will take about an hour or so no matter what?

RBF being enabled or disabled depends on the wallet and your personal settings. Ledger Live has it enabled by default but you still can't bump the fees from within that software. So even if your transaction is sent with RBF on, you can't send a second one with higher fees. With Electrum, you can. If the original transaction was RBF enabled and hasn't confirmed, all you need to do is right click on it and speed it up. I think the option you need to select is called "Increase fee" or something similar.

No, that's not how Bitcoin works. That's how Ethereum works. UTXOs are always spent entirely. Nothing is left in that address. You can see that here. Whatever funds didn't go to the recipient, were sent to the sender's change address.  

That's not always possible. Imagine if you consolidated all your inputs into a single one that now holds 50 BTC. If you have to send a transaction, you have to do it from that 50 BTC UTXO. You can of course use ChipMixer to break the connection, but in case you don't want to do that or know how to, you have to break up that 50 BTC UTXO. Whether or not that is a good way to use Bitcoin is an entirely different dicussion.

Well, you obviously want it to say that the privacy is high. I have never paid attention to the ratings blockchain explorers make though.

So when the site sent me funds, it showed rbf not enable.  But when i sent funds to another gambling site, does shows rbf enable when i sent.


i see that transaction example you posted... so that person who sent btc isn't very private then correct?


Because they sent 0.00096483 BTC on the transaction correct?  But the btc was sent from a btc address containing 0.11660278BTC?  So the remaining balance in that btc address still contains 0.11563795 BTC.  So the receiver knows okay that person still has that btc balance?  But is obviously not that much compare to if it showed btc address that have lot more right?  So normally when you send btc to someone you want to send them from a btc address where the remaining amount after you have sent them btc is a lower balance right?


Also on bottom left corner shows privacy is low with number two.  What do you want that to show or not show?   


Thanks.

Click on the transaction details in Ledger Live and see if it mentions RBF enabled or not anywhere. If it doesn't, copy and paste the transaction ID to blockchair explorer. Click on Additional info and it will tell you RBF Enabled - YES or NO.
Here > https://blockchair.com/bitcoin/transaction/e2d75ac5ee10425a44bd40c74bd840784057cfc0efcd4214135bd319ce1fe877
Just an example of a random transaction.

For your own safety, don't accept 0-confirmation transactions as final no matter if they are RBF enabled or not. 1-2 confirmations for pocket money, at least 3 for larger transactions.

That's a feature of some gambling sites. They have instant deposits. That means your money gets credited to your account as soon as the transaction is broadcasted (before the first confirmation). But you can't withdraw it until the deposit is confirmed on the blockchain.   

I showed you how with the blockchair explorer previously. But it doesn't really matter now. Your past transactions are in the past and what is done is done.   

You are asking the same things again. Wait for the transaction to get 3-6 confirmations. A transaction with 6 confirmations is generally considered as final. End of story. A double-spend doesn't cost any money. A 51% attack would probably cost millions of dollars daily and isn't worth it because you could only affect your own transactions and not random ones from the blockchain. 

Okay so when i give someone a new btc address generated by ledger live, the moment i see the transaction show up in latest operations, check to see if replace by fee is used correct?  But how do i check this?  You click on that transaction an look at what exactly?  So no replace by fee used, then its impossible to reverse?  So 0 confirmations would work for that?  Or not?



Well when i send btc to a gambling site, they usually credit my account in a reasonable time.  How do check if i sent btc with the replace by fee?  does ledger live auto check it?  can i check if my previous btc sent transactions had the replace by fee and how?



My concern is receiving btc from someone that i haven't established trust with.  Wait so for someone to double spend, does this cost them money?  Read that if someone tries it after a certain amount of time or hours, the more it cost them?  So if you wait for three confirmations, then how safe are you?  How much confirmations to be impossible to reverse?



That make sense it depend the type of customer an the amounts.  So if you were to trade with a stranger who you never traded with, let say 500 dollars is a decent amount of money for you ... whereas for someone it could be 5k or more, how much confirmations would a regular person require?  



i want to be protected for any amount, whether its 50 dollars or 500 dollars or 5000 dollars.  How many should you require? 3 or 6 confirmations no matter what?

---

So coffee shops that accept btc for coffee, let say its for 5 dollars, how do coffee shops protect against double spend?  Wouldn't that be impossible because that person would need to do two transactions at once while at the counter paying for it?  Also the other place they trying to double spend... has to be the same exact coffee amount?  Also when they to this, that payment has to empty their btc wallet correct?  Such that you can't double spend 0.0005 btc for coffee if your btc balance is higher than that?  So for a scammer who wants to double spend say 0.05 btc for swapping their btc for someone elses gambling funds or neteller funds, they would
need to make sure that wallet has exactly that 0.05 btc to double spend and double spend it to the other trader and to their other own btc wallet... is that correct?  So the person that is going to sen neteller fun to the other person, make sure it has or 6 confirmations even if the amount is say just 50 or 500 dollars or would that be ridiculous to the other person having to wait? Again, don't want to get scammed no matter the amount.



So say someone wants to buy a used car from someone for 0.5 btc.  They meet the seller for the car but when would that scammer do the double spend?  So say they met last time for him to look at car an now he agrees to buy it.  Tells the other person he will pay btc.  Would that scammer make payment say before they meet?  Say hours before?  No way they can do that right because by the time they meet hours later, it will have enough confirmations where it can't be reverse?  Or if it still show 0 confirmations, well the seller isn't giving the buyer the keys to the car right?  So that scammer would make payment right on the spot when they meet and the seller would check their phone and go okay... i see payment but i have to wait for 3 or 6 confirmations before he gives the buyer the keys?



What do you mean unreliable people you don't trust or know yet, need six confirmations for it depositing and ten for withdrawal.  don't understand the withdrawal part.



So when people buy or sell big dollar amount items, say a laptop, thus anything that is at least 500 dollars or more, the seller always should wait for 6 confirmations?  But if its a big money transaction, such as a house or car, that should be minimum 6 or even 10 confirmations?

We cannot give a general answer to your question, but the number of confirmations required depends on several factors.


For example, I can divide them into:

 - The type of customer and the degree of trust in him: In the end, if you are dealing with an unknown customer to buy a cup of coffee, is not the same as dealing with an unknown customer to buy a car, not like dealing with a trusted customer to buy a house, but in general the possibility of reversing 6 confirmations is considered impossible in the normal situation.
 - Network status: For example, there are some rare cases such as soft fork, hard fork, sudden drop in mining, and others.
 - The nature of your activity: Economic activity varies, for example, trading platforms are looking for speed to seize opportunities, and therefore the number of confirmations varies according to the risk, but in general and for the speed factor, one confirmation is sufficient for most cases.


So in short and according to your system, you can divide the system into several degrees, trusted people need one confirmation for depositing, two to three for withdrawals and unreliable people with large amounts of money need six confirmations for depositing and ten for withdrawal.

Yes, check ---> https://forkmonitor.info/stale/btc/666833 (Multiple blocks were produced at height 666833.)

Since it's you who sends the funds, it's really up to you. Wallets (possibly depending on settings) allow you send even unconfirmed inputs.


Yes, that's how most businesses go.


Yes, big amount of money means different thing from person to person.


You're looking to the problem to a completely wrong angle. The acceptable number of confirmation is not related to sending, it's related to accepting, really.
Let me explain: if you consider the funds "good" even without confirmation, you have the chance to get scammed. If you send unconfirmed funds and those will never be confirmed, you also can be seen as trying to scam the one expecting funds from you.
So, at least in theory, it's you who have to see for yourself if 1 confirmation is enough for your peace of mind or not. It's you who has to check if the funds sent out got confirmed as expected.
In reality, if the recipient of your funds is a business (casino, sportsbook, whatever), they do have their own rules and will credit your account based on those rules, so you can easily be, let's say, less careful on that.


Yes, it can happen, but the more the number of confirmations, the tinier the chance for that to happen.
Technically it can happen that for various reasons there's a chain split - some nodes see block X mined, some others don't see X, see Y instead. Then, depending if miners/pools mine the next block, then longer chain is accepted by all; if the new chain is built on top of X, then block Y is lost, if the new chain is built on top of Y, then X is lost.
The "lost" transactions are not necessarily lost, some were present in both X and Y already, some will be mined afterwards.
But there are no example to be seen on the blockchain because the bad (orphaned) block is discarded/not stored; it's like it never happened.

All in all, even if you accept only 1 confirmation there's a huge chance you'll get no troubles and, if the sender is legit, the tx will get confirmed sooner or later.
If you go for 3+ you are on a really safe side.


It not really reverse, instead they send the same money somewhere else faster. If you want to avoid getting scammed you should never consider you've received money unless there's at least one confirmation.


Again, you're looking from the wrong side. The one accepting the payment has to be careful on that, not the sender.

Chain reorg can occur. Chain reorg means a single block mined at once by two miners or mining pool. Between the block mined at the same time by the two miners, newly mined blocks will be building on one which will become he longest chain and the valid one while the abandon one will become invalid.

Assuming miner A and B mined the same block at the time, but newly mined blocks are building on the block mined by miner A, blocks mined by miner B will become invalid, the transactions included in the block mined by miner B will become unconfirmed again. What was confirmed before then become not confirmed.

The unconfirmed (that was confirmed before but become unconfirmed) transaction would be in the mempool of the node of the miner B and later would be included in the block mined by miner B. It would still happen in a way that the confirmed transaction that become unconfirmed would still later be included in a block and become confirmed.

But when the confirmed transaction become unconfirmed as a result of chain reorg which I have just explained above and the mempool become congested, if low fee is used for the transaction that can not get the transaction confirmed early and not confirmed in the next 14 days, it is possible that the transaction will later dropped out from mempool in a way the sender would be able to respond the coin in another transaction.

For a transaction to be reversed and become not confirmed, let us relate it to 51% attack or when mining pool is having more hashrates. Click on this link and scroll down, check the table and the chances of a transaction to be reversed. The more the confirmation, the lesser the chance of a transaction to be reversed.

But with the distribution of bitcoin hashrate in relation to mining pool, 1 confirmation is enough, but thinking of youself to be on a secure side, you can wait from more confirmations.

---

Also it depends on if a transaction is a replace-by-fee transaction or not. Do not be surprised if someone like me tell you not to flag a transaction to be replaced by fee in a way you can not pump the fee, in a way you can not double spend the coin, but yet using the fee that can let the transaction to get confirmed very early. So far you follow the criteria that I need, it is possible for me to accept unconfirmed transactions just like some other people that think of it this way. But this should not be recommended for ordinary people that do not know much about bitcoin.

If replace-by-fee is used, an unconfirmed transactions having a replace-by-fee unconfirmed parent can be double-spent in a way the child transaction which nis definitely not yet confirmed will become not valid again.

Generally, 1 is sufficient. There are two cases where someone can execute a double spend after a single confirmation;

1) With the participation of a miner, the adversary can ask the miner to include another transaction from the one that you see. This method is contingent on if your transaction gets included in another block first. Generally, this wouldn't happen because the chances of it is so low and it is such a hassle to contact a miner to get your transaction confirmed and you'llll still have to pay them on top of this. As such, you probably won't encounter this.

2) Orphaned blocks. Competing blocks might be on the same chain and the orphaned block might contain your transaction while the longest chain doesn't. You'll see a single confirmation, and then it goes back to unconfirmed afterwards. Orphaned blocks are pretty rare nowadays so it doesn't actually happen, once or twice in recent history but it wasn't intentional.

Tl;dr: 1 confirmation is enough, most exchanges and some service wait for 3 out of an abundance of caution.

Generally 6 confirmations will make a transaction almost impossible to reverse.

I don't think we've had blocks be orphaned in a row though so 1 conformation is mostly safe.

I don't tyone would mind waiting the 6 (or the 3) if you ask them too, some blocks can take a while to be found anyway so a block could take half an hour to be mined sometimes.

Just don't accept payments with 0 confirmations unless you really trust the person.

Will be trading some of my online gambling funds and neteller funds with few others for btc. 



As of right now, how much confirmations should i require after receiving btc before i send funds to the other person?  Someone mentioned a while back just 1 confirmation is fine but ifs its bigger amounts, you need 3 to 6 confirmations?  How much is considered big amount?  Does it depend on the person?  How many confirmations before its safe to send the funds to the other person?



Will be using ledger live and thus generate a new btc receiving address each time i swap with someone.  Could a transaction still be cancelled after one or two confirmations?   Are there examples of this happening?  Heard people would scam by sending btc and then getting you to send... but when they sent btc, it had 0 confirmations and they later reverse it?  How do you make sure here?  Example imagine you had some online funds or were to sell something to someone even... how much confirmations before you send or ship them whatever they want to buy?  Does it matter ii its for 5 dollars vs 500 dollars vs 5000 dollars?