https://blockchain.info/charts/hash-rate
Let's assume the latest block is mining for 10 minutes. Use the block's Nonce / 600s. that is 30E hash/s. Is it right ?
If it is right, that is only one client's hash rate. it is not the whole bitcoin network's hash capability.
It's impossible to know the exact network hash rate at any one time. But since we know the difficulty and the rate at which blocks are being mined, we can deduce an estimate.
Christian Decker provided a good response to this question a few years ago:
So let's calculate the average hash_rate for a single day:
expected_blocks = 144
difficulty = 11187257.461361 # this is on May 22nd 2013
blocks_found = 155 # Also May 22nd 2013
hash_rate = (blocks_found/expected_blocks*difficulty * 2**32 / 600)
The reason we use a day to average out the hash_rate is that taken block by block the variance would be really high and we would not get anything meaningful.
According to WolframAlpha this gives us an averagge hash_rate for the 22nd May 2013 of 86.19 THashes/s. Numbers of course may vary depending on how you chose your interval, which appears to be the reason numbers don't match with the ones on Blockchain.info