So you have to multiply this number by the number of pools, as they also calculated hashed, but that work was lost as someone found the nonce first
This is wrong in so many ways, I can't even
I think I get what bitmover was trying to say.
There are 2 answers to the question, depending on how you look at the question.
1 answer, which you can get a reasonably close guess to is how much work of SHA-256 hashes has gone into the blockchain that we can calculate. That you can get a reasonably close answer to.
What we can NEVER know, is how many miners spent how much time hashing away never finding a hash that solved a block. Can't even reasonably guess since we don't know what we don't know. i.e. I could have 1PH sitting here mining to a private solo pool for a year and never found a bock so nobody outside of me would know it existed. And if it DID find a block you would never know if I had a massive farm or 1 USB stick and just got really really lucky.
-Dave