Bitcoin Forum

As seen in the top all black nav bar of http://bitcoincharts.com/markets/

How do they determine the number of terahashes/second, currently at 0.735 Thash/s???

Isn't a hash only submit to the network once it has been deemed to be below the target value?

Is the network hash rate just an estimate?

I think it's guessing using a combination of the known current difficulty and how quickly blocks are currently being solved.

Yes, and a very tough one to make.

Ok thank god. I thought I was going nuts there for a second.

I'm a man of logic. And the speed with which that number sways every few hours doesn't make sense given the nature of mining.

Actually, it's not that hard.

The wiki states somewhere (can't find the page now, of course) that the probability of a single hash calculation solving a block is

      target
     -------
       2²⁵⁶

and the average number of hashes required to solve a block is its inverse:

       2²⁵⁶
     -------
     target

Let's call that last formula A. On average, then, it takes A hashes to solve a block. We know how long it takes to create a particular block (just take its creation timestamp and subtract the timestamp of the previous block from it). So the network hash rate is simply A / timespan.

Of course, individual blocks will be highly variable due to the random nature of finding a solution (there were about 3 or 4 blocks recently that were created within 5 minutes; the block before that series took 20 minutes, as did the block after). To get a more accurate estimate, you'd want to take the time over 50 or 100 blocks. The formula is:

      2²⁵⁶ * n
-----------------------
target * (ts_N - ts_N-n)

where
n is the number of blocks you're sampling
ts_x is the timestamp of block number x
N is the latest block number (or whatever block number you want to end your sample range with)
and target is currently 0xCBBD000000000000000000000000000000000000000000000000