See the following page for more details:
https://en.bitcoin.it/wiki/DifficultyThe time required to find 1 block is given by:
t = D * 2^32 / H
(D = Difficulty, H = hashrate, t = time in seconds).
So the number of blocks per seconds is given by:
BPS = H / (D * 2^32)
You get 25
BTC per block, so the income per second in
BTC is
I = 25 * H / (D * 2^32)
or per day:
I = 2160000 * H / (D * 2^32)
1 Ghash/s would do:
I = 2160000 * 10^9 / (D * 2^32)
or:
I = 502914 / D
At current difficulty of 2,193,847,870, that would be:
I = 0.000229238 BTC per day or 0.229238 mBTC per day.