How many BTC will be mined in 24 hrs?

BTC = (H x B / D) x (60 x 60 x 24 x 65535 x 10^6 / 2^48)
where H = your hash rate (Mh/s), B = block reward (BTC), D = difficulty

How to calculate the difficulty?

D = H / 7158388.055
where H = network hash rate (Mh/s), D = difficulty