Focal lengh calculation due to cropping

Hello,

I wanted to know, if somebody could help me with a calculation - rule?
So, I have a 9 mio pixel photo with appr. 112 mm focal lens (on 35mm base).

So how much more zoom/focal lengh would this give, if I make a crop whre there stays 6, 4, 2 mio pixels for example. Are there any calculation rules?