Formula for number of bricks per sq/m?

[Swindlehurstguy]

OK folks,  need to know how many bricks to buy to build a double wall to close up a window that I want to build closed. There is some sort of formula, but I don't have it, just don't want to buy too many bricks?
Anyone?

 

[Schmuck]

 

[willieopperman]

 

[FranZAR]