Disclaimer: The proceeding lines will probably only make sense to you if you're familiar with Glenn Tinseth's Hop Utilization formula. This is about to get very geeky.
O = Oz. of hops
I = IBUs
U = Utilization Factor (Bigness factor*Boil time factor)
G = Gallons of Finished Beer
A = Alpha Acid percentage
(O*A*U*74.9)/G = I
This is not the original formulation that Tinseth provides, but it produces the same numbers regardless.
If you had a target IBU in mind, and wanted to determine what amount you would need to reach it, here's the formula you would use:
I/[(A*U*74.9)/G] = O