Comparative figure for bitterness are the International Bittering Units, defined as 1 IBU = 1 milligram of iso-alpha-acid per liter. There are different calculation models and tables available for estimating this value. I'm using the formulas supplied by Glenn Tinseth but take a higher utilization for hop pellets into account (I like to muck around). With the Java-Script simulation below (works offline!) you may observe, how bittering is affected by variating different parameters of 3 hoppings. Calculating the required weight of hop for given IBU-targets is also possible.

Notes:

A usual way for calculating hoppings looks like this. First spot the amount of aroma hops based on the recipe. AA and the short boil will result in a few IBUs. The difference to the IBU-target has to be produced by earlier hoppings. To calculate the necessary amounts, enter AA, boil time and required IBU. Take into account that utilization proceeds until you start chilling. Distinguishing between Knock out wort and Original Wort enables high gravity brewers to consider effects by high extract content and dilution.