The IBU Calculator is an applet, written in Java, which calculates Homebrew Bitterness Units (HBU) and International Bitterness Units (IBU) of a given beer.
HBU's are simply the amount of alpha acids in a particular batch of beer, ie, the amount of hops added times the percent alpha acid. Each hop addition is calculated separately then summed to give total HBU's. This calculation is relatively easy, but is not as useful as IBU's since boil time is not taken into account.
IBU's are a more useful, and more accurate, measure of hop bitterness in a beer or wort. However, it is much more difficult to calculate.
With increased boil time comes increased hop utilization, ie, the longer the hops are boiled, the higher the utilization rate and the more alpha acids departed to the wort. Since the relationship of boil time to hop utilization is not a constant, calculating IBU's becomes a bit more involved than HBU calculation. To assist with this, I've taken values from the Hop Utilization graphs in The Brewer's Companion, by Randy Mosher, and used interpolation to calculate percent utilization for each hop addition. I then sum IBU calculations for each hop addition to get the composite, or Total, IBU's.