Here is what i have found.
HBU ( Home Bittering Units)
For a Five Gallon Batch- Multiply the ounces of hops used for Bittering by there Alpha Acid Number. For Example: 2 ounces of Kent Golding Hops at 5% apha acid per ounce equals 10 HBU or 2oz. times 5 %(AAU) = 10 HBU
IBU ( International Bittering Units) To calculate Total IBU's from per gallon AAU's, first determine the approximate efficiency of hops isomerization. Hops boiled for 50 to 60 minuted will have 30% of the alpha acid converted. Hops boiled for 30 minutes will have 21% of the alpha acid converted. Hops boiled for 5 minutes will have 10% of the alpha acid converted.
also there is a calculator herehttp://www.djmphotography.com/www/Br...lets/Hops.html