Basic homebrewing has fixed amounts of hops that follow a recipe or even use Homebrew Bittering Units (HBU's) to keep things simple. Notice though that on some bottles of commercial beer and in some reference material the term IBU's or International Bittering Units is used? Calculating IBU's that hops will provide for a beer can be found in the Papazian (page 266), but I will place those caluculations here as it is what we do for every beer. As mentioned prevously in the hop section, I use a 90 minute boil with a pre-boil of 15 minutes. I highly recommend this method. We do a rough gravity estimate using a refractometer and then increase the gravity reading by around 10% to account for some of the boil off (boil off is usually more than that depending on the humidity of the atmosphere during the boil.
If you have some money to spend, a refractometer allows for instant gravity measurments at any time during the boil or during wort sparging. These can come with a Brix scale (degrees plato) alone, or both Brix and Specific Gravity. There are calculations to convert between the two so both are not necessary.
% Hop Utilization (%U) = (Isoalpha Acid Present/Alpha Acid used) X 100
In English Units
Weight is the amount of hops in ounces
Volume is the quantity of wort being boiled
IBU's are International Bittering Units that are required or desired in the batch
% Utilizaton (%U) is the expected amount of hop utilzation from the boil
% Alpha Acid (%AA) is the Alpha Acid percent on the hop package
Weight = (Volume X IBU X 1.34) / (%U X %AA)
IBU = (Weight X %U X % AA) / (Volume X 1.34)
Below is a basic hop utilzation chart that you can find on line or in publications. There are graphs that I have seen as well. On line calculators may also be found on line.