There are two ways to measure hops for use in brewing.
The first way measures the bittering potential of the hops going into the boil.
Alpha Acid Units (AAUs) or Homebrew Bittering Units (HBUs), are the weight of hops (in ounces) multiplied by the percentage of Alpha acids.
This unit is convenient for describing hop additions in a recipe because it indicates the total bittering potential from a particular hop variety while allowing for year to year variation in the %AAs.

The second way estimates how much of the alpha acid is isomerized and actually dissolved into the beer.
The equation for International Bittering Units (IBUs) takes the amount of hops in AAUs and applies factors for the boil gravity, volume, and boiling time.
IBUs are independent of batch size, and to a large extent, independent of style, unlike the AAU.

Hop resins act like oil in water.
It takes the boiling action of the wort to isomerize them, which means that the chemical structure of the alpha acid compounds is altered so that the water molecules can attach and these compounds can dissolve into the wort.
The percentage of the total alpha acids that are isomerized and survive into the finished beer, i.e. utilized, is termed the "utilization".
Under homebrewing conditions, utilization generally tops out at 30%.

Several factors in the wort boil influence the degree to which isomerization occurs.
Unfortunately how all these factors affect the utilization is complicated and not well understood.
Empirical equations have been developed which give us at least some ability to estimate IBUs for homebrewing.

The utilization is influenced by the vigor of the boil, the total gravity of the boil, the time of the boil and several other minor factors.
The vigor of the boil can be considered a constant for each individual brewer, but between brewers there probably is some variation.
The gravity of the boil is significant because the higher the malt sugar content of a wort, the less room there is for isomerized alpha acids.
The strongest bittering factors are the total amount of alpha acids you added to the wort, and the amount of time in the boil for isomerization.
Understandably then, most equations for IBUs work with these three variables (gravity, amount, and time) against a nominal utilization.
As mentioned earlier, the utilization for alpha acids in homebrewing is generally accepted as topping out at about 30%.
The utilization table on the next page lists the utilization versus time and gravity of the boil.
This allows you to estimate how much each hop addition is contributing to the total bitterness of the beer.
By incorporating a factor for gravity adjustment, the IBU equation allows for direct comparisons of total hop bitterness across beer styles.
For instance, 10 AAUs in a Pale Ale would taste pretty bitter while 10 AAUs would hardly be noticed in a high gravity Stout.
Gravity is not the total difference between styles however, the yeast also yields a particular flavor and sweetness profile which the hop bitterness balances against.
As the maltiness of the beer increases, so does the relative balance between hop bitterness and malt sweetness.
A very sweet American Brown Ale needs about 40 IBUs to yield the same balance of flavor as a Bavarian Oktoberfest of the same gravity does with 30 IBUs.

This brings up a good question, how bitter is bitter?
Well, in terms of IBUs, 20 to 40 is considered to be the typical international range.
North American light beers, like Coors, have a bitterness of only 10-15 IBUs. More bitter imported light beers, like Heineken, have a bitterness closer to 20-25.
American microbrews like Samuel Adam Boston Lager have a bitterness of about 30 IBUs.
Strong bitter ales like Anchor Liberty Ale and Sierra Nevada Celebration Ale have bitterness of 45 or more.

While more experimentation and analysis needs to be done to accurately predict hop bittering potential, the IBU equations described on the next page have become the common standard by which most homebrewers calculate the final bitterness in the beer.
Everyone who uses these equations is in the same ballpark and that is close enough for comparison.

John Palmer is the author of "How to Brew" (2001, Defenestrative).