What is the formula for AAU?

AAU = hop weight in ounces × alpha acid percentage (as a whole number). For example, 1.5 oz of a 6% alpha hop gives 1.5 × 6 = 9 AAU. AAU and HBU are the same figure under two different names.

What is the difference between AAU and HBU?

There is none. Alpha Acid Units (AAU) and Homebrew Bitterness Units (HBU) are identical — both equal weight in ounces multiplied by alpha acid percentage. HBU is just the older homebrewer name for the same number.

How do I substitute one hop for another using AAU?

Keep the AAU constant. If a recipe calls for 2 oz of a 5% hop (10 AAU) and you only have a 10% hop, use 1 oz to keep the same 10 AAU and roughly the same bitterness.

Does AAU account for boil time or utilization?

No. AAU is a quick approximation of bitterness potential only. It ignores boil time, gravity, and hop utilization. For accurate International Bitterness Units, use a Tinseth or Rager IBU calculator.

Is my recipe data stored anywhere?

No. The calculation runs entirely in your browser. Nothing you enter is uploaded or saved on any server.

Hop AAU (Alpha Acid Units) Calculator

The Hop AAU Calculator converts a hop addition’s weight and alpha acid content into Alpha Acid Units (AAU) — the same figure homebrewers also call Homebrew Bitterness Units (HBU). It is the fastest way to compare hop charges and swap one variety for another without running a full IBU model.

How it works

Bitterness in beer comes from the alpha acids in hops being isomerised during the boil. A larger weight of hops, or hops with a higher alpha acid percentage, delivers more bittering potential. AAU captures that potential in a single number:

AAU (HBU) = weight (oz) × alpha acid %

For example, 1.5 oz × 6% = 9 AAU. The alpha acid percentage is entered as a whole number (6, not 0.06), which is how it is printed on hop packaging.

To find a recipe’s total, add the AAU of every hop addition together. Because the figure is just weight times strength, it scales linearly and is trivial to work with on brew day.

Substituting hops

The main practical use is hop substitution. If you keep total AAU constant, the bittering contribution stays roughly the same even when you change varieties:

new weight = original AAU ÷ new hop’s alpha acid %

So 2 oz of a 5% hop (10 AAU) becomes 1 oz of a 10% hop. This protects you from the classic mistake of over-bittering when swapping a low-alpha hop for a high-alpha one.

Notes

AAU is an approximation. It does not account for boil time, wort gravity, or hop utilization, so two beers with identical AAU can finish at different IBU values depending on when the hops were added. Treat AAU as a planning and substitution shortcut, and use a Tinseth IBU calculator when you need a real bitterness figure.