Hop bitterness is measured in International Bitterness Units, IBUs for short. One IBU equals one mg of isomerised alpha acid per litre of beer.

The bittering compounds in hops are known collectively as alpha acids. However for the alpha acids to properly impart their bitterness to the beer they must be isomerised. This just means  the atoms in the alpha acid molecules are re-arranged to create new structures. This is achieved by boiling the hops in the wort. The longer the hops are boiled, the more isomerisation, and the more bitter the beer becomes.

The formula for calculating IBUs is:

IBU    =    grams x alpha acid x utilisation rate
                   -------------------------------------------------
                                     Volume x 10


The formula can be transposed to give:

grams   =             IBU x volume x 10
                    --------------------------------------
                    alpha acid x utilisation rate

Grams is the weight of hops, alpha acid is the percentage supplied for the particular hop, and the volume is in litres.

The utilisation rate is an estimate of the proportion of alpha acids which are isomerised for a particular time of boiling, and end up in the finished beer.

Utilisation rates vary. Utilisation rates fall as the wort gravity, and amount of hops increase. Procedures and equipment will also effect utilisation rates.  Yeast numbers effect utilisation rates. When you taste spent yeast from the bottom of the fermenter it is bitter. A proportion of the bittering compounds, isomerised alpha acids, stick to the yeast. The more yeast, the more bitterness is pulled out of the beer. If yeast numbers are down because of poor yeast health etc then the beer will taste more bitter than planned.

The table below shows different sets of utilisation rates. Across the five sets there are quite a few differences, and similarities. I don't think any one set is correct. For example, the first column shows the rates I have come to use over the years, they reflect my experience. They are also the ones I use in the shop with customers.


 


Minutes
AHB/Casey
Rager
Garetz
Tinsteth
Daniels
60

30%

30%

20%
23%
24%
45
22%
27%
18%
21%
22%
30
18%
15%
11%
18%
19%
15
12%
8%
2%
11%
12%
10
10%
6%
0
8%
12%
5
6%
5%
0
5%
5%
Flame out
5%
5%
0
5%
5%
Whirlpool
4%
-
-
-
-