IBU stands for International Bitterness Unit. 1 IBU = 1 milligram of isomerized alpha acid in 1 liter of wort/finished beer. The higher the IBU #, the more bitterness in a given beer. Although this can be slightly deceiving because a malty beer with 50 IBU’s will be perceived as being less bitter than a pale ale with 50 IBU’s. The heavy malt character would balance the bitterness, so it seems less bitter when in fact it has the same bitterness levels as the pale ale.

IBU’s can only be 100% accurate when tested in a lab using expensive equipment. Home brewers can come close to IBU numbers using calculations which were made from experimental data taken from various worts. There are three main calculations used by home brewers to determine IBU.(2) They are:

• Rager
• Tinseth
• Garetz

These three sets of calculations are similar except for how they determine hop utilization.

Utilization is the percentage measurement of hop’s alpha acids “utilized” during the boiling process. This gives you a formulated number to measure the bitterness of a final beer known as IBU’s (International Bittering units), AAU’s (Alpha Acid Units), or HBU’s (Home brew Bitterness Units). (2)

There are many factors determining hop utilization including:

• Boil length – A longer boil means more utilization, though after 90 minutes there is generally no more increase in utilization. 10-15 minute boils generally don’t contribute to any bitterness (just flavor/aroma), so utilization generally starts after this time. (3)
• Vigor of boil – the more vigorous the boil, the greater the utilization.
• Blow-off losses – The use of a blow-off tube during fermentation results in the loss of precious alpha-acids, which means less utilization.
• Trub / Proteins – Alpha acids have a slight electric charge which can cause them to bind to proteins and trub which will settle to the bottom of the fermenter during fermentation. The wort is racked off these settlements, resulting in alpha acid loss. (1)
• Absorbtion by yeast – The alpha acids can be taken in through the cell walls by yeast. When the yeast settles, the beer is racked off, resulting in alpha-acid loss. (1)

The three methods are quite different in how they determine IBU’s, and my best advice would be to just pick one that you think makes more sense, and stick with it. Remember that the homebrewer cannot accurately determine the total IBU’s and that all numbers are estimates. I personally use Glen Tinseth’s method, but know people who use the others and are completely satisfied. Whatever works best for you, stick with it, and adjust if necessary.

Glen Tinseth method:
This method is the newest of the three for determining IBU’s, and in my opinion the best because of the data backing up the equations, and his ability to use equipment and conduct research at Oregon State University (5). The graph of his data looks completely different than the other two methods, and seems to make more sense and be less random.

According to his method, you must first determine the amount of alpha acids (AA) added to the wort for EACH hop addition using the following equation:

After that calculation is done for each hop addition, you can use the following equation to determine the IBU’s for each addition:

The AA utilization that is needed in the IBU calculation can either be taken from the table found here, or by completing the utilization calculation.

The bigness factor can be determined by:

The Boil Time Factor can be determined by:

The equations may seem to be really confusing at first..(at least I was…) but once you get the hang of it, it is really simple. If you don’t feel like doing the math, there is an online calculator which will do it all for you. The calculator can be found by clicking this link.