How to Calculate IBU's

International Bitterness Units (IBUs) often show up on packaged craft beer and just as often in conversations involving IPAs.  In this post I will share how home brewers can easily calculate the amount of IBUs in their recipes..  As a home brewer you must know how to calculate these if you hope to make consistent recipes.

Alpha acids in hops come from soft resins in the lupulin glands of the hop flower.

An IBU is a measurement of the isomerized alpha acids that are present in finished beer.  This is measured using a spectrometer that looks at the absorption of light at certain wavelengths.  While that is awfully scientific, there are a few methods of estimating the amount of alpha acids that are extracted into the final beer.

In his book Designing Great Beer, Ray Daniels presents the following equation for determining the amount of IBUs contributed by a single hop addition in the kettle (basically the only time when alpha acids will be extracted): 

where W is the weight of the hop addition, U% is the utilization which comes from a table that factors in the gravity of the boil and duration of the boil, A% is the alpha acid percentage of the hop variety, V is the volume of the final wort in liters, and Cgravity is a correction factor to be used only when the wort in the boil has a specific gravity higher than 1.050.

The correction factor can be neglected to keep things simple, but in case you tend to brew higher gravity brews then it can be determined as follows:

where the Gboil must be greater than 1.050.

Remember that this is used to calculate the contribution of IBUs for each hop addition.  Typical home brewing that involves a hop addition at 60 minutes, 30 minutes, 15 minutes, and 5 minutes would require four separate calculations – dry hopping does not contribute IBUs.  That being said, if you are calculating your hop bill for a recipe then consider the following:

1.    What are the target IBUs for the recipe (total)?

2.    How much flavor and aroma hops will you add (the 30-, 15-, and 5-minute additions)?

3.    Calculate the IBUs contributed by flavor and aroma additions.  Subtract from the target IBU total.  This gives the amount that must come from bittering hops (60 minute addition).

4.    Use the equation below to determine the weight of bitter hops needed.

Rearranging the previous equation:

So, for an example, I recently made an American Pale Ale that had a target of 30 IBUs.  Here’s the rest of the necessary information:

Flavor and aroma additions -

Centennial (8.5% AA):  7g – 15 min

Cascade (5.5% AA):  28g – 15 min;  28g – 5 min

Gravity of boil:  1.058 à correction factor = 1.004

Utilization for 1.060 boil:  0.211 (60 min); 0.105 (15 min); 0.042 (5 min)   *Palmer, 58.

Using the IBU equation, the flavor and aroma additions contribute the following IBUs:

Subtracting this from the target of 30 IBUs, we are left with 17 IBUs to be provided by the bittering addition.  For a 60 minute boil, to find out how much Centennial needs to be added we use the second equation:

So for this recipe we should start with 21 grams of Centennial along with the pre-determined amount of flavor and aroma additions.  (Admittedly I often start with an even amount for the bittering and then figure out how much more I can add for the flavor and aroma).  I will also note that this recipe falls at the bottom of the range of acceptable IBUs for the American Pale Ale style (30 – 50 IBU).  For these calculations I used the utilization table from John Palmer’s How to Brew text (p.58), which he gathered from empirical data compiled by Glenn Tinseth on his site www.realbeer.com/hops. 

Remember that every brewery and brewer is different and the utilization values will also be very different for each.  Keep in mind that the point for calculating IBUs is only to ensure consistency with your own recipes.  If I taste this APA at 30 IBU and I find it to be more bitter than expected then I will know that I need to shoot for 20 IBU next time (consider that we can only taste with a resolution of about 5 IBUs so 25 IBU might taste similar).  If I send a sample of my beer to a lab they may find that I only have 15 mg of iso-alpha acids per liter (15 IBU).  Does it really matter?  What should matter is that I know how to achieve the flavor profile that I want in the finished beer.  Trial and error is necessary in this case.  If you want to take it to another level you would need to determine your own utilization curve, which is where numbers from Tinseth’s site are derived.

I’ll leave you with one question to ponder then:  Should the number of IBUs even be included on labels?  I plan to discuss this in a follow-up post.

--  Mr. Jackson

Palmer, John.  How to Brew.  Brewers Publications: Boulder, CO. 2006.