Beer Smith's IBU's

In figuring out IBU's for recipies... I've been using the formula on pg. 353 of The Brewmaster's Bible (oz's of Hops x AA x utilization / 7.25).

Last night I downloaded Beer Smith, and have been punching in some of the stuff I've been working on, and quickly realized that it figures out the IBU differently.

For Instance in one which I had figured had 31 IBU's, Beer Smith is telling me 22.5 IBU's... its like that for all of them... Beer Smith's IBU's comes out significantly lower that what I'm getting throught the TBB formula.

Why is that?

There are many different formulas for estimating IBUs. BeerSmith gives you the option of 3 different ones - Rager, Tinseth and Garetz, I believe. Each gives quite different results, and the actual IBU may be significantly different from what any of these formulas tell you.

Also there are certain things that affect what the IBU's are. For example how many gallons you are boiling, the AA% levels (they vary everywhere), and of course what Broadway said ^.

Well I took out "How to brew" by Palmer, and he states the formula is:
IBU=AAU x U x 75/V

AAu = Alpha Acids of the hops

U = Utilization, which is coefficient based on the isomerization of alpha acids in specific gravites for a set amount of time. (You'll have to look it up on a table)

75 is a constent used as a conversion from English units to metric

V = volume of boil(gallons)

In Palmer's book, He uses a table for utilization published by Glenn Tinseth. So, If you are using a table with slightly different values you will get different IBUs. Palmer's book is online free at howtobrew.com

If you check out Beertools.com, they have different ways to calculate hop utilization based on several formulas. The answers are all in the same ball park, but they are different.

Cool, thanks. This clears it up a bit.

I've got Palmer's book, I just haven't started reading it yet. Just finished The Brew Masters Bible, and started Tasting Beer by Mosher. Was gonna hit the Palmer book next. When I start something new, I tend to dive headfirst into the shallow end.