Thursday, June 7, 2007

I'm Almost Certain Someone Has Pointed This Out Before

While I am the sort of person who doesn't mind doing calculations, I'd rather have a quick idea of what's going on before reaching for the calculator. Of the things that I consider when designing a recipe, Original Gravity and ABV come first. They're obiviously quite closely related. However, it hadn't dawned on me until recently how easy it is to take a really good guess as to what you're ABV will be like just by knowing your OG and your apparent attenuation rate without doing any calculations.

Let's say you want to make an APA with an OG of 1.055. Let's also say that you're going to use Wyeast 1056 American Ale. You know that it has an apparent attenuation range of 73-77%.

An apparent attenuation rate of 76% will yield a terminal gravity of 1.0132 (we can just round to 1.013). This would make a beer of just around 5.5% ABV. Interestingly, this is the same number as the OG expressed in GUs x 0.1. I've found that they tend to hang really close to one another.

OG = 1.055
55 x 0.1 = 5.5
GU = 55

The ABV Calculation*:
1.055 - 1.013 = 0.042
0.042 x 131 = 5.5% ABV

Likewise this will work with worts of different gravities. How about a high gravity wort like 1.093?

OG = 1.093
GU = 93
93 x .76 = 70.68
93 - 70.68 = 22.32 (Terminal Gravity in GUs)
1.093 - 1.02232 = 0.07068, round to 0.071
0.071 x 131 = 9.3% ABV

How about a low gravity wort like 1.032?

OG = 1.032
GU = 32
32 x .76 = 24.32
32 - 24.32 = 7.68
1.032 - 1.00768 = 3.18592, round to 3.19%
Pretty close!

Anyway, it is important to keep in mind that this is just something to give you a quick idea of what your ABV will be like if you plan on having a wort that is attenuated to around 76%. If you plan on making something much less attenuated, or much more highly attenuated, this clearly won't do you much good. However, apparent attenuations around 76% are common enough to make this a helpful mental device.

*I would also like to point out that your results may differ if you use another calculation to determine ABV%. Not everyone uses (OG - FG) x 131 = ABV%. Your mileage may vary.


jonathan said...

OK, so does GU = Gravity Units?

joe said...


in search said...

gnawsome, thanks!

Jonathan said...

sorry, that was me

Todd said...

Haha. Yeah we were just talking about that over the weekend. The phenomenon is also mentioned in Michael Jackson's "World Guide to Beer." He did not provide any math or even mention of the estimates dependence on an apparent attenuation around 76%, but it was mentioned as a good way to take a guess on how strong your beer is.

Anyway, good call, man!

p.s.- how did blogspot know my name is Todd?

joe said...

I imagine it has something to do with Google owning Blogger.

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