Measuring Alcohol in Homebrewed Beer

This is my third post in the “other hobbies” category. The other two are “The Climate Journeys of Thomas and Larry” and “Eurovision Victory for Sweden”. This category has nothing to do with Leonbergers and dogs, well almost nothing. I guess it is a get to know the blogger kind of category.

In this post I am explaining how you measure the alcohol level in a home beer brew. You can find this information online and in books, but I am explaining it very succinctly. It is quite simple, and you don’t need to read a ten-page explanation. A few photos with a brief explanation will do.

When you brew beer at home you start by boiling the wort. You boil water and you add the malts and the hops for the flavoring and the aroma at specific times. This all depends on the recipe you are following. The wort easily boils over, which upsets my wife, but luckily my dog Rollo loves to lick the wort off the kitchen floor. He’ll lay there and wait for me to screw up. Don’t worry, the wort contains no alcohol at this point, which makes this a good point to measure what is called gravity. I should mention that you need to let the wort cool off before doing your measurements and before adding the yeast (or you’ll kill it). I use an ice bath to do this.

It is difficult to measure the alcohol directly. You need to set up a chemistry lab in the kitchen, which would upset your wife. Therefore, you use an indirect method using a hydrometer. During the fermentation process, yeast converts sugars into alcohol (and carbon dioxide). As the sugar is used up, the wort slowly becomes less dense. By measuring the density before and after fermentation, you can calculate how much alcohol is in the finished beer. In the beer world this is called measuring the gravity. You can buy a hydrometer in a lot of places including Amazon.

The density/gravity of water is used for reference as 1.000. To be exact, it also depends on the temperature, but for now we’ll ignore that. You measure the gravity before fermentation has started, just before you pitch (add) the yeast. The temperature at this point should be around room temperature, 72 degrees (60 to 75 degrees). This is called the original gravity (OG). Then after fermentation (in your container, carboy, whatever) you measure it again. This is called the final gravity (FG).

I should add that after the fermentation in your container/carboy is done you add a little bit more sugar (called priming sugar), you bottle the beer, and you let it ferment a little bit more, which will add a little bit more alcohol as well as carbon dioxide. You want some carbon dioxide in the beer but not too much. This extra amount of alcohol is not accounted for using the final gravity. However, it is typically around 0.2% and if you wish to include it, you can just add that number.

Using the original gravity (OG) and the final gravity (FG) you can now calculate the ABV, Alcohol By Volume, by using the formula below. For my latest brew, an IPA (India Pale Ale), which I bottled yesterday, I got OG = 1.072 and FG = 1.018. Ideally FG is around 1.010, but for whatever reason I did not get there.

ABV = (OG – FG) x 131.25 = 0.054 x 131.25 = 7.1%

So that would be 7.3% with the bottle fermentation.

There is a more exact formula:

ABV = (76.08 x (OG – FG) / (1.775 – OG)) * (FG/0.794) = which in my case yields ABV = 7.23% which would yield 7.43% with the bottling. I can add the recipe predicted ABV = 7.5%. There are also formulas that account for the temperature at the point of measurement of original gravity and the final gravity. But I think this is good enough.

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Finally, if you would like to learn more about my book and find out where to buy it, click here or here. You can also click the image below to buy it from Amazon. All royalties are donated to the Leonberger Health Foundation International.