As a home brewer at Fifth Season Gardening, I get fairly frequent requests about how to properly calculate the ABV (Alcohol By Volume) of beer/ wine. In this blog, I’ll walk through the measurements and calculations necessary to determine ABV of your brew.
First let’s take a look at what these numbers mean on a standard hydrometer (an instrument used to measure the density of liquids). One side of the hydrometer shows Specific Gravity, which is the ratio of the density of a substance to the density of a reference substance. In brewing terms, this is the amount of measurable sugars in the wort we created. The higher numbers will represent more sugar in the solution, which in turn will create a higher ABV.
The second set of numbers stands for degrees Plato, or Brix. This measurement is on the metric system, so whole numbers are used versus the decimal in the Specific Gravity. These numbers also represent the amount of sugar in your solution. I like to think of it as each Brix level represents the percentage of sugars in your wort. In my opinion using the Brix to calculate our ABV is simpler than using Specific Gravity. I’ll explain why I feel this after we talk about the third set of numbers on the hydrometer. This last set of numbers gives you the potential ABV if all your sugars are fermentable.
Now that we know what the numbers mean on a hydrometer, let’s talk about how to find out the potential ABV. After you have completed brewing your beer, collect around 8oz of wort before adding your yeast, and let it cool down to room temperature. 68F is the temperature that your wort should be when you take your reading with your hydrometer. There is a chart online that will tell you how to make a correction if your temperature is higher or lower than this temperature. Depending on if it is too hot or too cold, you will either have to add or subtract to do the correction.
Once your wort is at 68F, fill up your hydrometer test jar with wort and drop the hydrometer into the jar. When the hydrometer has settled, you want to write down the number just below the meniscus. This first reading is called OG (original gravity). We will also be taking the FG (final gravity) after our beer has finished fermenting.
Using the OG and FG, we will now calculate what our ABV is. If you are using Specific Gravity, substract the OG from the FG. Take this number and multiply it by 131.25. The resulting number will be you ABV. (Example: My OG was 1.052 and my FG was 1.008. 1.052-1.008= .044. .044 x 131.25 = 5.775% ABV) If you don’t like writing down all those decimal places, use the Brix scale. We still subtract the OG from the FG, but instead of multiplying, we simply divide by 2. (Example: My OG is 13.4 and my FG was 2.2. 13.4-2.2 = 11.2. 11.2/2 = 5.6% ABV)
I hope this will help you understand how to calculate your ABV. Using Specific Gravity is slightly more accurate, but requires a bit more math and also having to remember the standard multiplication number for it. In my opinion, using the Brix scale is slightly easier and still gives you ABV within .1% of Specific Gravity.