The alcohol by volume (or ABV) is a standard measure of how much of the total volume of the liquid is alcohol.  Most beers contain the ABV on their label.  If not, you can find the ABV online.  But how do you find the ABV of the first batch of a new beer?

ABV is determined by comparing the specific gravity measurement of a beer before fermentation and after fermentation.  Measurements are taken using a hydrometer, a device that free floats in a liquid.  The specific gravity of water at 60° F is 1.000.  The denser the liquid, the higher the number.

The first measurement taken before fermentation is called the original gravity.  At this point in the brewing process, the liquid is known as wort and made up of sugars extracted from the grains.  The fermentation process starts when the yeast is added.  The yeast consume the sugar and create carbonation and alcohol.  At the completion of the brewing process, another measurement is taken to determine the final gravity.  It is important both readings are taken at 60° F to produce consistent results.

A simple mathematical equation is used to convert the original gravity and final gravity readings into an ABV percentage.

(Original Gravity – Final Gravity) x 131 = ABV as a percentage

For example:

The original gravity = 1.10
The final gravity = 1.02

(1.08 – 1.02) x 131 = 7.86%

You can purchase a hydrometer at your local home-brew store, or order online here.

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