During the home brewing process a hydrometer is used to monitor the change in original gravity (OG) and final gravity (FG). This measurement, applied to a simple formula, will give you the amount of alcohol by volume (ABV) in your finished beer.
At the beginning of the fermentation process (before the sugar starts to convert to alcohol) you will place your thoroughly sanitized hydrometer into your fermentation bucket and take a reading. To do this you simply place the hydrometer into the liquid and allow it to float. You may want to gently spin it to dislodge the bubble around it to see the numbers. After it stops bobbing you log where the liquid measure on the scale. Water has a specific gravity of 1.000. Since your unfermented beer contains levels of fermentable solids it will have a gravity higher than 1.000. For example, an American Pale Ale I recently made had an OG of 1.051 and a FG of 1.012.
So here comes the part that might seem just a little tricky but if you simply follow the formula it's a snap. Alcohol by Volume % (ABV) is calculated by taking the Original Gravity and subtracting the Final Gravity and multiplying by 131.25. For the American Pale Ale example above it would look like this:
- (1.051 - 1.012) x 131.25 = 5.19% ABV
I hope this post was informative and helped take some of the mystery out of this piece of the home brewing puzzle.
To make the process even easier, you can use this ABV calculator.