Having brewed your own beer, you will likely want to test its alcohol content. To do this, you will need to use a simple tool called a hydrometer. A hydrometer determines the level of fermentation that has taken place in your brew by measuring the liquid density. Alcohol is a denser liquid than water, so measuring your brew before and after fermentation using a hydrometer will allow you to calculate the alcohol content with the help of a simple formula.
Place 1 cup of water in a test jar. Ensure the water is at room temperature (around 20 degrees Celsius or 20 degrees Celsius). The temperature of a liquid affects its density. Place the hydrometer into the glass and check the reading. It should rest at 1.000 -- the specific gravity (SG) of water. If it does not, it is likely something in the test jar has contaminated the water, or the liquid is too hot/cold.
Pour 1 cup of your brew before the yeast has been added into an empty test jar. Insert the hydrometer and make a note of the reading. This number represents the original gravity (OG) of your brew.
Once fermented, pour one cup of your brew into an empty test jar, insert the hydrometer and take a note of the reading. This is the final gravity (FG) of your brew.
To calculate the alcohol percentage by weight, you must first subtract the original gravity from the final gravity. Multiply this number by 105 to calculate the alcohol percentage by weight.
Alcohol % by weight = (FG - OG) x 105
To calculate the alcohol percentage by volume (ABV), the standard commercial alcohol content measure, subtract the original gravity from the final gravity as before, and then multiply by 131. Alternatively, multiply the alcohol percentage by weight by 1.25.
Alcohol % by volume (ABV) = (FG - OG) x 131
Alcohol % by volume (ABV) = alcohol % by weight x 1.25