It is easy to calculate percent alcohol in beer and it only requires two important measurements.  A beers alcohol level is measured in Alcohol by Volume (ABV), it is the calculated amount of the total volume of liquid that is alcohol.  The density of a liquid is often measured in unit of specific gravity, where water has an approximate density of 1.00 g/mL or a specific gravity of 1.000 at 60F.  The specific gravity of a liquid is a comparison of its density vs the density of water.

Gravity Readings

During the fermenting process, yeast is added and the yeast eat the sugars in the wort(sugar water) producing alcohol and CO2.  After fermentation, since alcohol is less dense then wort (sugar water) there will be a change in the specific gravity of the liquid.  ABV is calculated by taking a specific gravity reading of the wort, beer before fermentation, and beer after the fermentation.  The two measurements are referred to as the Original Gravity(wort) and Final Gravity(beer post fermentation).

Measuring Specific Gravity

Measuring the specific gravity is often either done by a hydrometer or a refractometer.   All measurements should be done at what the hydrometer or refractometer is calibrated to, usually 60 °F.

graduated cylinder with hydrometer

graduated cylinder with hydrometer


To measure the specific gravity using a hydrometer fill the cylinder until about 2-3 remain of head space, drop in the hydrometer slowly and spin it.  When reading the hydrometer make sure it is not clinging to the side of the cylinder, it should be completely floating.

fill cylinder with 2-3 inch head space

fill cylinder with 2-3 inch head space

read specific gravity on hydrometer

read specific gravity on hydrometer

Using a refractometer is much easier and requires only a drop of liquid.  A refractometer measures the density of a liquid in units of brix, amount of sugars present in a liquid.  There is a coversion equation below to go from brix to specific gravity.

refractometer

refractometer

Equations to Determine ABV

Here is a basic equation that can be used to determine ABV of a beer.  Two contants are used.

1.05 g — represents the amount of CO2 produced for every gram of ethanol produced
0.79 g/mL  — represents the density of ethanol alcohol, drop the g/mL to get the specific gravity.

 (  ( 1.05  x  ( OG – FG )  )  / FG  )  / 0.79  x  100 = % ABV

*For temperature correction use the following  ~50 °F subtract .001  /~ 70 ° F add .001 / ~ 80 ° F add .002

For example, lets say you are brewing up a dry irsh stout and your OG=1.052 with your FG=1.014.

 (  ( 1.05  x  ( 1.052 – 1.014 )  )  / 1.014  )  / 0.79  x  100 = 4.98% ABV

Here is an equation that can be used to convert units of brix to units of specific gravity.  This is an estimate equation, refer to the USDA Brix Measurements doc for the extact conversion numbers.

Specific Gravity = 1 +(0.004 x °Brix)