How To Use a Hydrometer To Calculate ABV

A hydrometer is a tool that measures the specific gravity of a liquid. Basically, it tells you the density or sugar content of a beer, mead or cider sample.
Yeast convert sugar into alcohol during fermentation. By comparing a beer’s sugar content before & after fermentation, you can determine its ABV!
Before fermentation your apple juice, beer wort or wine must will contain lots of sugar. The hydrometer will float higher in a more sugary liquid and have a higher Original Gravity (OG) reading. After fermentation the yeast will have converted most of the sugar into alcohol. The hydrometer will sink lower in a less dense liquid sample and have a lower Final Gravity (FG reading).
With your OG & FG readings, you can easily calculate the ABV! Let’s use this Hard Cider, fermented with EC-1118 yeast, to demonstrate:
(OG minus FG) multiplied by 131.25 = ABV%
(1.044 - 0.996) x 131.25 = 6.3% ABV
NOTE: don’t pour samples from the hydrometer back into your fermenter! Doing so can risk contamination.