© Denzil Green
Alcohol content is measured with a hydrometer.
A hydrometer looks like a buoy that you would spot out at sea with a thermometer stuck on top. When it’s placed in a liquid, it floats upright, with the base weighing it down.
When it’s in plain water, a measurement on the hydrometer marked “1.000” will be right at the water line. When there’s sugar in the water, the water is “denser”, so the hydrometer will float higher. When in fermenting alcohol, as the sugar gets eaten up by yeast, it floats lower again.
The first measurement, with the original sugar present before yeast is added, is called the Original Gravity (or sometimes “Specific Gravity”.) The second measurement, after the yeast has finished, is called the Final Gravity. Ideally measurements should be taken close to 60F (15.5C), as hydrometers are calibrated for that temperature.
See separate entries on measuring alcohol content by volume, versus measuring by weight.