ABV, alcohol by volume, is the standard way to measure the alcohol content of beer or wine. Simply stated, it's the volume of ethanol (alcohol) in a liquid divided by the total volume of liquid. For instance, if a 750 cubic-centimetre bottle contains 75 cubic centimetres of ethanol, the liquid is 10% alcohol by volume. ABV is expressed as a percentage.
Brewers, distillers and vintners measure alcohol content by comparing the specific gravity of the liquid before and after fermentation, in which yeast eats the sugar and replaces it with carbon dioxide and ethanol.
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Measure the specific gravity of the unfermented liquid with a hydrometer. This measurement must be made at a standard temperature, usually 15.6 degrees Celsius. Record this value, which is called the "original gravity" or OG.
Pitch the yeast and allow the liquid to ferment. This takes from several days to several weeks, depending on the spirit being made.
Measure the specific gravity of the liquid with the same hydrometer after fermentation is complete. Again, measure the specific gravity at standard temperature of 15.6 degrees Celsius. Record this measurement, termed the final gravity (FG).
Calculate the ABV. Subtract the final gravity from the original gravity and multiply the difference by 131. For example, if OG = 1.085 and FG = 1.016, multiply the difference (0.069) by 131 to get an ABV of very slightly over 9, for an ABV of 9 per cent.
Tips and warnings
- ABV differs from ABW, or alcohol by weight. Because of the different densities of alcohol and water (which makes up most of the remaining liquid), ABW is usually about 80% of ABV. A beer that is 4.0% ABV is approximately 3.2% ABW.
- ABV can only be calculated from specific gravity if the original and final specific gravities are known.
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