The specific gravity of a solution is the ratio of its density to that of water at the same temperature. Since the specific gravity of a fermenting solution will drop as alcohol is produced, and since specific gravity is easily measured, this is often used to calculate the alcohol percentage of beers and wines. A simple equation can be used to convert specific gravity readings to measure alcohol content, either by volume or weight.
- Skill level:
- Moderately Easy
Things you need
- Specific gravity hydrometer
Take a specific gravity reading of the fermentation mixture -- for example, beer wort or grape juice mixture. Take the reading after all ingredients but yeast are added and therefore before any fermentation occurs. This is the original gravity (OG) of the mixture. A typical value is around 1.05 for beer mixtures, with wines being higher.
Take a specific gravity reading of the mixture after fermentation is complete (but before bottling). This will be your final gravity (FG). Final gravity readings should be lower than original gravity, in the range of 1.015 for beers or closer to 1 for wines.
Calculate the alcohol content as the percentage of alcohol by volume. For beers, subtract final gravity from original gravity. Then multiply the result by 130. For wines, subtract final gravity from original gravity, then divide the result by 7.36. The percentage of alcohol in beers is normally in the range of 4 to 6 per cent, and for wines is usually between 10 per cent and 14 per cent. This is known as alcohol by volume, or abv. This is the most common means of displaying alcohol content worldwide.
Convert the alcohol content to the percentage of alcohol by weight by multiplying the volume amount by 0.814. This manner of reporting alcohol content is used by some states. Reporting alcohol in this manner sometimes gives the incorrect impression that the drink is weaker, since the alcohol content appears lower.
Tips and warnings
- Most hydrometers are calibrated for a temperature of 15.6 degrees Celsius, so taking a reading at a different temperatures will produce inaccurate results.
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