Wine and beer makers use a hydrometer to measure alcohol levels and tell them when fermentation is complete. A simple, though fragile, glass instrument, a hydrometer measures the specific gravity of a liquid, or its density in relation to water. A must, which is the crushed grapes that become wine, and wort, the unfermented malt that becomes beer, are both high in sugars, making their specific gravity high. As they ferment, the sugars are changed into alcohol, decreasing their specific gravity. Use a hydrometer's potential alcohol scale to easily determine the amount of alcohol a wine or beer will contain once it's fermented. Smaller numbers, such as 3 or 5 percent, indicate a low alcohol level, while higher numbers, such as 10 or 12 percent, indicate a higher level.
Sanitize all equipment that will come in contact with the must or wort. Soak the equipment in the sanitizing solution and allow it to air-dry.
Use a wine thief, a tool designed to draw samples of wine or beer with minimal contact, to remove must or wort for the first reading. Fill the test jar three-quarters of the way to the top. Take an initial reading of the specific gravity of the must or wort at the start of the project, before fermentation begins.
Place the hydrometer in the test jar and gently give the hydrometer a spin using the thumb and index finger to remove any air bubbles clinging to the sides of the test jar. Be sure not to drop the hydrometer into the test jar, as it is very fragile. Read the hydrometer's potential alcohol scale at the point at which the liquid contacts the glass of the hydrometer. The potential alcohol scale determines how much alcohol could be produced in a beer or wine if fermented to dryness based on how much sugar is present.
Take another reading as fermentation is nearing completion to determine the amount of sugar remaining in the beer or wine. Subtract the second reading from the first to determine the amount of alcohol produced during fermentation. For example, if the first reading was 14 percent alcohol and the final reading 2 percent alcohol, the final alcohol reading would be 12 percent.