Calculating Specific Gravity and ABV in Liquids

For homebrewers and winemakers, the ABV (Alcohol By Volume) calculation is the definitive measure of a successful fermentation. Alcohol is produced when yeast consumes sugar in a liquid (wort or must). Because alcohol is less dense than sugar water, the density of the liquid changes during fermentation. By measuring this density change using a hydrometer or refractometer, we can calculate precisely how much alcohol was created.

Understanding Specific Gravity

Specific Gravity (SG) is the ratio of the density of a liquid to the density of pure water. Pure water has an SG of 1.000. As you add sugar (creating wort for beer or must for wine), the SG rises. As the yeast converts that sugar into ethanol, the SG drops. The Original Gravity (OG) is the reading taken before yeast is pitched, and the Final Gravity (FG) is the reading taken once fermentation has completely stopped.

The Standard ABV Formula

This calculator uses the industry-standard linear formula for approximating alcohol content:

The constant 131.25 accounts for the density of ethanol relative to water and the conversion of weight-to-volume. While more complex non-linear formulas exist for high-gravity beers (above 1.100 OG), this standard formula provides accuracy within +/- 0.2% for the vast majority of brewing applications.

Factors That Affect Accuracy

• Temperature Correction: Hydrometers are calibrated to a specific temperature (usually 60°F or 68°F). If your sample is warmer or colder, you must adjust the reading.
• Refractometer Alcohol Offset: If using a refractometer, the presence of alcohol distorts the reading of light. You must use a correction factor once fermentation has begun.
• Degassing: Carbon dioxide bubbles in the liquid can "lift" a hydrometer, giving a false high reading. Always spin or swirl the hydrometer to release gas bubbles.

Frequently Asked Questions