ABV from Specific Gravity Calculator
During fermentation, yeast convert sugar into alcohol and carbon dioxide, which lowers the density of the liquid. By measuring specific gravity before fermentation (original gravity) and after (final gravity) with a hydrometer, you can estimate alcohol by volume. This calculator reports ABV using both the simple linear estimate and a more accurate cubic correction, plus apparent attenuation and alcohol by weight. Specific gravity readings should be temperature corrected to your hydrometer calibration point before entry. Legal alcohol content for sale is governed by the TTB; this tool is for brewing and recipe estimates.
ABV from gravity formula
ABV simple = (OG - FG) * 131.25
ABV refined = (76.08 * (OG - FG) / (1.775 - OG)) * (FG / 0.794)
apparent attenuation = (OG - FG) / (OG - 1) * 100
ABW approx = ABV * 0.79336
Original and final gravity must each be greater than the other appropriately: OG should exceed FG and FG should exceed 1.000 in practice. The refined formula tracks the simple one closely at low strength and corrects upward as alcohol rises.
Brewing measurement notes
- Pure water has a specific gravity of 1.000 at the hydrometer calibration temperature.
- The simple linear factor 131.25 is a well known homebrew approximation, best below about 7 percent ABV.
- Temperature correct every reading before entering it.
- Alcohol by weight is roughly ABV times 0.79336, the density of ethanol relative to water.
- Federal labeling and tax of alcohol for sale are regulated by the TTB, not estimated here.
ABV from gravity: frequently asked questions
How is ABV calculated from gravity?
A widely used homebrew estimate is ABV percent = (OG - FG) times 131.25, where OG is original gravity and FG is final gravity. This linear formula is accurate for typical beer strengths up to about 7 percent ABV. For stronger beverages, a refined cubic form gives a closer result; this calculator shows both.
What is specific gravity?
Specific gravity is the density of the liquid divided by the density of water at the same temperature, so pure water reads 1.000. Sugars raise the gravity above 1.000; fermentation converts sugar to alcohol and carbon dioxide, lowering the gravity. The drop from original to final gravity drives the alcohol estimate.
What is apparent attenuation?
Apparent attenuation is the percentage of the original gravity points that fermented out: (OG - FG) / (OG - 1) times 100. It tells you how much of the sugar the yeast consumed. It is called apparent because alcohol is lighter than water, so the hydrometer reads lower than the true sugar remaining.
Why are there two ABV formulas?
The simple linear formula (OG - FG) times 131.25 is easy and accurate at low strength. A more accurate alternative published by homebrew references is ABV = (76.08 times (OG - FG) / (1.775 - OG)) times (FG / 0.794), which corrects for the curvature at higher alcohol levels. This page reports both so you can compare.
Should I correct gravity for temperature?
Yes. Hydrometers are calibrated at a specific temperature, often 60 deg F (15.6 deg C) or 20 deg C. Readings taken warmer or cooler need a correction before you enter them, or your ABV estimate will be off. Use a temperature correction chart or a temperature-compensating instrument.
Official sources
- Alcohol and Tobacco Tax and Trade Bureau: Beverage Alcohol.
- eCFR: Title 27, Alcohol, Tobacco and Firearms.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.