What is the ABV Calculator?
This calculator estimates the alcohol by volume (ABV) of fermented beverages — wine, beer, mead, cider or kombucha — from two hydrometer readings: the original gravity (OG) measured before fermentation and the final gravity (FG) measured after fermentation. As yeast converts sugar into alcohol and CO₂, the liquid becomes less dense, so the drop in gravity is directly proportional to the amount of alcohol produced.
How to use it
Float a calibrated hydrometer in your unfermented must or wort and record the reading — that is your OG (e.g. 1.050). When fermentation finishes, take a second reading for your FG (e.g. 1.010). Enter both numbers above and the calculator returns the estimated ABV plus the apparent attenuation, which tells you how much of the sugar the yeast consumed.
The formula explained
The widely used homebrew estimate is $$\text{ABV\%} = \left( \text{OG} - \text{FG} \right) \times 131.25$$ The constant 131.25 converts the specific-gravity difference into a percentage of alcohol by volume. It is accurate for typical beers and wines up to roughly 6–7% ABV and slightly overestimates for very strong brews; more complex formulas exist but this one is the standard quick reference.
Apparent attenuation, \(( \text{OG} - \text{FG} ) \div ( \text{OG} - 1 ) \times 100\), shows the fraction of fermentable sugar the yeast actually fermented.
Worked example
A wine starts at OG = 1.090 and finishes at FG = 1.000. The difference is 0.090. Multiply by 131.25: $$0.090 \times 131.25 = 11.81\% \text{ ABV}$$ The attenuation is \(( 0.090 \div 0.090 ) \times 100 = 100\%\), meaning the yeast fermented all available sugar.
FAQ
Why is my reading temperature dependent? Hydrometers are usually calibrated at 20°C/68°F. If your sample is much warmer or cooler, apply a temperature correction before entering values.
Is this exact? No — it is a reliable estimate. Actual ABV can vary with yeast strain, sugar type and measurement precision. Expect accuracy within a few tenths of a percent for normal-strength drinks.
Can I use this for spirits? No. Distillation concentrates alcohol, so gravity readings no longer reflect ABV after distilling.
