ABV Calculator

The ABV formula

The standard hobbyist formula estimates alcohol by volume directly from the drop in specific gravity:

ABV%         = (OG − FG) × 131.25
ABW%         = ABV% × 0.79336
attenuation% = (OG − FG) ÷ (OG − 1) × 100

The factor 131.25 converts the gravity drop into a percentage of alcohol by volume. Alcohol by weight (ABW) is lower than ABV because ethanol is less dense than water, roughly 0.79 g per ml. Apparent attenuation tells you what fraction of the available sugar the yeast actually fermented.

Worked example

Say you brewed a pale ale with OG 1.054 that finished at FG 1.012.

• ABV = (1.054 − 1.012) × 131.25 = 5.51%
• ABW = 5.51 × 0.79336 = 4.37%
• Attenuation = (1.054 − 1.012) ÷ (1.054 − 1) × 100 = 77.8%

That attenuation sits right in the healthy 75 to 80% range for an ale yeast, so fermentation went as expected.

Pitfalls to avoid

• Temperature throws off hydrometers. Hydrometers are calibrated to a specific temperature (often 20 °C / 68 °F). Reading a warm sample reports a falsely low gravity. Correct your readings or let the sample cool first.
• This is an estimate, not a lab assay. The 131.25 factor is an approximation that drifts at very high gravities; strong meads and barleywines may read a touch high.
• Apparent vs. real attenuation. The attenuation here is apparent, measured with a hydrometer that is still influenced by residual alcohol. Real attenuation is a few points lower.
• The calorie figure is approximate. It assumes a 355 ml / 12 fl oz serving and estimates calories from both residual sugar and alcohol; sweetened or adjunct-heavy brews will vary.