ABV & Attenuation Calculator
Alcohol by volume and by weight from original and final gravity, by the Hall formula and the linear one, with apparent and real attenuation.
ABV by Michael Hall’s formula and by (OG − FG) × 131.25, ABV by weight, apparent and real attenuation, real extract, and both gravities in SG and °Plato — entered in either.
Example: OG 1.050 to FG 1.010 is 5.34 % ABV by the Hall formula (5.25 % linear) and 80 % apparent attenuation; 1.090 to 1.018 is 10.25 % against 9.45 %.
Sugar in,
alcohol out.
How the drop in gravity becomes alcohol, why two formulas disagree at high gravity, and what attenuation measures.
Two formulas
The familiar (OG − FG) × 131.25 treats every point of gravity lost as the same amount of alcohol. Michael Hall’s formula (Zymurgy, 1995) — 76.08 × (OG − FG) ÷ (1.775 − OG) × FG ÷ 0.794 — follows how the relationship bends as gravity rises, so the two agree for ordinary beers and part for strong ones: 10.25 % against 9.45 % for 1.090 to 1.018. The page leads with Hall.
By weight
Alcohol by weight is ABV × the specific gravity of ethanol (0.794) ÷ the beer’s final gravity — always lower than ABV, because alcohol is lighter than water.
Attenuation
Apparent attenuation is the share of the gravity points that disappeared: (OG − FG) ÷ (OG − 1). It is “apparent” because alcohol is lighter than water and makes the final reading look lower than the sugar left. Real attenuation uses Balling’s real extract, 0.1808 × original extract + 0.8192 × apparent extract, in °Plato.
What the numbers do not settle
Readings must be corrected to the hydrometer’s calibration temperature first, and a refractometer reading after fermentation needs its own correction. Degrees Plato use the cubic fit to the ASBC extract table. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.
SOURCES
- Michael L. Hall, “Brew by the Numbers — Add Up What’s in Your Beer”, Zymurgy, Summer 1995
- ASBC Methods of Analysis — extract (°Plato) table, as its cubic fit in specific gravity
- Balling’s real-extract relation, RE = 0.1808 × OE + 0.8192 × AE
Last reviewed 24 September 2026. How results are checked: How we verify.