ABV Calculator (OG & FG)
Two gravity readings in — alcohol by volume and attenuation out, with the standard and alternate formulas shown side by side.
The short answer
Formula. ABV = (original gravity − final gravity) × 131.25.
Example. A beer fermenting from OG 1.050 down to FG 1.010 finishes at (1.050 − 1.010) × 131.25 = 5.25% alcohol by volume.
Your readings
Hydrometer or refractometer reading before fermentation.
Reading when fermentation is finished.
How the math works
Yeast converts sugar to alcohol, and alcohol is lighter than the sugar-water it replaces — so the wort's density falls as it ferments. The standard formula is a linear fit over normal beer strengths: ABV = (OG − FG) × 131.25. A 1.050 beer finishing at 1.010 drops 40 "gravity points" and lands at (0.040 × 131.25) = 5.25% alcohol by volume. Wine and mead calculators sometimes use 131 or 135 — same idea, slightly different curve fits — and 131.25 is the value most brewing software defaults to.
The factor quietly overestimates on big beers, because alcohol's effect on density is not linear. The alternate formula — 76.08 × (OG − FG) ÷ (1.775 − OG) × (FG ÷ 0.794) — accounts for the actual amount of ethanol produced (the 0.794 is ethanol's density in g/mL) and agrees with the standard formula within a few hundredths at session strength while staying honest past 8–9% ABV, where the simple version reads high by half a point or more. This calculator prints both so you can quote whichever your label rules require; commercial labs measure alcohol directly (distillation or GC), which is why packaged labels occasionally differ from your hydrometer math by a tenth.
Apparent attenuation rides along free: (OG − FG) ÷ (OG − 1) × 100, the fraction of extract the yeast ate. It reads yeast character at a glance — 75–80% is a clean, dry finish typical of ale strains; below 70% means body, sweetness or a stressed/stuck fermentation; 85%+ means highly attenuative yeast or a long warm rest. Two measurement notes that prevent wrong answers: hydrometer readings are temperature-corrected (read at the calibration temperature, usually 60°F/15°C, or correct), and refractometers must use an alcohol-corrected formula post-fermentation — a plain Brix-to-SG conversion of the FG refractometer reading is the most common bad input this calculator receives.
Sources & review date
- ABV = (OG − FG) × 131.25 is the standard homebrewing formula; the page also shows the alternate gravity formula, which reads higher on big beers.
Last reviewed: October 2026. Numbers that drift (fees, prices, densities) are editable inputs on this page.
ABV FAQs
Is 131.25 or 7.5 × (OG − FG) × FG the right formula?
Both are approximations of the same curve. The 131.25 factor is the common linear version; the alternate formula on this page is the more defensible fit, and they agree closely below ~7% ABV. Pick one, stay consistent, and your notes will compare fairly batch to batch.
Can I calculate ABV without an OG reading?
Not accurately from gravity alone — you can back-estimate from FG plus known recipe efficiency, or boil a degassed sample to expel alcohol and compare, but both carry real error. Take the OG; it is the cheapest data in brewing.
Why is my ABV lower than the kit claimed?
Usually a lower actual OG than the recipe assumed (extract volume topping-off, mashing temperature) — the kit's "5.5%" is arithmetic from an assumed OG, not a promise. Your numbers are the real beer.
Does bottling sugar change ABV?
Slightly: the priming sugar ferments too, typically adding 0.2–0.4% ABV depending on how much sugar per volume. The calculator's FG is pre-priming, so add a touch mentally for bottled beer.