FermentoryABV calculator
Two hydrometer readings are all it takes: the gravity before fermentation and the gravity after. Enter them below and get your beer's alcohol by volume, plus how fully it fermented.
Quick answer: ABV ≈ (OG − FG) × 131.25. A beer that starts at 1.050 and finishes at 1.010 is about 5.25% ABV, with 80% apparent attenuation.
OG & FG → ABV
Use specific gravity readings, e.g. 1.050. Take both at the same temperature your hydrometer is calibrated for (usually 20 °C).
How the calculation works
A hydrometer measures density. Before fermentation your wort is dense with sugar; as yeast turns that sugar into alcohol and CO₂, the density falls. The drop between the two readings tracks how much sugar became alcohol, and the standard homebrew shortcut converts it directly:
ABV = (OG − FG) × 131.25
So 1.050 → 1.010 is 0.040 × 131.25 = 5.25%. The multiplier bakes in the density of alcohol and a couple of chemistry constants, and it's the same formula used by most brewing software — and by the Fermentory extension's built-in calculator.
Typical gravities by style
| Style | OG | FG | ABV |
|---|---|---|---|
| Session ale / mild | 1.035–1.042 | 1.008–1.012 | 3.2–4.2% |
| Pale ale | 1.045–1.058 | 1.010–1.014 | 4.5–6% |
| IPA | 1.056–1.070 | 1.010–1.015 | 5.8–7.5% |
| Lager / pilsner | 1.044–1.052 | 1.008–1.013 | 4.4–5.5% |
| Dry cider | 1.045–1.060 | 0.998–1.004 | 5.5–8% |
| Imperial stout | 1.075–1.115 | 1.018–1.030 | 8–12% |
When the simple formula drifts
Honest caveat: × 131.25 is an approximation that's excellent up to ordinary beer strengths and slowly loses accuracy as gravity climbs. Above roughly 8% ABV it starts under-reading by a few tenths of a percent. For big imperial beers, alternate high-gravity formulas exist, but for the vast majority of homebrew the standard formula is the right tool — every calculator drifts more than your hydrometer technique does anyway.
Reading gravity properly
- Temperature matters. Hydrometers are calibrated at a set temperature, usually 20 °C. A warm sample reads low; correct it or cool the sample.
- Degas the FG sample. Dissolved CO₂ lifts the float. A gentle stir until it stops fizzing gives a truer reading.
- Refractometer users: once fermentation starts, alcohol skews refractometer readings badly — the FG number needs a correction formula, or just use a hydrometer for the final reading.
ABV, IBU and carbonation in one popup
The free Fermentory browser extension carries these calculators with it, spots homebrew recipes on any page, and scales them to your batch size — no tab-switching mid-brew.
＋ Add Fermentory to Chrome Free to install · optional one-time Pro upgradeABV FAQ
What's a normal final gravity?
Most ales finish between 1.008 and 1.014. Dry ciders and seltzers can finish at or below 1.000, because nearly all their sugar is fermentable. If your beer stalls well above the recipe's expected FG, give it warmth and time before assuming it's done.
My gravity hasn't moved for days but it's higher than expected. Is it finished?
Stable readings two to three days apart mean fermentation has stopped — but stopped isn't always finished. A stuck fermentation can restart in the bottle and over-carbonate, so if you're far above the expected FG, rouse the yeast or warm it a few degrees and re-check before bottling.
Does priming sugar change the ABV?
Slightly. The sugar you add at bottling ferments too, adding roughly 0.1–0.2% ABV depending on the dose. Most brewers ignore it; add it mentally if you're chasing precision.
Can I measure ABV without an OG reading?
Not accurately with home equipment. The alcohol estimate needs the before-and-after density difference. If you forgot the OG, use the recipe's predicted OG as a stand-in and treat the result as an estimate.