The ABV Calculator turns two hydrometer readings — original gravity (OG) and final gravity (FG) — into the alcohol content of your finished beer, wine, mead, or cider. It shows both the popular simple formula and the more accurate advanced formula, plus the apparent attenuation of your yeast.
How it works
Alcohol is produced as yeast eats sugar, and dissolved sugar makes a liquid denser. By measuring density before fermentation (OG) and after (FG) you can infer how much sugar was converted to alcohol.
The standard estimate is the simple formula:
ABV = (OG − FG) × 131.25
The constant 131.25 comes from the density and energy relationship between sugar
and ethanol. For an OG of 1.050 and FG of 1.010 this gives
(1.050 − 1.010) × 131.25 = 5.25%.
For stronger brews the relationship is non-linear, so this tool also computes the advanced (alternate) formula:
ABV = (76.08 × (OG − FG) ÷ (1.775 − OG)) × (FG ÷ 0.794)
which accounts for the way ethanol’s contribution changes at higher starting gravities.
Apparent attenuation
The tool also reports apparent attenuation — how much of the available sugar the yeast fermented:
Apparent attenuation = (OG − FG) ÷ (OG − 1) × 100
Most ale strains reach 70–80%. A low figure can mean a stuck fermentation; a very high one suggests a highly attenuative yeast or added simple sugars.
Worked examples for common brew styles
| Style | Typical OG | Typical FG | Simple ABV | Attenuation |
|---|---|---|---|---|
| Session bitter | 1.038 | 1.008 | ~3.9% | ~79% |
| Pale ale / IPA | 1.055 | 1.012 | ~5.6% | ~78% |
| Strong ale / stout | 1.080 | 1.016 | ~8.4% | ~80% |
| Dry cider | 1.050 | 0.998 | ~6.8% | ~105%* |
*Apparent attenuation can exceed 100% in dry ciders and wines where residual sugar is below the hydrometer’s water baseline (gravity below 1.000).
For the west-coast IPA at OG 1.064 finishing at FG 1.012, the simple formula gives (1.064 − 1.012) × 131.25 ≈ 6.8%, while the advanced formula returns a touch higher — the gap widens meaningfully above about 7% ABV, which is where the simple formula starts to understate the real alcohol content.
Reading your hydrometer accurately
- Take readings at or near the calibration temperature (often 20°C / 68°F) printed on the hydrometer. Warmer wort reads lower; colder reads higher. A 10°C deviation can shift the reading by 0.002 to 0.003, adding or removing roughly 0.25% ABV from the result.
- Read the gravity at the bottom of the meniscus (the liquid surface curve), not the top.
- Confirm FG is stable over two to three consecutive days before calling fermentation complete — an early reading that changes later will give an inaccurate ABV.