What is the formula for ABV?

The simple formula is ABV = (OG − FG) × 131.25. For example, an OG of 1.050 and FG of 1.010 gives (1.050 − 1.010) × 131.25 = 5.25% ABV. The advanced formula corrects for high gravities.

Which formula should I use?

The simple formula is accurate up to about 6–7% ABV. Above that, gravity drop no longer scales linearly with alcohol, so the advanced (alternate) formula gives a more accurate result. Both are shown side by side.

What is apparent attenuation?

Apparent attenuation is the percentage of the original sugar the yeast has converted, measured by hydrometer. It equals (OG − FG) ÷ (OG − 1) × 100. Most ale yeasts attenuate 70–80%.

Do I need to correct for temperature?

Hydrometers are calibrated to a reference temperature (often 20°C / 68°F). Readings taken warmer or colder need a small correction before you enter them here, or the ABV will be slightly off.

Is the data sent anywhere?

No. Every calculation runs locally in your browser. Your gravity readings never leave your device.

ABV Calculator — Gera Tools

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.

Example and notes

A west-coast IPA at OG 1.064 finishing at FG 1.012 gives roughly (1.064 − 1.012) × 131.25 ≈ 6.8% on the simple formula, with the advanced formula landing a touch higher. Always take your FG reading after gravity has been stable for two to three days, and correct hydrometer readings to their calibration temperature first for the most reliable result.