ABV Calculator
Work out the alcohol content of a homebrew from its original and final gravity, by the common 131.25 rule and by Hall’s more detailed equation.
Result
Enter original gravity (og) and final gravity (fg) to see the result.
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Results are arithmetic on the figures you enter. Cooking times are estimates: ovens, cuts and starting temperatures vary, so check doneness with a thermometer. Safe temperatures are the USDA's; guidance differs by country, so check your own. Read the full disclaimer
How to use the ABV Calculator
- Enter the original gravity, the hydrometer reading taken before the yeast went in.
- Enter the final gravity, the reading once fermentation has finished.
- Read the alcohol by volume from the 131.25 rule, with Hall's more detailed figures beside it.
Two ways to estimate alcohol
Yeast turns sugar into alcohol and carbon dioxide. Sugar makes the liquid denser and alcohol makes it less dense, so the drop in specific gravity from before fermentation to after it tracks how much alcohol was made.
The common rule multiplies that drop by a fixed factor:
ABV % = (OG − FG) × 131.25Michael Hall’s 1995 article in Zymurgy built a more careful estimate from Balling’s work on sugar solutions:
ABW % = 76.08 × (OG − FG) ÷ (1.775 − OG)ABV % = ABW × FG ÷ 0.794The first line gives alcohol by weight. The second converts it to alcohol by volume, using 0.794 as the specific gravity of ethanol.
Why the page shows both
The 131.25 rule is the figure homebrew kit instructions give, so it heads the result. Hall’s equation grows with the original gravity rather than staying fixed, and the gap between the two widens for strong brews. Seeing both side by side shows how much the estimate depends on the method.
Getting good readings
Take the original gravity after the wort has cooled and been mixed, before adding yeast. Take the final gravity once fermentation has finished, just before bottling. Spin the hydrometer to shake off bubbles, read at eye level, and correct each reading for temperature.
With your numbers
- Hall’s equation gives 4.23% by weight and 5.4% by volume
- 1.055 − 1.015 = 0.04, and 0.04 × 131.25 = 5.25%
Quick reference
Estimates from hydrometer readings. Readings not corrected to the hydrometer’s calibration temperature will shift the result.
| Example | Alcohol by volume, (OG − FG) × 131.25 |
|---|---|
| 1.040 to 1.008 | 4.2% |
| 1.050 to 1.010 | 5.25% |
| 1.055 to 1.015 | 5.25% |
| 1.065 to 1.012 | 6.96% |
| 1.090 to 1.020 | 9.19% |
| 1.108 to 1.041 | 8.79% |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
1 source
A convention, not a derived equation. Brewer’s Best (LD Carlson), How To Use a Hydrometer, https://www.brewersbestkits.com/assets/PDF/HydroInstruct.pdf: "The equation to figure out the amount of alcohol is this: (Starting Gravity - Final Gravity) x 131.25."
Used for
- The homebrewing rule of thumb that multiplies the drop in specific gravity by 131.25 to give percent alcohol by volume. A convention, less accurate for strong beers.
Common questions about the ABV Calculator
- Where does 131.25 come from?
- It is a homebrewing rule of thumb, printed on kit makers' hydrometer instructions and widely repeated. It turns the drop in gravity straight into a percentage. No derivation of the exact figure was found for this page, which is why it sits beside a published equation rather than alone.
- Which number should I believe?
- For an everyday beer the two by-volume figures are within a few tenths of a percentage point, so either is fine. As the original gravity climbs, Hall's equation gives noticeably more alcohol than the simple rule, because the rule treats every point of gravity drop as the same amount of alcohol. For a strong brew the two can differ by a whole percentage point or more.
- Why do my readings need a temperature correction?
- A hydrometer is calibrated at one temperature, marked on the instrument, and liquid gets less dense as it warms. A warm sample reads low. Correct both readings to the calibration temperature before entering them, or cool the sample first.
- Does priming sugar add alcohol?
- A little, when it ferments in the bottle, but this page only uses the two readings you enter and does not add it.
- Can I use this for a legal alcohol label?
- No. It is an estimate from hydrometer readings. Labelling rules for alcohol content differ by country; check the ones that apply to you.