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Get the alcohol by volume of your brew (simple and advanced formulas) plus apparent attenuation from original and final gravity, for OG 1.000 to 1.170.

📘 How to Use

  1. Enter the original gravity (OG) measured before fermentation
  2. Enter the final gravity (FG) measured after fermentation
  3. Read the simple and advanced ABV plus apparent attenuation

ABV Calculator

The gravity measured before fermentation.

The gravity measured after fermentation.

percent Results

ABV (simple)

— %

ABV (advanced)

— %

Apparent attenuation

— %
warning

The shown strength is an estimate from gravity readings. Do not use it for legal or driving decisions such as whether it is safe to drive.

※ Simple formula is ABV = (OG − FG) × 131.25; the advanced formula stays more accurate at higher strengths.

※ Apparent attenuation = (OG − FG) ÷ (OG − 1) × 100.

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ABV Calculator | Alcohol by Volume from OG and FG

Turn your original gravity (OG) and final gravity (FG) into alcohol by volume using both the simple and the advanced homebrew formula at once. Apparent attenuation is shown alongside, for any brew from OG 1.000 to 1.170.

💡 About this tool

After a batch of beer, wine, mead or cider finishes fermenting, the first question is almost always "how strong did it actually come out?" You can't read ABV off a label you printed yourself — you work it out from two hydrometer readings: the original gravity before the yeast went in, and the final gravity once fermentation has stalled out. The gap between those two numbers is the sugar the yeast ate, and that sugar is what became alcohol.

This calculator takes those two readings and gives you two ABV figures at the same time. The simple formula, (OG − FG) × 131.25, is the one most homebrewers memorise; it is reliable up to roughly 6% ABV but reads a little low above that. The advanced formula stays more accurate for big beers, barleywines, imperial stouts, wines and meads where the relationship between gravity drop and alcohol stops being linear. On top of both, you get apparent attenuation so you can see how completely the yeast finished the job in the same view.

🧐 Frequently asked questions

How do I measure OG and FG? With a hydrometer: take one reading before pitching yeast (once the wort or must is fully mixed) and one after fermentation. A refractometer also works, but once alcohol is present its reading needs a correction.

Which ABV figure should I trust? For everyday brews up to about 6% the simple figure is fine. Above that the simple formula underestimates, so lean on the advanced figure. This tool shows both so you don't have to choose blind.

My FG is higher than my OG — what went wrong? That points to a measurement or entry slip. Fermentation removes sugar, so gravity should fall and FG should land below OG. When the values are reversed the calculation can't run and an error is shown.

What is apparent attenuation? It is a rough percentage of how much sugar was consumed, found from (OG − FG) ÷ (OG − 1) × 100. It is called "apparent" because alcohol is lighter than water, which nudges the reading higher than the real attenuation.

Does temperature affect the reading? Hydrometers are calibrated to a reference temperature (often around 20°C / 68°F). A large temperature gap skews the gravity, so treat the resulting ABV as an estimate.

📚 Where 131.25 comes from

The 131.25 constant that turns a gravity drop into a percentage is not arbitrary. It bundles together two factors: roughly 1.05 g of ethanol is produced for each gram of carbon dioxide released during fermentation, and ethanol's density (about 0.79 g/mL) converts alcohol-by-weight into alcohol-by-volume. Multiply through and round for everyday use and you land on 131.25. It's a working number polished over decades of brewing rather than a textbook constant — which is exactly why a second, less linear formula exists for the strong stuff.