How to calculate ABV

Alcohol by volume comes down to two gravity readings and one formula. Once you understand what those numbers are actually measuring, the math makes complete sense — and you can trust your results instead of guessing from a recipe target.

Brew Math Calculators · Last updated June 5, 2026

What gravity measures

Specific gravity (SG) is a dimensionless ratio that compares the density of your wort or beer to the density of pure water, which is defined as 1.000. When you add fermentable sugars to water — whether from malted barley, fruit, honey, or any other source — those sugars raise the density above 1.000. A typical American pale ale wort might read 1.050 before fermentation, meaning it is 5.0% denser than water.

As yeast eats the sugars and converts them to ethanol and CO2, the density drops. Ethanol is lighter than water (SG ~0.789), so the finished beer always reads lower than the starting wort. That drop in gravity is what you are measuring when you calculate ABV.

The two readings you need are:

• Original gravity (OG) — measured before pitching yeast, once the wort is chilled to your fermenter. This represents all of the fermentable and non-fermentable sugars at the start.
• Final gravity (FG) — measured when fermentation appears complete. Take readings on two consecutive days; if the number is stable, the beer is done.

Measuring gravity: hydrometer vs refractometer

Hydrometer

A hydrometer is a weighted glass float calibrated to read 1.000 in pure water at a reference temperature (usually 60°F / 15.6°C). Spin the tube gently to dislodge bubbles, let it settle, and read the scale at the bottom of the meniscus (the curved liquid surface). Hydrometers are accurate and inexpensive, but they require a meaningful sample volume — typically 100–200 ml — which you pour off rather than return to the fermenter.

Temperature correction: If your sample is warmer than the calibration temperature, the reading will be slightly low. Most homebrew calculators include a temperature correction field. The standard correction is roughly +0.001 for every 10°F above 60°F.

Refractometer

A refractometer measures how much light bends as it passes through a few drops of liquid. For pre-fermentation readings it is highly convenient — just put two drops on the prism, close the cover, and read the Brix scale. Brix and specific gravity are related: roughly SG ≈ 1 + (Brix / 258.6), or more precisely SG = 1.00001 + 0.003858·Brix + 0.00001285·Brix². The Brix to Gravity Calculator handles this conversion for you.

Important limitation: Alcohol distorts refractometer readings during and after fermentation. A raw refractometer FG reading will appear artificially low. You must apply a correction formula (the Sean Terrill or Novotný equation) to get the real FG — or simply use a hydrometer for final gravity. The ABV calculator on this site includes the refractometer correction if you enter both OG and FG as refractometer Brix.

The ABV formula

The standard homebrewer formula is:

ABV = (OG − FG) × 131.25

This is a linearized approximation derived from the more exact relationship between ethanol production and gravity drop. It is accurate to within about 0.1–0.2% ABV for normal gravity beers (OG up to roughly 1.080), which is plenty of precision for homebrewing.

For higher-gravity beers — barleywines, imperial stouts, or big Belgian ales — a slightly more precise alternate formula gives a better result:

ABV = (76.08 × (OG − FG) / (1.775 − OG)) × (FG / 0.794)

Both formulas assume the standard homebrew convention of expressing gravity as a decimal (1.050, not 50 "gravity points"). If you are working in Plato degrees, convert first.

Apparent attenuation vs real attenuation

Attenuation describes what fraction of the original fermentable sugars the yeast actually consumed. There are two versions of this number, and confusing them is a common beginner mistake.

Apparent attenuation (AA) is the simple gravity-drop percentage, and it is the number yeast manufacturers publish in their strain specs:

AA = ((OG − FG) / (OG − 1.000)) × 100

Using the worked example above:

AA = ((1.050 − 1.010) / (1.050 − 1.000)) × 100 = (0.040 / 0.050) × 100 = 80%

So the yeast attenuated 80% of the available gravity. A highly attenuating yeast (like most American ale strains) typically runs 75–82% AA; a less attenuative strain used for malty English ales might land in the 68–72% range.

Real attenuation (RA) corrects for the fact that ethanol is less dense than the unfermented sugars it replaces. The raw gravity reading of fermented beer is pulled down by the presence of alcohol, making the apparent attenuation look higher than the true sugar consumption. Real attenuation is roughly RA ≈ 0.82 × AA. For most practical homebrew purposes — checking yeast health, comparing batches, predicting body — apparent attenuation is the number you use.

Common ABV errors to avoid

• Using a refractometer for FG without correction. The ethanol in finished beer bends light differently than sugar does. An uncorrected refractometer FG will read about 30–40% lower than the true value, inflating your calculated ABV significantly.
• Reading the hydrometer before CO2 bubbles settle. Active fermentation deposits bubbles on the float, pushing it up and giving you a falsely high reading. Spin the tube and wait 30 seconds before reading.
• Not accounting for sample temperature. At 80°F, a hydrometer reads about 0.002 lower than the true value at its 60°F calibration point.
• Declaring FG too early. Take readings on two consecutive days. A stable reading is confirmation; a single low reading during active fermentation is not.