How to Calculate ABV: Original Gravity, Final Gravity and Alcohol

Your beer’s alcohol content is the gap between two numbers. Measure the wort’s density before the yeast goes in, measure it again when fermentation is done, and the drop tells you how much sugar became alcohol. One subtraction and one multiplication, and you have the ABV.

What gravity is measuring

“Gravity” here is specific gravity — how dense the liquid is compared to water (which is 1.000). Sugar dissolved in water makes it denser, so sugary wort reads well above 1.000, around 1.040–1.060 for a typical beer.

When yeast ferments, it eats that sugar and produces alcohol and CO₂. Alcohol is less dense than water, and the sugar is disappearing, so the liquid gets lighter and the gravity falls. That fall is the fingerprint of fermentation — and the bigger the fall, the more alcohol was made.

The two readings

• Original Gravity (OG) — taken before you pitch the yeast, when the wort is full of sugar. This is your starting point.
• Final Gravity (FG) — taken after fermentation finishes, when the yeast has converted most of the sugar. A typical beer lands around 1.008–1.014.

You need both, because ABV comes from the difference. Forget the OG and you’re left estimating from the recipe.

The formula

Once you have both:

ABV% = (OG − FG) × 131.25

For our example, OG 1.050 and FG 1.010:

(1.050 − 1.010) × 131.25 = 0.040 × 131.25 = 5.25% ABV

The 131.25 is a conversion constant (you’ll see 131 or 132 in some places) that turns the gravity drop into a percentage. It works because the alcohol produced is closely proportional to how far the gravity fell.

When the simple formula isn’t enough

That linear formula is accurate for everyday beers, but for strong beers — above roughly 1.070 OG — it reads a little low. For those, use Michael Hall’s more accurate equation:

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

It accounts for the non-linear relationship at high gravity. The ABV calculator shows both side by side, so you can see how much they diverge for your beer (they agree closely for session beers and separate for big ones).

The mistakes that throw it off

The formula is simple; the measurement is where it goes wrong:

• Temperature. Hydrometers are calibrated at one temperature (often 20 °C / 68 °F). A warm sample reads low. Either cool the sample or use a temperature correction.
• Using a refractometer for FG. Alcohol bends light differently from sugar, so a refractometer reads final gravity too high once fermentation has started. You must apply a refractometer correction.
• Reading FG too early. If fermentation hasn’t finished, FG is still falling and your ABV will be understated. Confirm with two stable readings a few days apart.

Let the tool do the arithmetic

Pop your two gravities into the ABV calculator and it returns the ABV (both formulas), the attenuation and the calories per serving — no mental maths on brew day. Like every LazyTools tool, it runs entirely in your browser, nothing uploaded, and works offline in the brewhouse.

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ABV from gravity uses the standard homebrew formula (OG − FG) × 131.25, with Michael Hall’s equation for higher-gravity beers. Both are estimates dependent on accurate, temperature-corrected gravity readings. Sources: Brewer’s Friend ABV Calculator, Michael Hall, “Brew by the Numbers,” Zymurgy (1995).