Beer ABV Calculator Formula

Understand the math behind the beer abv calculator. Each variable explained with a worked example.

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Formulas Used

ABV

abv = (og - fg) * 131.25

Apparent Attenuation

apparent_attenuation = (og - fg) / (og - 1) * 100

Calories (12 oz serving)

calories_12oz = round(((6.9 * ((og - fg) * 131.25 / 100)) + 4 * (fg - 1) * 1000 * 0.1) * 3.55 * 100) / 100

Variables

VariableDescriptionDefault
ogOriginal Gravity (OG)1.05
fgFinal Gravity (FG)1.01

How It Works

How to Calculate Beer ABV

Formula

ABV = (OG - FG) x 131.25

This is the standard homebrewing formula. OG (original gravity) is measured before fermentation, and FG (final gravity) is measured after fermentation is complete.

Apparent Attenuation

AA = (OG - FG) / (OG - 1) x 100%

Attenuation measures what percentage of the available sugars were fermented. Most beer yeasts achieve 65-80% apparent attenuation.

Taking Gravity Readings

Use a hydrometer or refractometer. Hydrometers are calibrated to 60°F; adjust for temperature differences. Refractometer readings after fermentation need a correction factor because alcohol changes the refraction.

Worked Example

A pale ale with OG 1.050 and FG 1.010.

og = 1.05fg = 1.01
  1. 01ABV = (1.050 - 1.010) x 131.25 = 5.25%
  2. 02Attenuation = (1.050 - 1.010) / (1.050 - 1) x 100 = 80%
  3. 03This is a well-attenuated beer at a standard strength.

Frequently Asked Questions

What is a normal original gravity?

Light beers: 1.030-1.040. Standard ales and lagers: 1.040-1.060. Strong ales: 1.060-1.080. Barleywines and imperial stouts: 1.080-1.120+.

Why is my final gravity too high?

Common causes: fermentation temperature too low, not enough healthy yeast, or too many unfermentable sugars. Warm the beer to the upper end of the yeast range and swirl gently. If FG is stable for 3 days, fermentation is done.

Is this formula accurate for high-gravity beers?

The simple formula works well up to about 7% ABV. For stronger beers, the alternate formula ABV = (76.08 x (OG - FG) / (1.775 - OG)) x (FG / 0.794) is more accurate.

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