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.

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