Food & Cooking⏱ 5 min read
How to Calculate Beer ABV Using Original and Final Gravity
Homebrewers need to know the alcohol content of their beer. Here's how the original gravity and final gravity formula works, how to use a hydrometer, and what affects attenuation.
Knowing your beer's alcohol content matters for understanding what you've made, comparing recipes, and responsible serving. The calculation comes from measuring how much fermentable sugar the yeast has consumed.
Gravity and What It Measures
Specific gravity (SG) measures liquid density relative to water.
Water = 1.000
Unfermented wort (pre-fermentation) = Original Gravity (OG)
Typically: 1.040 - 1.080 for most ales and lagers
Fermented beer (post-fermentation) = Final Gravity (FG)
Typically: 1.008 - 1.020
The difference = sugar consumed by yeast = alcohol produced
The ABV Formula
Simple formula (accurate within 0.3% for typical beers):
ABV % = (OG - FG) x 131.25
Example: OG = 1.052, FG = 1.012
ABV = (1.052 - 1.012) x 131.25
= 0.040 x 131.25
= 5.25% ABV
More precise formula:
ABV = (76.08 x (OG - FG) / (1.775 - OG)) x (FG / 0.794)
For most homebrewing purposes, the simple formula is sufficient.
Using a Hydrometer
A hydrometer floats at different levels depending on sugar content.
Read where the liquid surface intersects the scale.
Temperature correction:
Hydrometers are calibrated at 20°C (68°F).
If measuring at different temperature:
Correction = (Temp°C - 20) x 0.00033
Add this to your reading.
Measuring at 25°C:
Correction = (25 - 20) x 0.00033 = +0.00165
If hydrometer reads 1.050, true OG = 1.050 + 0.002 = 1.052
Refractometers can be used for OG (easier than hydrometer
in small samples) but are not accurate for FG after fermentation
— alcohol alters the refractive index.
Attenuation: How Efficiently Yeast Fermented
Apparent Attenuation % = ((OG - FG) / (OG - 1)) x 100
Example: OG 1.052, FG 1.012
= ((1.052 - 1.012) / (1.052 - 1.000)) x 100
= (0.040 / 0.052) x 100
= 76.9%
Typical attenuation by yeast type:
English ale yeast: 65-75% (leaves more body and sweetness)
American ale yeast: 73-80% (cleaner, more fermentable)
Lager yeast: 75-85% (very clean, drier finish)
Belgian yeast: 75-85%
Low attenuation = sweeter, fuller-bodied beer
High attenuation = drier, more alcoholic, thinner body
Estimating OG From Recipe
Points per pound per gallon (PPG) system:
Each grain contributes sugar based on its PPG value.
Pale malt: 37 PPG
Munich malt: 35 PPG
Crystal 60L: 34 PPG
OG = Sum of (grain_lbs x PPG x efficiency) / batch_gallons_US
Brew efficiency typically 70-75% for homebrew
Example: 10 lbs pale malt, 5 gallon batch, 72% efficiency
OG contribution = (10 x 37 x 0.72) / 5 = 266.4 / 5 = 53.3 points
OG = 1.053