How do I measure original gravity?

Take a hydrometer reading of your wort/must before pitching yeast. Cool the sample to the hydrometer's calibration temperature (usually 20°C/68°F). The reading where the liquid surface crosses the scale is your OG.

When should I use the alternate ABV formula?

Use the alternate formula for high-gravity brews (OG above 1.070). The standard formula underestimates ABV for strong beers, barleywines, and imperial stouts because it doesn't account for the density change of alcohol-water mixtures.

What is apparent attenuation?

Apparent attenuation measures what percentage of the original sugars were consumed by yeast. 75-80% is typical for most ale yeasts. Higher attenuation means a drier, less sweet beer.

Free Brewing ABV Calculator - Homebrew Alcohol Calculator

How It Works

Alcohol by volume (ABV) is calculated from the difference between Original Gravity (OG) and Final Gravity (FG). Before fermentation, dissolved sugars make the liquid denser than water. As yeast converts sugars to alcohol and CO₂, the density drops. The gap between these two readings tells us how much alcohol was produced.

The basic relationship:

ABV = (OG − FG) × 131.25

This works because each point of gravity drop corresponds to a predictable amount of ethanol production.

How to Use

Enter your Original Gravity — the hydrometer or refractometer reading taken before pitching yeast
Enter your Final Gravity — the reading after fermentation is complete (stable for 2-3 days)
Choose a formula — standard works for most brews; alternate is better for high-gravity beers
Read your results — ABV, attenuation, calories, and Brix conversions update instantly

Standard vs Alternate Formula

Standard formula: ABV = (OG - FG) × 131.25

Simple and accurate for normal-gravity beers (OG below 1.070). This is what most homebrewing software uses.

Alternate formula: ABV = (76.08 × (OG - FG) / (1.775 - OG)) × (FG / 0.794)

Accounts for the non-linear relationship between gravity and alcohol at higher concentrations. Use this for barleywines, imperial stouts, Belgian quads, and meads where OG exceeds 1.070. The standard formula can underestimate ABV by 0.5-1.0% for very strong brews.

Understanding Your Results

ABV — the percentage of your beer that is ethanol by volume
Apparent Attenuation — percentage of sugars consumed. 75-80% is normal for ale yeast; lager yeast often achieves 80-85%. Low attenuation means sweeter, fuller beer.
Calories (12oz) — estimated kilocalories per standard serving. Alcohol and residual sugars both contribute.
Brix — an alternative sugar scale used by refractometer owners. Note: refractometer readings after fermentation need a correction factor (alcohol skews the reading).

Tips for Homebrewers

Temperature-correct your hydrometer — most are calibrated at 20°C (68°F). Hot wort gives a falsely low reading.
Wait for stable FG — take readings 2-3 days apart. If the number doesn't change, fermentation is complete.
Record both readings — it's easy to forget your OG. Write it on the fermenter with a dry-erase marker.
Refractometer users — OG readings are direct, but FG readings in the presence of alcohol must be corrected with a refractometer calculator.
Carbonation adds ~0.2% ABV — bottle-conditioned beers ferment a small amount of priming sugar, adding slightly to the final ABV.

FAQ

My FG is higher than my OG. What happened?

This usually means OG was measured incorrectly (wort wasn't mixed well, or it was too hot). It can also happen if you accidentally measure water before adding extract. Re-check your process notes.

What's a good attenuation target?

Most ale yeasts attenuate 72-80%. If you're below 65%, fermentation may have stalled (temperature too low, underpitched yeast, or low nutrient levels). Above 85% is typical for saison yeasts and lager strains.

Can I use this for wine and cider?

Yes. The formulas work identically for any fermented beverage. Wine and cider often start at higher gravities (1.060-1.100+) and ferment drier (FG 0.996-1.005), so consider using the alternate formula for accuracy.

How do I convert Brix to specific gravity?

Use the formula: SG = 1 + (Brix / (258.6 - (Brix × 0.879))). Most refractometers show Brix directly. Note: this conversion only works for pre-fermentation readings. After fermentation starts, alcohol skews refractometer readings and a correction factor is needed.

What's the difference between ABV and ABW?

ABV (Alcohol By Volume) measures the percentage of liquid that is alcohol. ABW (Alcohol By Weight) is lower because alcohol is lighter than water — multiply ABV by 0.79 to get ABW. US regulations use ABW for beer labeling, while most of the world uses ABV.

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Last reviewed: June 2026