What is the formula for beer ABV from OG and FG?

The most common formula is ABV = (OG - FG) x 131.25. For example, OG 1.055 and FG 1.012 gives (1.055 - 1.012) x 131.25 = 5.64% ABV. A more precise alternate formula used for high-gravity beers is ABV = (76.08 x (OG - FG) / (1.775 - OG)) x (FG / 0.794). Both are available in the calculator above.

What is apparent attenuation?

Apparent attenuation (AA) measures how much of the fermentable sugars yeast has consumed, expressed as a percentage: AA = ((OG - FG) / (OG - 1)) x 100. A well-attenuating ale yeast typically achieves 73-80%. High attenuation means a drier, more fermented beer; lower attenuation leaves more residual sweetness and body.

How accurate is the OG/FG ABV formula?

The standard formula is accurate to within 0.1-0.2% ABV for beers below about 8% ABV. Above that, the alternate Balling formula (available in the selector) is more reliable because alcohol and residual sugars interact non-linearly at high concentrations. Refractometer readings after fermentation also require a correction factor because alcohol changes the refractive index.

How many calories are in homebrew beer?

Calories come from two sources: alcohol (7 kcal/g of ethanol) and residual fermentable sugars reflected in the final gravity. The calculator uses the Papazian / How to Brew formula: Cals/12 oz = 3.55 x (6.9 x ABV% + 4 x RE), where Real Extract (RE) is derived from original and final gravity converted to degrees Plato. A typical 5.6% pale ale (OG 1.055, FG 1.012) comes in at roughly 209 kcal per 355 ml serving, while a 10% imperial stout can exceed 370 kcal.

What is a normal FG for homebrew beer?

Final gravity depends on yeast strain, mash temperature and recipe. Most ales finish between 1.008 and 1.015; lagers typically 1.006-1.012. Beers mashed high (68-70 C / 155-158 F) leave more unfermentable dextrins and finish higher. High-attenuating yeast strains can drop a 1.060 OG wort to 1.006 or lower.

Can I use a refractometer reading for FG?

Not directly. After fermentation begins, alcohol lowers the refractive index, so a raw refractometer reading overstates the true gravity. Use the Sean Terrill correction: FG = 1.0000 - 0.0044993 x Brix_OG + 0.011774 x Brix_FG + 0.00027581 x Brix_OG^2 - 0.0012717 x Brix_FG^2 - 0.0000072800 x Brix_OG^3 + 0.0000063293 x Brix_FG^3. Alternatively, use a hydrometer for the final reading — it needs no correction.

Beer ABV Calculator — Gera Tools

A beer ABV calculator for homebrewers and craft beer enthusiasts. Enter your original gravity (OG) and final gravity (FG) — measured with a hydrometer or refractometer — and get the alcohol by volume, apparent attenuation and estimated calories for standard serving sizes, all calculated instantly in your browser.

How it works

Brewers measure wort density in specific gravity units relative to pure water (1.000). Sugar dissolved in water raises the gravity above 1.000: a typical pale ale wort might read 1.055 OG. As yeast ferments the sugars into ethanol and CO2, the gravity falls — the same beer might finish at FG 1.012.

ABV formula

The most widely used formula, popularised by Papazian and Daniels, is:

ABV = (OG − FG) × 131.25

The constant 131.25 encapsulates the density of ethanol (0.789 g/ml) and the relationship between specific gravity and sugar content. For beers above about 8% ABV, an alternate Balling-derived formula is more accurate:

ABV = (76.08 × (OG − FG) ÷ (1.775 − OG)) × (FG ÷ 0.794)

Both are available in the calculator — use the dropdown to switch.

Apparent attenuation

Apparent attenuation (AA) tells you what percentage of fermentable sugars the yeast converted:

AA = (OG − FG) ÷ (OG − 1) × 100

A session bitter attenuating to 75% will taste drier than one that only reaches 65%. Most commercial ale yeasts are rated 72–82% apparent attenuation; lager yeasts often push above 80%.

Calories

Calories in beer come from two sources: ethanol (7 kcal per gram) and residual fermentable carbohydrates tracked by the final gravity. The calculator uses the Papazian / How to Brew formula:

Cals/12 oz = 3.55 × (6.9 × ABV% + 4 × RE)

where RE (Real Extract, in degrees Plato) = 0.1808 × OG°P + 0.8192 × FG°P, and Plato is derived from specific gravity via the ASBC cubic polynomial.

Results are scaled to 100 ml, 330 ml can, 355 ml (12 fl oz), 500 ml, and a 568 ml UK pint.

Worked example

An American IPA with OG 1.065 and FG 1.012:

Step	Calculation	Result
Gravity drop	1.065 − 1.012	0.053
ABV (standard)	0.053 × 131.25	6.96%
Attenuation	0.053 ÷ 0.065 × 100	81.5%
OG in Plato	cubic polynomial	15.90 °P
FG in Plato	cubic polynomial	3.07 °P
RE	0.1808 × 15.90 + 0.8192 × 3.07	5.39 °P
Cals/12 oz	3.55 × (6.9 × 6.96 + 4 × 5.39)	≈ 247 kcal

Switch to the alternate Balling formula for this OG and the result shifts to about 7.24% — a noticeable difference even at this gravity, and more pronounced above 9%.

Measuring gravity accurately

Hydrometer: the gold standard. Correct for temperature — most are calibrated at 15 C (59 F) or 20 C (68 F). Add 0.001 per 5 C above calibration temperature.
Refractometer: fast and needs only a few drops. Fine for OG, but FG readings require an alcohol correction (see FAQ below).
Digital density meter (e.g. Anton Paar): most accurate, no temperature correction needed.

The gravity reference scale

Homebrewers often express gravity in “points” — simply the decimal places multiplied by 1000. OG 1.055 = 55 points. The German Plato scale (°P) approximates points ÷ 4, so 55 points ≈ 13.75 °P. Commercial craft beers commonly list original extract in °P on the label.