This ABV calculator converts two hydrometer readings — original gravity (OG) before fermentation and final gravity (FG) after — into alcohol by volume. As yeast eats sugar, it produces alcohol and CO2, so the liquid gets lighter; the size of that gravity drop tells you exactly how much alcohol was made.
Alongside ABV, this homebrew abv calculator reports apparent attenuation (what fraction of the sugars fermented, typically 65–85% for beer yeast) and an estimated calorie count per 12 oz serving. Two formulas are shown: the standard quick formula every brewer memorizes, and an alternate formula that stays accurate for big beers above about 1.070 OG.
The ABV Formula: (OG − FG) × 131.25
The standard alcohol by volume calculator formula is:
ABV = (OG − FG) × 131.25
The constant 131.25 comes from the density of ethanol relative to water and how much mass leaves the wort as CO2 during fermentation. It is accurate to within about 0.1–0.2% ABV for ordinary-strength beers.
For high-gravity brews (imperial stouts, barleywines, meads), the linear approximation drifts, so brewers use the alternate formula:
ABV = 76.08 × (OG − FG) / (1.775 − OG) × (FG / 0.794)
Both are computed here. For a session beer they agree almost exactly; for an OG 1.100 barleywine the alternate formula can read 0.5% or more higher — and it is the one to trust.
Gravity Readings: Hydrometer vs Refractometer
Specific gravity compares your wort’s density to water (1.000). Dissolved sugar raises it; alcohol lowers it.
- Hydrometer — floats in a sample tube; read at the surface line. Calibrated at 60°F or 68°F, so correct warm samples (about +0.001 per 10°F above calibration). Works for both OG and FG.
- Refractometer — needs only a few drops and reads in Brix, handy on brew day. But alcohol skews it after fermentation starts, so FG readings must go through a wort correction, or your og fg calculator numbers will be wrong.
Typical ranges: pale ales OG 1.045–1.060 / FG 1.008–1.014; stouts OG 1.050–1.075; imperial styles OG 1.080+. If your FG stalls high (above ~1.020 for a normal ale), fermentation may be stuck.
Worked Example: OG 1.050, FG 1.010
Say your pale ale started at OG 1.050 and finished at FG 1.010.
- ABV (standard): (1.050 − 1.010) × 131.25 = 0.040 × 131.25 = 5.25%.
- ABV (alternate): 76.08 × 0.040 ÷ (1.775 − 1.050) × (1.010 ÷ 0.794) = 5.34% — nearly identical at this strength.
- Apparent attenuation: (0.050 − 0.010) ÷ 0.050 = 80%, right in the sweet spot for a clean American ale yeast.
- Calories: roughly 163 per 12 oz, close to a typical craft pale ale.
If that same beer had finished at 1.014 instead, ABV drops to 4.73% and attenuation to 72% — a small gravity difference is a noticeable alcohol difference.
Frequently Asked Questions
How do you calculate ABV from OG and FG?
Subtract final gravity from original gravity and multiply by 131.25. For example, OG 1.050 minus FG 1.010 is 0.040, and 0.040 × 131.25 = 5.25% ABV. This standard formula is accurate for most beers; for strong beers above roughly 1.070 OG, use the alternate formula shown by this calculator.
What does an original gravity of 1.050 mean?
It means your wort is 5% denser than water because of dissolved sugars — about 12.4 degrees Plato. OG 1.050 is a classic starting point for pale ales and ambers, and with a typical 75–80% attenuating yeast it produces a beer around 5–5.5% ABV with a final gravity near 1.010–1.013.
What is a good final gravity for beer?
Most ales finish between 1.008 and 1.016. Dry styles like saisons can reach 1.002–1.006, while sweet stouts and Scotch ales may sit at 1.018–1.024 on purpose. If a standard ale stalls above 1.020, suspect a stuck fermentation — check temperature, rouse the yeast, and confirm with readings two days apart.
Why is my homebrew ABV lower than expected?
The usual causes are poor attenuation (yeast quit early from low temperature, low pitch rate, or low oxygen), a lower-than-planned OG from weak mash efficiency, or an uncorrected refractometer FG reading. Verify FG with a hydrometer, confirm gravity is stable across two days, and compare your attenuation to the yeast strain’s published 65–85% range.
How many calories are in homemade beer?
Roughly 150–170 calories per 12 oz for a 5% ABV beer, rising to 250+ for 8% imperial styles. About two-thirds of the calories come from alcohol and the rest from residual carbohydrates, which is why a dry, well-attenuated beer has fewer calories than a sweet one at the same ABV. This calculator estimates it from your OG and FG.