How do you calculate ABV from OG and FG?
Take a hydrometer reading before you pitch the yeast (original gravity, OG) and another when fermentation has finished (final gravity, FG). The gap between them is sugar that became alcohol and CO₂, and the standard homebrew formula turns that gap into a percentage:
ABV = (OG − FG) × 131.25
For a beer that went from 1.052 to 1.011: (1.052 − 1.011) × 131.25 = 5.4% ABV. That is the number on most recipe sheets and most calculators, and for ordinary beer it is close enough — every 0.001 of gravity the yeast eats is worth about 0.13% ABV.
It isn't magic. Each gravity point is roughly 0.26% sugar by weight, fermenting a gram of sugar yields about half a gram of ethanol, and ethanol is lighter than water (0.789 g/mL) — so a weight percentage becomes a bigger volume percentage. Chain those together and the constant lands around 131 for typical beer. It is a linearization, which is exactly why it drifts on big beers.
Which ABV formula is right — 131.25 or the "alternate"?
The second formula the calculator shows is Hall's (Zymurgy, 1995), the one Brewer's Friend labels "alternate":
ABV = 76.08 × (OG − FG) ÷ (1.775 − OG) × (FG ÷ 0.794)
It accounts for the fact that alcohol's density and the sugar-to-alcohol yield are not constant as gravity climbs. Below about 1.060 the two agree to a tenth of a percent; from 1.070 up the simple formula starts undercounting, and by imperial-stout territory the gap is more than a full percent:
| OG | FG | Standard | Alternate | Gap |
|---|---|---|---|---|
| 1.040 | 1.008 | 4.2% | 4.21% | +0.01 |
| 1.050 | 1.010 | 5.25% | 5.34% | +0.09 |
| 1.060 | 1.012 | 6.3% | 6.51% | +0.21 |
| 1.070 | 1.015 | 7.22% | 7.59% | +0.37 |
| 1.080 | 1.018 | 8.14% | 8.7% | +0.56 |
| 1.090 | 1.020 | 9.19% | 9.99% | +0.8 |
| 1.100 | 1.022 | 10.24% | 11.32% | +1.08 |
| 1.110 | 1.025 | 11.16% | 12.55% | +1.4 |
| 1.120 | 1.028 | 12.08% | 13.84% | +1.76 |
Computed by this page's engine. Rows shaded where the gap exceeds 0.3% — about the best precision a homebrew hydrometer can deliver anyway.
Use the standard formula for anything under 1.070 and the alternate above it — or, more usefully, just read both off the calculator and know that the truth for a barleywine is nearer the higher number. Neither is exact; both assume the gravity drop was all sugar becoming ethanol, which is nearly but not perfectly true.
ABV chart: OG and FG at a glance
For a quick answer without a calculator — a brew-day sheet, a tap-list card, a fridge door — here is the standard formula across the range most beers land in. Above 1.070, add the gap from the table above.
| OG ↓ FG → | 1.000 | 1.005 | 1.008 | 1.010 | 1.012 | 1.015 | 1.018 | 1.020 |
|---|---|---|---|---|---|---|---|---|
| 1.030 | 3.9 | 3.3 | 2.9 | 2.6 | 2.4 | 2.0 | 1.6 | 1.3 |
| 1.035 | 4.6 | 3.9 | 3.5 | 3.3 | 3.0 | 2.6 | 2.2 | 2.0 |
| 1.040 | 5.3 | 4.6 | 4.2 | 3.9 | 3.7 | 3.3 | 2.9 | 2.6 |
| 1.045 | 5.9 | 5.3 | 4.9 | 4.6 | 4.3 | 3.9 | 3.5 | 3.3 |
| 1.050 | 6.6 | 5.9 | 5.5 | 5.3 | 5.0 | 4.6 | 4.2 | 3.9 |
| 1.055 | 7.2 | 6.6 | 6.2 | 5.9 | 5.6 | 5.3 | 4.9 | 4.6 |
| 1.060 | 7.9 | 7.2 | 6.8 | 6.6 | 6.3 | 5.9 | 5.5 | 5.3 |
| 1.065 | 8.5 | 7.9 | 7.5 | 7.2 | 7.0 | 6.6 | 6.2 | 5.9 |
| 1.070 | 9.2 | 8.5 | 8.1 | 7.9 | 7.6 | 7.2 | 6.8 | 6.6 |
| 1.075 | 9.8 | 9.2 | 8.8 | 8.5 | 8.3 | 7.9 | 7.5 | 7.2 |
| 1.080 | 10.5 | 9.8 | 9.5 | 9.2 | 8.9 | 8.5 | 8.1 | 7.9 |
| 1.085 | 11.2 | 10.5 | 10.1 | 9.8 | 9.6 | 9.2 | 8.8 | 8.5 |
| 1.090 | 11.8 | 11.2 | 10.8 | 10.5 | 10.2 | 9.8 | 9.5 | 9.2 |
| 1.095 | 12.5 | 11.8 | 11.4 | 11.2 | 10.9 | 10.5 | 10.1 | 9.8 |
| 1.100 | 13.1 | 12.5 | 12.1 | 11.8 | 11.6 | 11.2 | 10.8 | 10.5 |
Percent ABV by the standard formula, (OG − FG) × 131.25. Read the row for your OG and the column for your FG. Correct warm hydrometer readings to the calibration temperature first; above OG 1.070 the alternate formula reads 0.4–1.8% higher.
Why is my ABV lower than the recipe said?
Almost always one of three things, and the calculator can tell you which by how far you missed:
1. Your OG came in low. The recipe assumed a brewhouse efficiency you did not hit — the commonest cause for all-grain brewers, and the reason this site keeps your real efficiency in your equipment profile. Five gravity points short of the target is 0.7% ABV gone before the yeast even started.
2. Your FG stopped high. Under-pitched yeast, a cold spot, a mash that ran hot and left unfermentable dextrins: each point the beer stalls above the recipe's FG costs the same 0.13%. Compare your apparent attenuation (in the result panel) with the yeast's published range — if you are 10 points under it, the fermentation is the problem, not the recipe.
3. One reading was wrong. Read the top of the meniscus instead of the bottom, sample warm, or take FG before the beer was done, and the error goes straight into the answer. The "if the FG had landed differently" rows in the calculator show how much a couple of points move the result.
Do I need to correct hydrometer readings for temperature?
Yes if the sample was noticeably warmer than the hydrometer's calibration mark — usually 60°F (15.6°C), sometimes 68°F (20°C); it's printed on the paper scale inside. Warm liquid is less dense, so a warm reading comes out low, and since brewers routinely take OG from wort that is still cooling and FG from beer at room temperature, the two errors don't even cancel. Here is what happens to a 1.050 wort read warm, with the FG taken correctly:
| 1.050 read at | Actually | ABV error if uncorrected |
|---|---|---|
| 60°F / 16°C | 1.0500 | none |
| 70°F / 21°C | 1.0511 | −0.1% |
| 80°F / 27°C | 1.0524 | −0.3% |
| 90°F / 32°C | 1.0541 | −0.5% |
| 100°F / 38°C | 1.0561 | −0.8% |
Standard water-density correction (calibration 60°F), computed by this page's engine. Use the "correct for temperature" box in the calculator to apply it to your own readings.
A hydrometer read at the bottom of the meniscus, at (or corrected to) its calibration temperature, in a sample with the bubbles knocked out, is good to about ±0.001 — call it ±0.15% ABV. A refractometer reads a single drop and skips the sample-jar cleanup, but once alcohol is present its FG reading is wrong until you correct it, and most people don't. A $10 hydrometer read correctly beats a $60 refractometer read wrong. The label on a homebrew is honest to about ±0.3% either way — treat the second decimal as decoration.
Attenuation, ABW and calories — the other numbers in the panel
Apparent attenuation is the share of the original gravity the yeast removed: for 1.052 → 1.011 that's 79%, which is what yeast labs quote. Real attenuation corrects for the alcohol now in the beer thinning the reading, and comes out lower — 64% here. It's the truer number, but the apparent one is the one everyone compares. ABW (alcohol by weight) is what some countries print and older American texts used; it's ABV × 0.794 ÷ FG, so 5.4% ABV is 4.2% ABW. Calories follow the ASBC method from ABW and real extract: this example beer carries about 170 kcal per 12 oz or 239 per 500 mL, roughly two-thirds from the alcohol.