From Two Gravity Readings to Your ABV
Every homebrew starts sweet and finishes drier because yeast eats sugar and gives back alcohol and CO2. This calculator turns that transformation into a number. Give it your original gravity — the density of the wort or must before pitching — and your final gravity once fermentation settles, and it returns the alcohol by volume along with alcohol by weight and apparent attenuation. The whole method rests on a single, well-worn brewing formula, so the readings you take with your hydrometer are the only thing standing between you and an accurate result.
A Worked Example: 1.055 Down to 1.010
Say you brewed a pale ale that started at an original gravity of 1.055 and finished at a final gravity of 1.010. Subtract the final from the original to get the gravity drop: 1.055 − 1.010 = 0.045. Multiply that by the standard factor: 0.045 × 131.25 ≈ 5.9% ABV. That is a classic session-to-standard strength beer. Notice how much the small numbers matter — if you had misread the FG as 1.015 instead of 1.010, your gravity drop shrinks to 0.040 and the ABV falls to about 5.25%. A single hydrometer point moves the answer by roughly 0.13% ABV.
Why the ×131.25 Factor Isn't Perfect
The 131.25 constant is an empirical fit, not a law of physics. It assumes alcohol production scales linearly with the gravity drop, which holds up beautifully across the ordinary beer range but begins to overstate strength on big brews. Ferment a 1.100 wort down to 1.010 and the simple formula reads high by a noticeable margin because the real relationship curves at those extremes. For strong ales, wines, and meads, a nonlinear formula tracks better. You will also see some brewers use ×125 rather than ×131.25, which yields a slightly more conservative number; both are common in the hobby.
Homebrew ABV Calculator
How to Use This Calculator
- Record Your Original Gravity: Before pitching yeast, take a hydrometer reading of the wort or must at calibration temperature and enter it as your original gravity — for example 1.055.
- Record Your Final Gravity: Once fermentation has stopped and the reading is stable across a couple of days, measure the final gravity and enter it — for example 1.010.
- Click Calculate: The calculator finds the gravity drop, multiplies by 131.25, and returns your ABV instantly below.
- Read Your Results: Check the ABV %, alcohol by weight, and apparent attenuation. Compare the attenuation figure against your recipe to judge whether the yeast performed as expected.
How It Works
Alcohol by volume is worked out from how much sugar the yeast ate during fermentation. You take two hydrometer readings — one before the yeast goes to work (original gravity) and one after fermentation finishes (final gravity) — and the drop between them tells you how much sugar turned into alcohol.
The basic rule:
- ABV = (OG - FG) × 131.25
The ×131.25 constant is a linear shortcut that fits standard beer- and wine-strength brews well. It slowly overshoots on very strong batches, so treat a reading above roughly 10% ABV as a close estimate rather than a lab number.
Tips & Considerations
- Take OG and FG at the same temperature your hydrometer is calibrated for — usually 60°F (20°C) — or correct the reading, since a warm sample reads low.
- Spin the hydrometer and let CO2 bubbles clear before reading; clinging bubbles lift the float and inflate the number.
- Read at the bottom of the meniscus with your eye level to the liquid surface for a consistent value.
- Confirm fermentation is done by taking two FG readings a few days apart — a steady number, not the airlock, tells you it is finished.
- For strong wines, meads, or barleywines above about 10% ABV, treat the ×131.25 result as an estimate that reads slightly high and consider a nonlinear formula.
Frequently Asked Questions
What are OG and FG?
OG is original gravity, the specific gravity of your wort or must before you pitch the yeast — it reflects all the fermentable sugar you started with. FG is final gravity, the reading once fermentation has stopped and the yeast has converted what it can into alcohol and CO2. Both are dimensionless numbers close to water (1.000), like 1.055 and 1.010. The bigger the gap between them, the more sugar was fermented and the higher the ABV.
Why multiply by 131.25?
131.25 is an empirical constant that converts the gravity drop into a percentage of alcohol by volume. It bundles together the density of ethanol and the relationship between sugar consumed and alcohol produced into one tidy number. Some brewers use 125 instead, which reads a touch lower; the difference is small at session strength but widens on big beers. Neither is exact physics — both are practical approximations that fit the typical homebrew range.
How do I read a hydrometer?
Fill the test tube or float the hydrometer in a sample of wort, give it a gentle spin to shake off clinging bubbles, and let it settle. Read the number where the liquid surface crosses the stem, looking at the bottom of the meniscus (the little curved dip) at eye level. Foam and carbonation will throw the reading off, so degas fizzy samples first. A refractometer is an alternative for OG, but it needs a correction once alcohol is present, so a hydrometer is simpler for FG.
Does temperature affect the reading?
Yes. Hydrometers are calibrated to one temperature, usually 60°F (20°C), and a warm sample reads artificially low because the liquid is less dense. If you measure a hot sample straight off the boil, correct it upward or cool it to calibration temperature before reading. A few tenths of a gravity point from temperature can shift your final ABV by a couple of tenths of a percent, so it is worth getting right on both the OG and FG readings.
Why does the ×131.25 factor drift at high ABV?
The formula assumes a straight-line relationship between gravity drop and alcohol, but that relationship curves as strength climbs. On a strong barleywine, mead, or wine — say an OG of 1.100 fermenting down to 1.010 — the simple formula tends to overstate the result slightly. For those big batches, an alternate formula such as ABV = (76.08 × (OG − FG) / (1.775 − OG)) × (FG / 0.794) tracks reality better. For everyday beers this drift is negligible.
How do I know fermentation is finished?
Take gravity readings two or three days apart. When the final gravity holds steady across those readings and matches the ballpark you expected for the recipe, fermentation is done. A stable FG, not the calendar or airlock activity, is the reliable signal that the yeast has finished and the beer is ready to package.