How to Calculate ABV From Your Gravity Readings

The formula

OG minus FG, times 131.25

Alcohol by volume for a homebrew is calculated from the difference between the gravity before fermentation and the gravity after it. Sugar is denser than water; alcohol is less dense. The drop in specific gravity is therefore a proxy for how much sugar the yeast converted.

The standard formula is: ABV equals original gravity minus final gravity, multiplied by 131.25. An ale starting at 1.055 and finishing at 1.012 has dropped 0.043, which multiplied by 131.25 gives 5.6 percent. That is it. The multiplier is an empirical constant derived from the density relationship between sugar, water and ethanol.

For session beers through ordinary IPAs, 131.25 is accurate to within a couple of tenths of a percent, which is far better precision than the readings themselves justify. There is no reason to use anything more complicated for everyday brewing.

The exception

Strong beer and mead need a bigger multiplier

The 131.25 constant assumes a modest alcohol content. As ethanol concentration rises the relationship between gravity drop and alcohol produced stops being linear, and the simple formula starts to underestimate. For a 5 percent ale that error is negligible. For a 14 percent mead it is not.

Above roughly 8 percent, use one of the published alternative formulas that account for the non-linearity -- most brewing calculators offer an option labeled for high gravity or for a cubic correction. The practical effect is that strong beers and meads come out a few tenths of a percent to a full percent higher than the simple formula suggests.

This matters most for mead, where a dry finish from a high starting gravity routinely lands in the teens. A traditional mead at 1.110 finishing at 1.000 is listed above as 14.4 percent by the simple formula; the high-gravity formulas put it meaningfully above that.

Error source one

Temperature, and the correction you forget

A glass hydrometer is calibrated at a specific temperature, usually 60 or 68 F depending on the instrument. Read a sample at 100 F straight out of the kettle and the figure is wrong, because warm liquid is less dense, and the hydrometer floats lower.

The error is not trivial. A reading taken 30 degrees above calibration temperature can be off by several thousandths of specific gravity, which on a 0.043 gravity drop is a meaningful fraction of your ABV. Either cool the sample to calibration temperature, or apply the temperature correction printed on the hydrometer's paper insert.

This is one of the quiet advantages of a floating Bluetooth hydrometer or a refractometer with automatic temperature compensation: the correction happens without you remembering to do it. The broader instrument comparison is in hydrometer vs refractometer.

Error source two

A refractometer after fermentation reads high

This is the single most common way homebrewers end up with a wrong ABV, and it is worth stating plainly: a refractometer is accurate on wort and inaccurate on beer.

A refractometer measures how much a liquid bends light, which correlates tightly with sugar concentration in a sugar-and-water solution. Ethanol bends light differently from both water and sugar, so once alcohol is present the instrument is reading a mixture it was not calibrated for. The result reads high, often by a large enough margin to suggest a fermentation has stalled when it has finished perfectly.

1. Use the refractometer before fermentation. Mash gravity, pre-boil gravity and original gravity are exactly what it is good for, and two or three drops is a far nicer sample than a hydrometer cylinder full.
2. Use a hydrometer for final gravity, or apply a published alcohol correction formula to the refractometer reading, which needs the original Brix as an input.
3. Zero the refractometer on distilled water before each session. It drifts, and a zero offset propagates into every reading you take afterwards.

Doing it properly

A sequence that gives a trustworthy number

1. Take OG after cooling, from a well-mixed wort. Wort stratifies, and a sample drawn from the top of an unmixed kettle reads differently from one drawn after the transfer. Stir, then sample.
2. Record the temperature with every reading. Not the corrected value, the actual temperature. It lets you re-check your arithmetic later.
3. Confirm FG is terminal, not just low. Two readings the same across two or three days means fermentation has finished. One low reading means one low reading.
4. Apply the right multiplier for the strength. 131.25 below about 8 percent, a high-gravity formula above it.

Where this came from

The basis for this

The 131.25 multiplier and the limitations of the simple formula at high gravity are standard brewing-science practice. Instrument behavior -- hydrometer temperature calibration, refractometer response to ethanol, automatic temperature compensation -- reflects the manufacturers' published specifications for the instruments named above. See how we pick.