What this calculator does
Alcohol by volume is estimated from the drop in specific gravity during fermentation. The standard formula multiplies the difference between original and final gravity by 131.25, so a beer going from 1.050 to 1.010 comes out at 5.25% ABV.
The multiplier is an empirical approximation rather than a derivation, and it drifts at high alcohol. It is reliable to within a couple of tenths of a percent below about 8% ABV and progressively overstates above that, which is why stronger beers and wines are often calculated with a more elaborate formula instead.
The formula
Final gravity is subtracted from original gravity and the difference multiplied by 131.25 to give alcohol by volume as a percentage. Alcohol by weight is derived from that using the density ratio of ethanol to water, and comes out at roughly four fifths of the ABV figure.
| Term | Meaning |
|---|---|
| Original gravity (OG) | Specific gravity before fermentation, when sugars are at their highest. |
| Final gravity (FG) | Specific gravity after fermentation, once the yeast has finished. |
| 131.25 | The empirical multiplier converting gravity drop to percentage ABV. |
| ABW | Alcohol by weight, about 0.79 times the ABV figure because ethanol is less dense than water. |
The inputs explained
| Field | What to enter |
|---|---|
| Original gravity | Original gravity, typically between 1.040 and 1.090 for beer. |
| Final gravity | Final gravity, typically 1.008 to 1.015 for a fully fermented beer. |
When to use it
Working out the strength of a homebrew
Two hydrometer readings taken before and after fermentation are the standard way of knowing what a batch actually came out at.
Checking whether fermentation has finished
A gravity that stops falling across several days indicates the yeast has finished, and the reading at that point is the final gravity.
Planning a recipe to a target strength
Working backwards from a target ABV and an expected attenuation gives the original gravity a recipe needs to hit.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What ABV does each gravity drop give?
A range of original gravities all fermenting down to the same final gravity.
| Original gravity | Alcohol by volume | Approx. alcohol by weight |
|---|---|---|
| 1.040 | 3.94% | 3.11% |
| 1.050 | 5.25% | 4.15% |
| 1.060 | 6.56% | 5.18% |
| 1.090 | 10.50% | 8.30% |
Questions
How accurate is the 131.25 formula?
Good to within about 0.2% ABV below 8%, which covers most beer. Above that it progressively overstates, and for wines and strong beers an alternative formula accounting for the non-linear relationship is more reliable. The simple version persists because it is close enough for most brewing and easy to remember.
Do I need to correct for temperature?
Yes, if the sample is not at the hydrometer calibration temperature, usually 20°C or 60°F. A warm sample reads low, and the correction is a few thousandths of a gravity point which translates to a few tenths of a percent ABV. Most hydrometers carry a correction scale.
What is the difference between ABV and ABW?
Alcohol by volume measures the fraction of the liquid that is ethanol by volume; alcohol by weight measures it by mass. Since ethanol is less dense than water, ABW is about 0.79 times ABV. Most countries label by volume, and the distinction matters mainly when reading older American sources.
Can I calculate ABV from one reading?
Not reliably. You need both the starting and finishing gravity, because the alcohol comes from the difference between them. A refractometer reading after fermentation can be corrected to estimate the drop, but that correction needs the original reading too.
For diluting a finished spirit to a target strength, see the alcohol dilution calculator. For priming sugar at bottling, see the priming sugar calculator.