Home brewing
ABV estimator
Estimates alcohol by volume from original and final gravity, and shows apparent attenuation — the more diagnostic number, and the one rarely displayed.
The question this answers: “What strength is this, and has it actually finished fermenting?”
Result
- Estimated ABV
- 4.9%
- From a gravity drop of 0.037. An estimate, not a measurement.
- Apparent attenuation
- 77%
- The proportion of available sugar the yeast consumed. Typically 70–80% for most ale yeasts.
- Batch size
- 23 litres
- Recorded so you can scale priming sugar and dosing against it.
Worth knowing about this answer
- Two identical readings two days apart is the only evidence that fermentation has finished. A single reading, however low, is not — and bottling on an unfinished ferment is what produces bottle bombs.
- Hydrometers are calibrated at a specific temperature, usually printed on the instrument. A reading taken on warm wort will be off, and that error carries straight into this result.
Show the working
- Gravity drop
1.048 − 1.0110.037 - ABV
0.037 × 131.254.9% - Apparent attenuation
0.037 ÷ 0.04877%
Estimated ABV: 4.9%. Apparent attenuation: 77%. Batch size: 23 litres
The standard linear approximation: the gravity drop multiplied by 131.25. Apparent attenuation is the drop as a proportion of the original gravity points.
- Both readings were taken at, or corrected to, the hydrometer’s calibration temperature.
- The 131.25 constant is a linear approximation that holds well in the ordinary beer range.
- Above roughly 1.090 original gravity the approximation drifts and reads low.
- Reported to one decimal place, because a hydrometer read by eye does not support more precision than that.
- It cannot tell you fermentation has finished. Only two identical readings two days apart can, and bottling without them is what causes bottle bombs.
Runs entirely in your browser — nothing you type is sent anywhere. Last reviewed 2026-08-14. More on Home brewing.