ABV Calculator
Alcohol by volume is worked out from how much sugar the yeast consumed, measured as the drop in specific gravity between the start and end of fermentation.
Formula
ABV % = (original gravity − final gravity) × 131.25
- Apparent attenuation = (OG − FG) ÷ (OG − 1) × 100
- Gravities are read with a hydrometer, corrected to its calibration temperature
OG 1.050 down to FG 1.010: (0.040) × 131.25 = 5.3% ABV
The Formula
ABV % = (Original Gravity − Final Gravity) × 131.25
An OG of 1.050 falling to an FG of 1.010 gives (0.040) × 131.25 = 5.25% ABV.
Specific gravity compares the density of your liquid to water at 1.000. Sugar raises it, alcohol lowers it, so the fall during fermentation is a direct measure of sugar converted.
Taking Good Readings
Take the original gravity before pitching yeast, and the final gravity when the reading has held steady for two or three days — a still hydrometer is the only reliable sign fermentation has finished.
Hydrometers are calibrated to a specific temperature, usually 20°C. A warm sample reads low, so either cool it or apply the correction printed on the instrument.
Typical Gravities and Attenuation
- Session beer: OG 1.035–1.045, around 3.5–4.5% ABV
- Standard ale: OG 1.045–1.055, around 4.5–5.5%
- Strong ale or IPA: OG 1.060–1.075, around 6–8%
- Wine: OG 1.080–1.100, around 11–14%
The share of sugar consumed is called attenuation, typically 70–80% for ale yeasts. A final gravity stuck well above expectation usually means a stalled fermentation rather than a strong beer.
Quick Reference
| Original gravity | Final gravity | ABV | Attenuation |
|---|---|---|---|
| 1.040 | 1.010 | 3.9% | 75% |
| 1.050 | 1.010 | 5.3% | 80% |
| 1.060 | 1.012 | 6.3% | 80% |
| 1.070 | 1.015 | 7.2% | 79% |
| 1.090 | 1.020 | 9.2% | 78% |
Frequently Asked Questions
- How do I calculate ABV?
- Subtract final gravity from original gravity and multiply by 131.25. From 1.050 down to 1.010 gives 5.25 per cent.
- When should I take the final reading?
- When the gravity has been unchanged for two or three consecutive days. A steady hydrometer is the only reliable sign fermentation has finished.
- Does temperature affect the reading?
- Yes. Hydrometers are calibrated to around 20°C and a warm sample reads low. Cool the sample or apply the correction chart supplied with it.
- Why is my final gravity too high?
- Usually a stalled fermentation from low temperature, underpitched yeast or insufficient nutrient, rather than an unusually strong brew.