The formula
ABV ≈ (original gravity − final gravity) × 131.25
Where it comes from
Yeast turns sugar into alcohol and carbon dioxide, and alcohol is lighter than water: that is why the wort gets less dense as it ferments. The gap between original and final gravity measures how much sugar has gone, and the alcohol follows from it. Multiplying by 131.25 is a straight-line approximation that works well up to about seven per cent; above that it reads low, and the corrected formula takes over.
How to work it out by hand
- Take the gravity of the wort before pitching the yeast
- Take it again once fermentation has stopped
- Subtract the final from the original
- Multiply by 131.25, or use the corrected formula above seven per cent
What is worth knowing
Attenuation tells you what share of the available sugar the yeast has eaten: 70 to 80 per cent is normal in a beer, and below 60 usually means a fermentation that stalled. Watch the hydrometer: it is calibrated at 20 °C and a warm sample reads low, so let it cool or correct for it. And taking a final gravity while the beer is still bubbling gives a low reading too, because the gas clinging to the glass lifts it.