Measuring alcohol with a hydrometer
Yeast turns sugar into alcohol and carbon dioxide. Sugar makes a liquid denser than water, and alcohol makes it less dense, so the drop in density during fermentation tells you how much alcohol was made. Homebrewers measure it with a hydrometer or refractometer: the original gravity (OG) before pitching the yeast and the final gravity (FG) once fermentation is done.
The formulas
Standard formula: ABV = (OG − FG) × 131.25
It is accurate for most beers and ciders up to about 6% to 7%.
Advanced formula: ABV = 76.08 × (OG − FG) ÷ (1.775 − OG) × FG ÷ 0.794
It accounts for the lower density of alcohol and gives better results for strong beers, wines and meads, where the standard formula underestimates.
The calculator also shows:
- Apparent attenuation = (OG − FG) ÷ (OG − 1), the share of the sugar the yeast appears to have fermented
- Calories per 12 oz, from the alcohol and the remaining extract
- OG in degrees Plato, the scale commercial brewers use
Worked example
A pale ale starts at 1.050 and finishes at 1.010.
- ABV = 0.040 × 131.25 ≈ 5.3%; the advanced formula also gives 5.3%
- Attenuation = 0.040 ÷ 0.050 = 80%
- About 162 kcal per 12 oz
- 1.050 is about 12.4 °P
Typical values
| Style | OG | FG | ABV |
|---|---|---|---|
| Light lager | 1.040 | 1.006 | 4.5% |
| Pale ale | 1.050 | 1.010 | 5.3% |
| IPA | 1.065 | 1.012 | 7.0% |
| Imperial stout | 1.090 | 1.020 | 9–10% |
| Dry cider | 1.050 | 1.000 | 6.6% |
| Dry wine | 1.090 | 0.995 | 12.5–13% |
Most ale yeasts reach 70% to 80% apparent attenuation. A wine or cider can finish below 1.000 because alcohol is lighter than water.
Tips for accurate readings
- Temperature: hydrometers are calibrated for 60 °F or 68 °F. A warm sample reads low; cool it or use a correction chart.
- Degas the final sample by pouring it back and forth, since bubbles lift the hydrometer.
- Refractometers need a correction once alcohol is present; do not read FG directly from one.
- Check that the FG has been stable for two or three days before bottling, to avoid over-carbonation.
- Priming sugar added for bottle conditioning raises the final ABV slightly, usually by a few tenths of a point.