Alcohol by volume (ABV) is the percentage of pure ethanol in a fermented beverage. Understanding how much alcohol will be produced from a given starting sugar content is essential for homebrewing beer, wine, and fermented ciders. The calculation relies on the fact that yeast converts sugar into alcohol and carbon dioxide during fermentation.
The Formula
ABV (%) = (OG - FG) × 131.25
Where:
- OG = Original Gravity (specific gravity before fermentation)
- FG = Final Gravity (specific gravity after fermentation)
- 131.25 is an empirical conversion factor
Specific gravity is the density of liquid compared to water (which has SG = 1.000). Higher gravity means more dissolved sugar.
Simplified Formula
For quick estimates when starting with sugar content:
ABV (%) ≈ (Fermentable Sugar in grams / 1,000) × 0.131
Or simply: each gram of sugar per 1,000 mL of liquid produces about 0.13% ABV when fully fermented.
Worked Example
Making apple cider from juice:
- Starting gravity: 1.050 (measured with hydrometer)
- Final gravity: 1.000 (after fermentation complete)
ABV = (1.050 - 1.000) × 131.25 = 0.050 × 131.25 = 6.56%
The cider will be about 6.5% alcohol.
Gravity-to-Sugar Conversion
Original gravity roughly correlates to sugar content:
| OG | Apparent Sugar (Brix) | Potential ABV |
|---|---|---|
| 1.010 | 2.5° | 1.3% |
| 1.020 | 5° | 2.6% |
| 1.040 | 10° | 5.2% |
| 1.060 | 15° | 7.8% |
| 1.080 | 20% | 10.4% |
| 1.100 | 25% | 13% |
Brewers often use the rule: OG points × 0.0131 = ABV. (For OG 1.050, that's 50 × 0.0131 = 0.656% = 6.56%).
Measuring Gravity
Hydrometer: Float it in a sample of liquid. The scale reads specific gravity directly. Take readings at the same temperature (calibrated to 20°C or 68°F).
Refractometer: Faster, requires less liquid. Must convert from Brix to gravity: OG = 1 + (Brix / 200).
Calculation: Dissolve known sugar amount in water to calculate expected gravity.
Fermentation Efficiency
Yeast doesn't always ferment all sugar (residual sweetness remains). Apparent attenuation is what percentage of the way from OG to FG the fermentation goes:
Apparent Attenuation (%) = (OG - FG) / (OG - 1.000) × 100%
Most ale yeasts attenuate 70–80%; lager yeasts 80–90%.
Tips
Take gravity readings at the same temperature to avoid errors. Also remember that gravity measures dissolved solids (not just sugar — proteins, minerals, etc.) so estimates are rough. Stronger beverages (higher ABV) take longer to ferment and may stick (stop fermenting prematurely), especially if you pitch low yeast quantities.
Use our Fermentation ABV Calculator to calculate alcohol content from original and final gravity.