ABV, Proof, Gravity, and Dilution: Alcohol Math for Drinkers and Homebrewers
October 29, 2026
Alcohol by volume (ABV) is the percentage of a drink's volume that is pure ethanol. In the United States, proof is twice the ABV, so a 40% ABV spirit is 80 proof. In the UK the historical proof scale differs (57.1% ABV was 100 proof), but labels now use ABV.
A standard drink in the US contains 0.6 fluid ounces (14 g) of pure alcohol. 12 oz × 5% = 0.6 oz for beer; 5 oz × 12% = 0.6 oz for wine; and 1.5 oz × 40% = 0.6 oz for spirits.
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Brewers measure specific gravity before fermentation (original gravity, OG) and after (final gravity, FG) with a hydrometer. A common formula is ABV ≈ (OG − FG) × 131.25. For OG 1.050 and FG 1.010: (1.050 − 1.010) × 131.25 = 0.040 × 131.25 = 5.25%. For a strong ale with OG 1.090 and FG 1.020: 0.070 × 131.25 = 9.2%. The formula is accurate to a few tenths of a percent for typical beers, with less accuracy for high-gravity beers (alternate formulas exist). Wine: a must at 1.090 fermenting to 0.995 gives (1.090 − 0.995) × 131.25 = 12.5%.
| Beverage | OG | FG | Estimated ABV |
|---|---|---|---|
| Pale ale | 1.050 | 1.010 | 5.25% |
| Strong ale | 1.090 | 1.020 | 9.19% |
| Table wine | 1.090 | 0.995 | 12.47% |
| Light lager | 1.040 | 1.008 | 4.20% |
Diluting to a Target Strength
Use the rule C₁V₁ = C₂V₂. To dilute 750 mL of 40% spirit to 20%, the final volume is 750 × 40 ÷ 20 = 1,500 mL, so add 750 mL of water. To bring a 60% (120 proof) spirit to 40%: final volume = V × 60 ÷ 40 = 1.5V, so add 0.5 volume of water per volume of spirit, for 500 mL of water per liter of spirit. In practice, ethanol and water contract slightly when mixed, so final volumes are a bit less than the sum (about 1% to 3%), which matters for commercial labeling and is usually negligible for home use.
Pure Alcohol and Calories
Pure ethanol has 0.789 g per mL and provides 7 calories per gram, so a standard drink with 14 g has about 98 calories from alcohol, with additional calories from sugar in beer, wine, and mixers. To find the pure alcohol in a drink: volume (mL) × ABV × 0.789. A 750 mL bottle of 13% wine has 750 × 0.13 × 0.789 = 76.9 g, about 5.5 standard drinks.
Health and Legal Notes
Health guidelines suggest limits (for example, up to 1 drink a day for women and 2 for men in US dietary guidelines, and less is better). Homebrewing and distilling laws vary: home distillation is illegal in many places without a permit. This article is for general information only.
Frequently Asked Questions
How do I convert ABV to proof in the US?
Multiply by 2.
How do I estimate ABV from gravity?
(OG − FG) × 131.25.
How much pure alcohol is in a standard US drink?
14 g, or 0.6 fluid ounces.
How do I dilute a spirit to a lower ABV?
Use C₁V₁ = C₂V₂ to find the final volume.
How many calories come from alcohol in a standard drink?
About 98.