Spirits Proof Calculator
The US proof system defines proof as exactly twice the ABV percentage. This calculator converts between ABV and proof in both directions, and also calculates the dilution required to bring a high-strength spirit down to a target bottling or serving strength. Distillers, bartenders, and home enthusiasts use dilution calculations when proofing down barrel-strength whiskey, adjusting cocktail spirit strength, or blending spirits of different strengths. The calculator uses the simplified Pearson Square dilution formula, which is accurate for ABV below 60%. For higher strengths, actual volume contraction must be accounted for.
Proof and dilution formulas
US Proof = ABV% * 2
Water to add = spirit_volume * (current_ABV / target_ABV - 1)
Final volume = spirit_volume + water_added
Verification: final_ABV = (spirit_volume * current_ABV) / final_volume
Dilution notes
- Add water slowly and in small increments when proofing down spirits. Stir gently after each addition.
- Allow diluted spirit to rest for 24-48 hours ("marrying") before final tasting or bottling.
- Use demineralized or reverse osmosis water for the purest result in craft distilling.
- At very high ABV levels (above 60%), the volume contraction upon mixing with water (due to hydrogen bonding) means the final volume is slightly less than the sum of parts.
- Commercial distillers use proofing hydrometers with temperature correction tables for precise results.
Spirits proof: frequently asked questions
What is the relationship between proof and ABV?
In the United States, proof is exactly twice the ABV (alcohol by volume) percentage. A spirit at 40% ABV is 80 proof. A spirit at 50% ABV is 100 proof. This US system was standardized by the Bureau of Alcohol, Tobacco and Firearms.
How do I dilute a high-proof spirit to bottling strength?
Use the Pearson Square formula: V1 * C1 = V2 * C2, where V is volume and C is concentration (ABV). To find water to add: water = (spirit_volume * current_ABV / target_ABV) - spirit_volume.
What is chill filtration and how does it affect proof?
Chill filtration removes fatty acids and esters that can cloud the spirit when water is added during dilution. It does not change the ABV or proof. Many craft distillers skip chill filtration to preserve flavor compounds.
What is bottling proof for whiskey?
Legally, whiskey in the US must be bottled at a minimum of 40% ABV (80 proof). Straight whiskeys cannot exceed 62.5% ABV (125 proof) during aging. Barrel-proof or cask-strength whiskeys are bottled without dilution, often 55-65% ABV (110-130 proof).
How do I measure ABV at home?
A hydrometer (distiller's hydrometer or alcoholmeter) measures ABV directly by floating in the spirit. A refractometer can estimate ABV but is affected by sugar content. Temperature matters: hydrometers are calibrated for specific temperatures (usually 60 F or 68 F).
Official sources
- Alcohol and Tobacco Tax and Trade Bureau (TTB): Proofing Distilled Spirits.
- TTB: Beverage Alcohol Manual.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.