Proof Conversion Calculator — US, UK, ABV & Gay-Lussac
Different countries have used different systems to measure alcohol strength. The United States uses a simple doubling of ABV; the United Kingdom historically used the Sikes system based on specific gravity; and France introduced the Gay-Lussac scale, which is now effectively identical to ABV. This calculator converts between all four systems instantly.
Convert Between Proof Systems
US proof = ABV × 2. UK Sikes proof = ABV × 1.75 (approximately; the exact Sikes scale uses a specific gravity table). Gay-Lussac is identical to ABV. The UK abandoned the Sikes system in 1980 in favor of ABV. These conversions are based on the standard definitions used by the US TTB, UK HMRC, and OIV.
History of Proof Systems
US Proof (1848–present)
The US proof system was codified in 1848 and is defined by 27 CFR §5.37: proof equals ABV multiplied by two at 60°F. An 80-proof bourbon is 40% ABV. The system applies to all distilled spirits sold in the United States.
UK Sikes Proof (1816–1980)
The Sikes system, named after Bartholomew Sikes, measured proof based on specific gravity using a calibrated hydrometer. “100° proof” in the Sikes system corresponded to approximately 57.15% ABV — the concentration at which spirit-soaked gunpowder would still ignite. The UK abandoned this system on January 1, 1980, adopting ABV as mandated by European Community directives.
Gay-Lussac (°GL)
Joseph Louis Gay-Lussac developed the modern volumetric measurement of alcohol content in 1824. His system measures the percentage of alcohol by volume at 15°C, which for practical purposes is now identical to ABV at 20°C (the slight temperature difference causes negligible variation in most applications).
| Description | ABV | US Proof | UK Proof | Gay-Lussac |
|---|---|---|---|---|
| Minimum bottling strength (US spirits) | 20% | 40 | 35.0° | 20° |
| Standard spirits bottling | 40% | 80 | 70.0° | 40° |
| Navy-strength gin | 57% | 114 | 99.75° | 57° |
| UK “proof spirit” | 57.15% | 114.3 | 100° | 57.15° |
| Cask strength whisky (typical) | 60% | 120 | 105.0° | 60° |
| Overproof rum | 75.5% | 151 | 132.1° | 75.5° |
| Neutral grain spirit (US max) | 95% | 190 | 166.25° | 95° |
Frequently Asked Questions
Why was UK proof different from US proof?
The UK system predated the US system and was based on the physical property of gunpowder ignition rather than a simple mathematical ratio. When 100° UK proof was defined as the spirit concentration that could ignite gunpowder (approximately 57.15% ABV), the resulting scale diverged from the US convention of 100 proof = 50% ABV.
Does anyone still use UK Sikes proof?
Officially, no. Since 1980, the UK and EU have used ABV exclusively for labeling. However, Sikes proof appears in historical tasting notes, old bottlings, and some auction catalogs. Collectors and historians occasionally need to convert vintage labels.