Proof to ABV Converter

On the US scale, divide the proof by 2, because US proof is defined as exactly twice the alcohol by volume. So 80 proof is 40% ABV and 100 proof is 50% ABV. On the UK degrees proof scale, multiply by 4 and divide by 7 instead.

For US proof, multiply the ABV percentage by 2, so 40% ABV becomes 80 proof. For UK degrees proof, multiply the ABV by 7 and divide by 4 (the same as multiplying by 1.75), so 40% ABV becomes 70 UK proof. Switch the direction selector to ABV to proof and pick your scale.

US proof is simply twice the ABV (80 proof equals 40% ABV). UK degrees proof, used from 1816 to 1980 with the Sikes hydrometer, set 100 proof at the strength of spirit that weighed 12 thirteenths of an equal volume of water, about 57.14% ABV under the clean 4/7 convention. That means a UK proof number is always lower than the US proof number for the same drink.

In the United States, 80 proof is the most common bottling strength for whiskey, vodka, gin, and rum, which works out to 40% ABV. The proof figure is just the ABV doubled, a labeling convention kept from the historical US system. Many countries outside the US print only the ABV percentage and skip proof entirely.

ABV is a volume fraction, so it cannot exceed 100%. The matching ceiling is 200 US proof or 175 UK proof, both of which correspond to pure ethanol at 100% ABV. This converter rejects any proof or ABV value outside that range so you do not get a meaningless result.

It uses the clean teaching convention where 100 UK degrees proof equals 4/7 of the proof value in ABV, about 57.14%, so conversions round-trip exactly. The original 1816 legal definition fixed 100 UK proof at 57.06% ABV, a difference of roughly 0.15 percentage points that is noted alongside the result for reference.