The US rule: double the ABV
| Label proof (US) | ABV | Typical drink |
|---|---|---|
| 40 proof | 20% | Liqueurs, fortified aperitifs |
| 80 proof | 40% | Standard vodka, whisky, gin, rum |
| 100 proof | 50% | Bottled-in-bond bourbon, navy-strength gin |
| 151 proof | 75.5% | Overproof rum |
| 190 proof | 95% | Neutral grain spirit |
The rule is exact, not approximate: US proof = 2 × ABV, with no rounding quirks. That is why American bottles show both numbers or neither with confusion — halve one and you have the other. The one exception to watch is "bottled in bond" whisky, which must be exactly 100 proof (50% ABV) by law, a rule that survives from 1897 as a guarantee of authenticity.
The British gunpowder test
Before hydrometers, there was no easy way to measure a spirit's strength, and tax depended on it. The British solution was practical: pour spirit on gunpowder and try to light it. If the powder flashed, the spirit was "above proof"; if the water in a weak spirit soaked the powder, it would not ignite, and the spirit was "under proof". The reference point — 100° proof, the weakest spirit that still let powder burn — works out to about 57.15% ABV, which is why old navy rum at "100 proof" was far stronger than a modern American 100-proof bottle. Britain abandoned proof for ABV labelling in 1980, but the gunpowder story is where the word comes from: a spirit had to prove itself.
Reading a label today
Outside the US, bottles show ABV only — 40% vol on a European whisky is the same strength as an American 80-proof bottle. Inside the US, proof may appear alone or beside ABV. If you see only one number, convert it (halve proof, double ABV) and compare like with like — especially with overproof spirits, where 151 proof sounds like a marketing claim but is a precise 75.5% alcohol, flammable enough that bars treat it with respect and some airlines will not carry it.
For the volume conversions behind cocktail measures, see how many ounces are in a cup, and for the metric side of drinks measures, the ml in a cup guide.