What Does "Proof" Mean?

"Proof" is an older way of expressing a spirit's strength, and it means different things in the US versus the historical UK system — New Zealand labels use ABV instead.

"Proof" predates the modern ABV system, and confusingly, it doesn't mean the same thing everywhere it's used.

US proof

In the United States, proof is simply double the ABV. A spirit bottled at 40% ABV is "80 proof." A cask-strength whisky at 60% ABV would be "120 proof." This is a straightforward conversion: proof = ABV × 2.

Historical UK proof

The old British system was more complicated and based on the density of the spirit relative to water at a specific temperature — "100 proof" in the old UK system was actually about 57.1% ABV, not 50%. The UK officially replaced this system with ABV labelling in 1980.

Where the word comes from

The term dates back to a rough test where gunpowder was soaked in spirit and set alight — if it still burned, the spirit was "proved" to contain enough alcohol. Spirit that failed to ignite was considered under-proof.

What New Zealand uses today

New Zealand labelling uses ABV, not proof, so you generally won't see "proof" on a locally produced bottle. You'll mostly encounter the term when reading about American whiskey (bourbon, rye) or older/imported spirits.