Proof, ABV and What "Barrel Proof" Actually Means
Three numbers on a label, one idea underneath them.
By TMOWSA · 2026-09-20 · Reviewed 2026-09-20
ABV
Alcohol by volume is the share of the liquid that is ethanol. A bottle at 40% ABV is two-fifths alcohol by volume.
Proof
In the United States, proof is simply double the ABV. 40% ABV is 80 proof. 50% ABV is 100 proof. The word is a leftover from an old British test in which spirit was poured on gunpowder: if the powder still lit, the spirit was above a certain strength and had been "proved."
By federal standard, whisky, gin, vodka, rum, brandy and tequila must be bottled at no less than 40% ABV (80 proof).
Cask strength and barrel proof
Whiskey leaves the barrel stronger than it is usually sold. Most producers cut it with water to hit a consistent bottling strength. A bottle labelled cask strength or barrel proof has not been cut, or has barely been cut, so it arrives at whatever strength the barrel gave up, commonly somewhere in the 50s or 60s in ABV terms.
That is why barrel proof bottles vary batch to batch, and why many people add a few drops of water in the glass. You are doing at the table what the producer chose not to do at the plant.
Three fingers
Not a measurement, a gesture. It means pour until the liquid is about as deep as three fingers laid flat against the glass. Useful in a bar, useless on a spec sheet.
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