The Proofing Room

You bought a cask-strength bottle at 60% ABV, and you'd rather drink it at 45%. How much water do you add? There's a one-line formula for that, and it's almost right. Here's the formula, where it comes up short, and how to get closer, using a 750 mL bottle as the example. (This is about diluting a spirit you bought. It doesn't cover making spirits.) Step 1: the simple formula The alcohol in the bottle doesn't change when you add water; it gets spread through more liquid. So: Starting strength × starting volume = target strength × final volume For our bottle: 60% × 750 mL = 45% × final volume, so the final volume is 1,000 mL . That part is exact, as long as everything is measured at the same temperature (US proof is defined at 60°F, more on that below). The common shortcut then says: water to add = final volume − starting volume = 250 mL . That's the step that comes up a little short. Step 2: why it's short Alcohol and water shrink when you mix t...

Pick up a bottle of American whiskey and you'll often see two numbers: something like "45% alc./vol. (90 proof)." One is required. The other is optional, and it means something different depending on which country's history you ask. Here's what each number means, what the label rules say, and the conversions worth knowing. The short answer: in the US, proof is twice the ABV US law defines proof as the alcohol content at 60°F, "stated as twice the percent of ethyl alcohol by volume". So: % alcohol by volume US proof 40% 80 45% 90 50% 100 75.5% 151 95% 190 For US proof, that's the whole conversion: multiply ABV by two to get proof, or halve the proof to get ABV. What a US label has to say Alcohol by volume is mandatory. Federal rules require the alcohol content of distilled spirits to be stated as a percentage by volume. Proof is optional. A label can show it, as long as it's in the same field of vision as the ABV statement. TTB...