Dilution Math, Step by Step: Bringing a Cask-Strength Bottle Down to Your Proof
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...