Diluting a spirit down to a target proof or ABV is simple arithmetic once you know the formula, but it's easy to get backwards or to forget that water changes the total volume, not just the strength. This calculator handles the math so you can focus on getting the pour right.
How the calculator works
The calculator applies the standard dilution equation C1V1 = C2V2, where C is concentration (proof or ABV) and V is volume. Solving for the final volume gives finalVolume = startVolume × startProof / targetProof, and the water to add is just that final volume minus your starting volume. Because proof is always exactly double ABV in the US system, the ratio works identically whichever unit you enter — the calculator converts internally so you can mix and match without doing that conversion yourself.
Inputs and what they mean
Starting volume is the amount of spirit you have on hand, in fluid ounces or milliliters. Starting proof/ABV is the strength printed on the bottle — for anything you didn't distill yourself, this is a known, labeled value. Target proof/ABV is whatever strength you're aiming for; the Common Targets tab lists typical values like 80 proof (40% ABV) for standard spirits or 114 proof (57% ABV) for navy-strength. The target must always be lower than the start — this tool only dilutes down, it doesn't concentrate a spirit back up.
Limits and edge cases
This calculator assumes water and ethanol volumes add linearly, which is accurate enough for kitchen and bar use but not perfectly precise — mixing ethanol and water actually causes a small volume contraction (real mixed volume is slightly less than the sum of the parts), which is why commercial distillers use hydrometers and dilution tables rather than simple arithmetic for exact bottling proof. Temperature also affects hydrometer proof readings, since alcohol and water have different thermal expansion rates — most hydrometers are calibrated for a reference temperature (commonly 60°F/15.6°C) and need a correction if you're measuring at a different temperature. Finally, note that the legality of distilling spirits at home varies significantly by country and US state; this calculator is a math tool only and doesn't address legal questions.