Enter your spirit's starting ABV and target strength — the calculator tells you exactly how much water to add. Results update instantly.
The formula is based on conservation of pure alcohol: the amount of ethanol in the spirit before dilution equals the amount after. Adding water increases total volume while the absolute ethanol content stays constant, which lowers the ABV percentage.
For example: you have 700ml of a spirit at 55% ABV and want to bring it to 40%. The calculation: 700 × (55 − 40) / 40 = 700 × 15 / 40 = 262.5ml of water to add, giving a final volume of 962.5ml at exactly 40% ABV.
For spirits above 50% ABV, add water gradually rather than all at once. Rapid dilution of high-proof spirits can temporarily cloud the liquid (a phenomenon called "louching") as compounds that were dissolved in alcohol come out of solution. Slow addition and gentle stirring produces a cleaner result.
The formula above assumes room temperature. At very high proof levels (above 70% ABV), ethanol and water contract slightly when mixed — meaning the final volume is marginally less than the arithmetic sum. For home use this difference is negligible (under 1%). For production-level precision, use a calibrated alcoholmeter after mixing.
For cocktail preparation or serving, any neutral water works. For whisky or spirits where the dilution is part of the drinking experience, use filtered or soft water — mineral-heavy water can introduce off-flavors, particularly with delicate spirits like Scotch or Japanese whisky.
Note on contraction: When ethanol and water mix, the total volume contracts slightly due to molecular interactions — the final volume is marginally less than the sum of the two volumes. This calculator uses the standard C₁V₁ formula which does not account for contraction. For most home bar applications the variance is under 1% and can be disregarded.