Alcohol Dilution Calculator
Use this alcohol dilution calculator to proof down spirits, moonshine or cask-strength whiskey to a target ABV. Enter the starting ABV, your current volume, and the ABV you want — it tells you exactly how much water to add.
Enter the starting ABV, your current volume, and the ABV you want to get the water to add.
How to dilute alcohol to a target ABV
Adding water does not change the amount of pure alcohol, only the total volume, so the dilution equation holds with ABV as the concentration:
C₁ is the starting ABV, V₁ is your current volume, and C₂ is the ABV you want. The final volume is V₂ = C₁ × V₁ ÷ C₂, so the water to add is:
Worked example
You have 750 mL of 65% ABV spirit and want 40%: water = 750 × (65 ÷ 40 − 1) = 468.75 mL, giving about 1218.75 mL of 40% ABV. Add the water slowly and confirm with a hydrometer.
Related tools
For any solution rather than spirits, use the home dilution calculator. To work in percentages directly, see the percent solution calculator.
Frequently asked questions
›How much water do I add to dilute spirits to 40%?
Water to add = V₁ × (C₁ ÷ C₂ − 1). To bring 1000 mL of 60% spirit down to 40%: 1000 × (60 ÷ 40 − 1) = 500 mL of water, giving 1500 mL of 40% ABV.
›What is the alcohol dilution formula?
Alcohol content is conserved when you add water, so C₁V₁ = C₂V₂. The final volume is V₂ = C₁ × V₁ ÷ C₂, and the water to add is V₂ − V₁, where C₁/C₂ are the starting and target ABV and V₁ is your current volume.
›Does adding water change the volume exactly?
This calculator assumes ideal mixing (volumes add up). In reality, mixing ethanol and water causes a small volume contraction, so the true final volume can be slightly less than calculated. For proofing spirits the difference is minor, but for precise lab work account for it.
›Can I use this to proof down moonshine or cask-strength whiskey?
Yes. Enter the starting ABV, your current volume, and the ABV you want. It returns the water to add. Add the water gradually and re-check with a hydrometer, since alcohol meters read by volume and temperature matters.
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