Collecting and Hobby Calculators
Whisky dilution.
Cask strength whisky is often better with water. Enter how much spirit you have, its ABV and the ABV you want, and the calculator gives the water to add. Or enter the water you plan to add and see the resulting ABV and proof. Runs in your browser.
Spirit and target
Water to add
Fill the form and press Calculate.
Answer
To bring 700 mL of 58.6% ABV whisky down to 46%, add about 192 mL of water. The rule is water = volume x (current ABV / target ABV - 1).
Worked example
700 mL at 58.6% ABV holds 700 x 0.586 = 410.2 mL of ethanol. For 46%, the total volume must be 410.2 / 0.46 = 891.7 mL, so add 891.7 - 700 = 191.7 mL of water. That is 27.4 mL of water per 100 mL of whisky. Proof goes from 117.2 to 92.0 (US proof is twice ABV).
How it works
Adding water does not change the amount of ethanol, so ABV after = ABV x volume / (volume + water), and solving for water gives water = volume x (ABV / target - 1). ABV is the share of ethanol by volume at 20 C, and US proof is twice ABV.
Volume contraction. Ethanol and water take up slightly less room together than apart; 50 parts ethanol and 50 parts water make roughly 96 to 97 parts. Because ABV is measured on the mixed volume, the result above is an approximation: the real ABV runs a little above your target, and hitting it exactly takes slightly more water. The finished volume is also a little less than volume plus water. For a precise batch, measure the final ABV with a hydrometer.
Tip. Add water in stages and taste. Most people sip over a range of strengths rather than one exact number.
Sources and basis (checked 2026-10-05): Wikipedia, Alcohol proof (ABV definition, US proof = 2 x ABV, volume contraction).
FAQ
How much water do I add to cask strength whisky?
Use water = volume x (ABV / target - 1). For 700 mL at 58.6% down to 46% that is about 192 mL.
How do I dilute 100 proof whisky to 80 proof?
Proof is twice ABV, so that is 50% to 40%. Add 25 mL of water for every 100 mL of whisky.
Does adding water change the amount of alcohol?
No. The ethanol stays the same; its share of the total volume goes down.
Why is my final volume slightly less than expected?
Ethanol and water contract when mixed, so the combined volume is a little under the sum. The calculated ABV is therefore approximate: the real ABV will run a little above the figure shown, so confirm with a hydrometer.
Can I use it for other spirits?
Yes. Any spirit with a known ABV, such as rum, gin or vodka.
Disclaimer
Informational arithmetic only. Hobby dilution of bottled spirits, not distilling advice. Drink responsibly and follow local laws.