Alcohol Dilution Calculator

Dilution formula for reducing alcohol by volume (ABV).

The formula:

• Water to add = Volume x (Current ABV / Target ABV - 1)

Example:

• You have 750 ml of spirit at 60% ABV
• You want to dilute to 40% ABV
• Water needed: 750 x (60/40 - 1) = 750 x 0.5 = 375 ml
• Final volume: 1,125 ml at 40% ABV

Common dilution scenarios:

• Cask strength whiskey (55-65%) to drinking strength (40-46%)
• High-proof spirits (75%) to bottling proof (40%)
• Homebrew spirits to desired strength

Notes:

• Use distilled or filtered water for best results.
• Mix slowly and let it rest before measuring the final ABV.
• Temperature affects density, so measure at room temperature.
• There is slight volume contraction when mixing alcohol and water.

The units cancel out, which is worth knowing. The formula is a pure ratio, so whatever you feed it comes back in the same unit. Enter 750 and read the answer as millilitres; enter 5 and read it as gallons. The calculator labels its fields in millilitres because that is how spirits are sold, but the ratio it prints works at any scale, which is what you want when you are diluting a barrel rather than a bottle.

The formula works because diluting only adds water; the amount of pure alcohol in the bottle never changes. You’re spreading the same alcohol through more total liquid, which is why the math is a simple ratio of current strength to target strength. Distillers and home blenders use it to bring cask-strength spirits down to a drinkable proof without guesswork.

Two practical notes the numbers don’t show. Water and alcohol contract slightly when mixed, so the final volume comes out a touch less than the sum of the parts and the strength settles a hair higher than the formula predicts. The effect is real and easy to demonstrate: 52 ml of pure ethanol stirred into 48 ml of water gives about 96 ml, not 100. Diluting an already-mixed spirit is a much gentler case than that, because most of the contraction happened when the spirit was first made, so the shift here is fractions of a percent rather than several. It is still the reason commercial distillers work from proofing tables instead of this formula, and the reason to add most of the water, let it rest, then measure and finish by hand if the exact strength matters. And diluting some spirits below about 46% can turn them cloudy, a harmless effect called louching, as oils that were dissolved in stronger alcohol drop out of solution. It is the reason so many non-chill-filtered whiskies are bottled at exactly 46%. Distilled water added slowly gives the cleanest result, and letting the mix rest before a final measurement lets temperature and contraction settle.

Working in proof rather than percent? The alcohol proof converter turns US or UK proof into ABV first, and the numbers here all take ABV.