Proofing and dilution calculator

How to use it

1. Choose a job. Dilute brings a volume of spirit down to a target strength. Fill bottles starts from the bottles you want and works back to the spirit and water. Blend two spirits gives the strength and volume of a mix.
2. Enter your spirit. Its volume and strength, in % ABV or US proof. Strength is defined at 20 °C (68 °F): if you measured it warmer or cooler, correct the reading first with the alcoholmeter calculator.
3. Set the target. Most spirits are bottled at 40% ABV (80 proof); gins run from 37.5% to about 47%, overproof spirits higher, liqueurs lower. Tap the marker on the slider to jump to 40%.
4. Weigh the water. The calculator gives the water by volume and by weight. Weighing on kitchen scales is the most accurate way to measure it - a litre of water weighs almost exactly a kilogram.
5. Add it in stages. Add about nine-tenths of the water, stir, let the spirit rest for a few hours, then measure and add the rest a little at a time. Spirit warms slightly as it is diluted, so measure once it has cooled.

How it works

Proofing - or proofing down, cutting or reducing - is simple in principle: the alcohol stays the same and only the water changes. Because strength is the share of the volume that is pure alcohol, the final volume is exact: 2 L at 75% holds 1.5 L of alcohol, which makes 3.75 L at 40%.

The water is another matter. Alcohol and water pack together more tightly than either does alone, so a mixture takes up less room than its two parts did. The familiar formula - final volume minus starting volume - assumes they don't, and so always asks for too little water. This calculator works by weight instead, because weights simply add up. It uses the international (OIML) formula for the density of alcohol and water to turn each volume into a weight, works out the water that brings the mixture to the strength you want, and turns the answer back into litres or gallons.

In the United States “proof” is twice the strength by volume, measured at 60 °F: 80 proof is 40% ABV. Choose proof on any strength dial and the calculator does the conversion.

A worked example

You have 2 litres of spirit at 75% and want it at 40%. The simple formula says 3.75 − 2 = 1.75 L of water. The calculator asks for 1.81 L (1.81 kg): the extra 63 mL makes up for the shrinkage. You end with 3.75 L - five 750 mL bottles.

The difference grows with stronger spirit. Bringing 5 L of 92% reflux spirit down to 40% takes 6.80 L of water; with the simple formula's 6.5 L the spirit would finish at 41.1%, not 40%.

Working backwards, six 750 mL bottles at 40% need 2.40 L of 75% spirit and 2.18 L of water.

Questions

What does proofing a spirit mean?

Proofing, or proofing down, means adding water to bring a spirit from the strength it came off the still to the strength you will drink or bottle it at. The name comes from proof, the old way of stating strength.

Why does the calculator ask for more water than the simple formula?

Because alcohol and water shrink when they are mixed: the mixture takes up a little less space than the two liquids did separately. The simple formula ignores that, so it falls short - by around 3–5% of the water for typical spirits.

What water should I use?

Water with as few minerals as possible: distilled, deionised or reverse-osmosis water. Minerals in hard tap water can leave a haze or a deposit, and chlorine can be tasted.

Why did my spirit turn cloudy when I added water?

Oils that dissolve in strong alcohol come out of solution as it is diluted - usually heavier compounds from the tails, or essential oils in a gin or an anise spirit. Making the cuts a little earlier helps; so does diluting slowly and letting the spirit rest, or filtering it.

Is US proof the same as UK proof?

No. US proof is simply twice the ABV. The old British proof scale put 100° proof at 57.1% ABV; it is no longer used on labels, which give % vol instead.