Alcohol Dilution Calculator

What to work out

Volume of spirit, ml

Starting strength, % ABV

Target strength, % ABV

Result

Water to add, ml1,446
Strength you land on, % ABV40%
Finished volume, ml2,400
Contraction, ml45.6
Pure ethanol, ml960
What mattersPour the spirit into the water rather than the other way round, and let it rest a day before checking.

Calculated by mass, not by volume

Works out how much water to add to reach a target strength, and gives the real volume you end up with once the mixture contracts.

Ethanol and water take up less room together than they do apart: a litre of each gives about 1.96 litres, not two. That contraction is what breaks the obvious arithmetic for 'how much water do I add'.

The correct route goes through mass. The mass of pure ethanol does not change, so the total mass of a mixture at the target strength can be worked out from it directly. That is the Fertman method, and it is why you need to pour in more water than the volume sum suggests.

What this calculator shows

The millilitres of water to add for the strength you want
The real finished volume and the size of the contraction
The reverse case: the strength you land on after adding a known amount of water

What to keep in mind

Densities are OIML values at 20 °C, the temperature strength is always quoted at.
A warm mixture reads stronger on a hydrometer than it is, so let it cool and settle before measuring.

FAQs

How much water dilutes a litre of 96% spirit to 40%?

About 1.45 litres. The finished volume is exactly 2.4 litres — less than 1 plus 1.45, because the mixture contracts.

What is the Fertman method?

Working the dilution out through the mass of spirit and water rather than their volumes. It gives the right answer because it accounts for the different densities and for the contraction.

Spirit into water or water into spirit?

Spirit into water. The other way round the mixture heats more, blends worse, and loses some of the lighter aromatics.

Why does my hydrometer disagree?

Most likely the liquid is warm. The scale assumes 20 °C, and every degree above that reads low. Leave it a day and measure at room temperature.

Worked example

A litre of 96% spirit down to forty

Input: 1000 ml at 96% → 40%

Output: Add 1446 ml of water for 2400 ml

Note: Volume arithmetic would have you add 1400 ml, straight to 2400. But the mixture shrinks by about 46 ml, and instead of forty percent you land near forty-one.

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Alcohol Dilution Calculator

Result

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