Diluting spirit to drinking strength
The arithmetic of dilution, and why the real answer never quite matches the formula.
Freshly distilled fruit brandy usually comes off the still somewhere between 60 and 75 %ABV. That is undrinkably strong, so water brings it down to its final strength — typically between 40 and 52 %ABV.
The calculation
Adding water changes the volume but not the amount of alcohol. That is the one quantity that stays constant, and the whole calculation rests on it:
volume × abv = final volume × target abv
Rearranged for the number you actually want:
final volume = volume × abv / target abv
The water to add is simply the difference between the two.
For example, taking 10 litres at 70 %ABV down to 50 %ABV:
- Pure alcohol: 10 × 0.70 = 7 litres
- Final volume: 10 × 70 / 50 = 14 litres
- Water to add: 14 − 10 = 4 litres
Why it never comes out exactly right
The formula assumes volumes simply add up. They do not. Mixed together, ethanol and water molecules pack more tightly than they do apart, so the volume of the mixture is less than the sum of its parts.
This is volume contraction. It peaks around 50 %ABV and can reach 2–3%. So our 14-litre example will in practice give something closer to 13.6–13.8 litres — and a strength a fraction above the target.
How to do it in practice
Because of that, work in two steps:
- Calculate the water needed, then add 90% of it.
- Stir thoroughly and let the temperature settle.
- Measure with an alcoholmeter and correct the reading to 20 °C.
- Add the remainder a little at a time until you reach the target.
Temperature matters here: mixing releases heat, and a reading taken on warm spirit will mislead you. That is what the temperature correction guide covers.
Which water to use
Use soft water with a low mineral content. The calcium and magnesium salts in tap water can cause haze, and chlorine is detectable on the nose. Distilled water, or bottled water of low hardness, is the safe choice.
If the spirit turns cloudy after dilution, that is usually not a fault — it is essential oils coming out of solution, and it generally clears after a few weeks of resting.