Spirit Dilution Calculator (Proofing Down)

How it works

Alcohol is conserved: V₁ × ABV₁ = V₂ × ABV₂ (ABV at 20 °C). With contraction: mass is conserved, so water = (V₂ × ρ₂ − V₁ × ρ₁) ÷ ρ_water, with each density ρ read from ethanol-water tables for that strength. Simple ratio: water ≈ V₁ × (ABV₁ − ABV₂) ÷ ABV₂. US proof = 2 × ABV; UK proof ≈ 1.75 × ABV

Cutting a spirit to bottling strength is a conservation problem. The alcohol stays the same and only water is added, so the final volume is the alcohol divided by the target strength. The twist is that ethanol and water pack together more tightly than they do apart: a litre of each makes about 1.93 litres of mixture. The simple ratio ignores this and under-states the water by one to three percent near bottling strength. With contraction switched on, the calculator works the way distillers and official alcoholometric tables (OIML, TTB) do. It tracks mass, which is always conserved, using the density of ethanol-water mixtures at 20 °C for the strength before and after, then turns the mass difference back into a volume of water. The same tables correct an alcoholmeter read at the wrong temperature: a spirit reading 40% at 25 °C is really about 38% at 20 °C. You can also solve the other way round, for the strength after adding a set amount of water or for the spirit needed to make a finished batch. Under More options you can cut with a weaker spirit instead of water and split the cut into stages. Add water gradually, because mixing releases heat and can bring out haze-forming oils, and let the spirit rest before the final reading.

Worked example

2 L of new-make spirit at 62% cut to 43%: the 1.24 L of pure alcohol makes 1.24 ÷ 0.43 = 2.884 L at 43%. By mass, 2 L × 0.9046 kg/L = 1.809 kg goes to 2.884 L × 0.9431 kg/L = 2.720 kg, so you add 0.910 kg, which is 0.912 L of water. The simple ratio says 0.884 L; the 28 mL difference is exactly the volume that disappears on mixing. That is about four 700 mL bottles. If the alcoholmeter read 62% with the spirit at 25 °C, the true strength is about 60.3%, and the water drops to about 0.83 L.

Sources: OIML R 22 International alcoholometric tables · TTB Gauging Manual (27 CFR Part 30) · Ethanol–water mixture densities (Wikipedia data page, after Lange’s Handbook)

Frequently asked questions