How to use it
- Choose the mode: start from a known volume of spirit, or from the final volume you want.
- Enter the volume and its unit (the unit is only a label – ratios are the same in ml, litres or fluid ounces).
- Enter the spirit’s strength and the target strength in % ABV (divide US proof by 2).
- If you are blending with something other than water, enter the diluent’s strength; otherwise leave it at 0.
- Read the amount of water, the nominal final volume, the Pearson-square parts and the contraction estimate; copy the summary for your notes.
Example
1,000 ml of new-make spirit at 60% ABV, target 40%: water = 1,000 × (60 ÷ 40 − 1) = 500 ml, nominal total 1,500 ml containing 600 ml of pure alcohol. Because the mix contracts by about 3% at this strength, expect roughly 1,454 ml at about 41.3% – so hold back a little water, let it rest, measure, and top up. In reverse mode, asking for 1,500 ml at 40% from the same 60% spirit returns 1,000 ml spirit + 500 ml water.
Formulas and the contraction estimate
C₁ · V₁ = C₂ · V₂
water to add = V₁ · (C₁ − C₂) / (C₂ − D) D = diluent strength (0 for water)
spirit for V₂ = V₂ · (C₂ − D) / (C₁ − D); diluent = V₂ − spirit
Pearson square : parts spirit = C₂ − D, parts diluent = C₁ − C₂
contraction (approx.) ≈ 0.128 · x · (1 − x), x = target ABV / 100 (≈ 3.2 % at 50 %)The volumes are nominal, i.e. what you measure out before mixing. Real ethanol–water mixtures occupy less space than the sum of their parts: 52 ml of ethanol and 48 ml of water give about 96.8 ml, not 100 ml. The parabola above approximates that effect for a first estimate; the exact figures depend on temperature and are tabulated in the OIML and TTB alcoholometric tables. Strengths here are by volume at 20 °C. For legal labelling or duty purposes use a calibrated hydrometer or density meter and the official tables, not this calculator.
Sources: OIML R 22 – International alcoholometric tables, US TTB – Gauging manual and dilution tables (27 CFR Part 30).
Home distillation is illegal in many countries. This tool is for diluting legally obtained spirits, essences and laboratory solutions.
Questions
- Why is my diluted spirit stronger than the target?
- Volume contraction: the mix takes up less space than spirit plus water, so the alcohol share rises by about 1–1.5 percentage points near 40%. Add the water in stages, rest the mix, measure with a hydrometer and adjust.
- Can I dilute by weight instead?
- Yes, and it avoids the contraction problem, but you need density tables: 40% ABV spirit weighs 0.948 kg per litre at 20 °C, water 0.998 kg. This calculator works by volume only.
- What water should I use?
- Distilled, deionised or reverse-osmosis water. Tap water adds minerals and chlorine that cloud and flavour the spirit; very hard water can produce a haze.