Diluting a spirit to bottling strength is the one piece of distilling maths people get wrong most often — because nearly every dilution calculator online assumes volumes simply add up. They don't. Broover's free dilution calculator corrects for ethanol-water contraction using the official OIML R22 density tables, so the strength you measure in the bottle is the strength you asked for.
Mix one litre of water into one litre of high-proof spirit and you do not get two litres. Ethanol and water hydrogen-bond and pack more tightly together, so the mixture contracts by a measurable amount. A calculator that assumes additive volumes therefore tells you to add too much water, and you end up below your target ABV — sometimes by a full percentage point or more at cask strength.
The fix is to work from density, not volume. The OIML R22-1975 international tables give the true density of every ethanol-water mixture at 20 °C. Broover's calculator looks up the real densities at your starting and target strengths and returns the exact water addition, contraction already accounted for.
Enter your starting volume and its measured ABV, choose the target ABV you want in the bottle, and the calculator returns the precise volume of water to add. Work in litres and millilitres or US gallons and fluid ounces — the unit toggle handles both. It is instant, runs entirely in your browser, and needs no signup.
A worked example: 5 L of 65% ABV spirit diluted to 40% ABV. The naive additive answer overshoots the water; the contraction-correct answer adds slightly less, landing you on 40.0% in the glass instead of 39.x%. At scale, across a whole run, that difference is the gap between hitting your label strength and reprinting labels.
Dilution is one tool in Broover's free calculator set — alongside ABV from specific gravity (Hall formula), pure-alcohol (LPA) totals, sugar bills, and strip/spirit-run recovery planning. And if you want the numbers to flow into a real workflow, Broover is the distilling app that ties wash, run, lot, and bottled-spirit together with provenance end to end.
Open the free distilling calculators to dilute now — no signup, runs in your browser. Or see how Broover works for the full distilling workflow.