Proofing down: how much water to add to spirit
Bringing spirit down to drinking or bottling strength takes a simple sum, and a little more water than it gives.
Updated
In short
Water to add = volume × (strength now ÷ target strength − 1). For 2 L at 65% taken to 40%, that is 2 × (65 ÷ 40 − 1) = 1.25 L of water, for 3.25 L in all. Add most of it, then the rest slowly while you measure, because alcohol and water shrink a little when they mix.
The sum
The alcohol in the spirit stays the same when you add water; only the volume around it grows. So the final volume is the volume now × the strength now ÷ the target strength, and the water is the difference.
final volume = volume × strength now ÷ target
water = final volume − volume
Use the true strength, corrected for temperature. Warm spirit reads high on an alcoholmeter, and a strength a degree or two out shifts the water noticeably. The alcoholmeter correction on the home page does that.
Water for each litre of spirit
| Strength now | Water to 45% | Water to 40% |
|---|---|---|
| 60% | 333 mL | 500 mL |
| 65% | 444 mL | 625 mL |
| 70% | 556 mL | 750 mL |
| 80% | 778 mL | 1,000 mL |
| 90% | 1,000 mL | 1,250 mL |
Multiply by the litres you have: 3 L at 70% to 40% needs 3 × 750 mL = 2.25 L of water.
Why it takes a little more water
Alcohol and water shrink when mixed: a litre of each makes a little less than two litres. The sum gets the final volume right, but reaching it takes roughly 1 to 2% of that volume in extra water, and adding exactly the water it gives leaves the spirit a few tenths of a percent over target. So add about nine tenths of the water, let it settle and come to temperature, measure, and add the rest gradually until it reads right.
Tips for a clean result
- Use distilled, deionised or reverse-osmosis water. The minerals in tap water can cloud spirit or leave a sediment.
- Add the water to the spirit a little at a time, stirring. If it turns cloudy, oils from the tails have come out of solution; tighter cuts next time prevent it.
- Rest it for a few days, then check the strength again before bottling; it can drift as it settles.
- If you are licensed and selling, check the legal minimum for the drink: in the UK and EU, whisky must be at least 40%, and vodka, gin and rum at least 37.5%.