What this calculator does
Diluting a spirit is a proportion problem. The alcohol in the bottle does not change, only the volume it is spread through, so the water to add is the starting volume multiplied by the drop in ABV and divided by the target ABV.
Taking 750 ml of 40% spirit down to 20% means doubling the volume, so 750 ml of water. Down to 30% needs only 250 ml, because the proportional change is much smaller. The relationship is not linear in the way people expect, which is the main reason to calculate rather than estimate.
The formula
The water to add is the starting volume multiplied by the difference between the starting and target ABV, divided by the target ABV. The final volume is the sum of the two, and the check line recalculates the ABV from that final volume as confirmation. The calculation ignores volume contraction, which is discussed in the FAQ.
| Term | Meaning |
|---|---|
| ABV | Alcohol by volume, the percentage of the liquid that is ethanol. |
| Proof | A US measure equal to twice the ABV, so 80 proof is 40% ABV. |
| Volume contraction | The small reduction in total volume when ethanol and water mix, since the molecules pack more closely together than either does alone. |
The inputs explained
| Field | What to enter |
|---|---|
| Starting ABV (%) | The starting ABV of the spirit. |
| Target ABV (%) | The target ABV you want to reach. It must be lower than the starting figure, since adding water can only reduce strength. |
| Starting volume (ml) | The volume of spirit you are starting with, in millilitres. |
When to use it
Bottling a cask-strength spirit
Home distillers and anyone bottling from cask strength need to bring the spirit to a drinking strength, and the water addition is the whole of that step.
Batching cocktails
Pre-batched cocktails need dilution added deliberately, since they will not pick it up from stirring or shaking at the point of service.
Reducing a homemade infusion
Infusions made on high-strength spirit are often diluted afterwards, and calculating the water gets the final strength right first time.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much water reaches each target strength?
A 750 ml bottle at 40% diluted to three different target strengths.
| Target ABV | Water to add | Final volume | Check: final ABV |
|---|---|---|---|
| 20% | 750 ml | 1,500 ml | 20.00% |
| 30% | 250 ml | 1,000 ml | 30.00% |
| 40% | 0 ml | 750 ml | 40.00% |
Questions
How do I dilute spirits to a lower proof?
Multiply the volume by the drop in ABV and divide by the target ABV. To take 750 ml from 40% to 20%, that is 750 × 20 ÷ 20, which is 750 ml of water. Add the water to the spirit rather than the other way round, and let it rest before judging the flavour.
Does mixing alcohol and water lose volume?
Yes, slightly. Ethanol and water molecules pack more closely together than either does alone, so the mixture occupies a little less than the sum of the parts, with the effect strongest around 50% ABV. It is under 4% at worst and is ignored here, but it means a precisely measured dilution comes out fractionally stronger than calculated.
What water should I use?
Distilled or filtered, ideally. Tap water carries minerals and chlorine that can cloud a spirit or alter its flavour, and hard water in particular can produce a visible haze. The effect is more noticeable the more you dilute.
How do I convert between proof and ABV?
US proof is twice the ABV, so 80 proof is 40% ABV and 100 proof is 50%. UK proof used a different and more complicated definition historically and is no longer used. Enter ABV here rather than proof.
For calculating ABV from fermentation gravity readings, see the ABV from gravity calculator. For planning drinks for a party, see the party drink planner.