How to use it
- Enter the Strength of first liquid (ABV, %) and the Volume of first liquid (litres).
- Enter the Strength of second liquid (ABV, %) and the Volume of second liquid (litres). For water, type 0 as its strength; to leave the second liquid out, type 0 as its volume.
- Read the Strength of the mix (ABV, %) and the Total volume (litres).
- Type a Target strength (ABV, %, optional) to read the Water to add for the target (litres).
How the strength of a mix is worked out
Strength here is ABV, the alcohol content by volume: the volume of ethanol divided by the volume of the solution, both at 20 °C (Wikipedia, Alcohol by volume). Each liquid carries a share of alcohol equal to its strength times its volume, and the shares are added together.
Cm = (C1 × V1 + C2 × V2) ÷ (V1 + V2)
water to add = (V1 + V2) × (Cm ÷ Ct − 1)- C is the strength (ABV, %), V the volume, Cm the strength of the mix and Ct the target.
- Water has a strength of 0, so enter it as the second liquid with an ABV of 0.
- A target stronger than the mix cannot be reached by adding water, so the page shows an error.
Why the volumes are not exactly added
The calculator adds the two volumes as if the mix kept them. Wikipedia says this is not exact: mixing two solutions above 24 % by mass decreases the volume, and mixing pure water with a solution below 24 % by mass increases it slightly. Its example of 50 mL of ethanol and 50 mL of water says the result measures less than 100 mL. If the total volume matters, measure the finished mix.
What this calculator does not cover
- Sugar, syrup, juice or any other ingredient, which adds volume and taste. The page counts only alcohol and water.
- Strengths measured at a temperature other than 20 °C. ABV is defined at 20 °C.
- Legal strength or tax. The strength printed on the label, or the official figure, is the one that counts.
Worked example: 0.7 litres at 40 % with 0.3 litres of water
The alcohol in the first liquid is 40 % × 0.7 L = 0.28 L. Water carries none, so the mix holds 0.28 L of alcohol in 1.0 L: a strength of 28 %.
For 20 %, the total must be 0.28 ÷ 0.20 = 1.4 L, so add 1.4 − 1.0 = 0.4 L of water. Check: 0.28 ÷ 1.4 = 20 %.
| Result | Value |
|---|---|
| Strength of the mix (ABV, %) | 28.0 |
| Total volume (litres) | 1.000 |
| Water to add for 20 % (litres) | 0.400 |
Frequently asked questions
What does ABV mean?
ABV, alcohol by volume, is the volume of ethanol divided by the volume of the solution, both measured at 20 °C. A liquid at 40 % ABV holds 40 ml of ethanol in every 100 ml.
How much water do I add to weaken a spirit?
Multiply the total volume by the current strength divided by the target, then subtract 1: water = V × (Cm ÷ Ct − 1). One litre at 40 % brought down to 20 % needs one litre of water.
Can I mix two spirits of different strength?
Yes. Multiply each strength by its volume, add the two results and divide by the total volume. 0.5 litres at 40 % and 0.5 litres at 20 % make a 30 % mix.
Is the final volume exactly the two volumes added?
No. Wikipedia says that mixing solutions above 24 % by mass shrinks the total, and that pure water added to a weaker solution makes it slightly larger. The calculator adds the volumes, so the real total can differ.
Does this work for wine or beer?
It works for any liquid whose strength you know, but only alcohol and water are counted. Sugar and other solids are not, so use the strength printed on the label.
Sources
Figures and text last checked on Tuesday, November 10, 2026.