Spirit Dilution Calculator: Find Final ABV

A spirit dilution calculator estimates the final alcohol by volume (ABV) after you add water to a spirit. Enter the starting volume, starting ABV, and target ABV; it returns the water to add and the resulting total volume, assuming volumes are additive.

Formula

The calculation uses conservation of the amount of pure alcohol. Before dilution, pure alcohol volume is starting volume multiplied by starting ABV as a fraction. At the target strength, that same alcohol occupies the target fraction of the final volume.

Water to add = V × (A ÷ T − 1)
Final volume = V + water to add

• V is the starting spirit volume, in millilitres (ml) or another consistent volume unit.
• A is the starting ABV, entered as a percentage number, such as 40 for 40% ABV.
• T is the target ABV, entered in the same percentage format.

The percentage values cancel as a ratio, so they do not need to be converted to decimals in this expression. The calculation assumes the target is positive and no greater than the starting ABV. A target equal to the starting ABV means no water is added. A target above it would imply a negative water addition, which is not dilution.

Worked Example

Using the calculator’s defaults: 750 ml of spirit at 40% ABV, with a target of 40% ABV.

1. Water to add = 750 × (40 ÷ 40 − 1).
2. 40 ÷ 40 = 1, so water to add = 750 × (1 − 1) = 0 ml.
3. Final volume = 750 + 0 = 750 ml.

The calculator therefore returns 0.0 ml of water and a 750.0 ml final volume. This default illustrates the unchanged-strength case; enter a lower target ABV to calculate a positive water addition.

How to Use a Spirit Dilution Calculator

Measure the spirit volume and identify its ABV from a reliable label or measurement. Enter those values and choose the desired final ABV. The first result is the estimated water volume to add; the second is the calculated final volume. Keep all volumes in the same unit. If you enter litres instead of millilitres, the answer will also be in litres.

For example, if you have 750 ml at 40% ABV and want 20% ABV, the formula gives 750 × (40 ÷ 20 − 1) = 750 ml water. The estimated total is 1,500 ml. The alcohol amount represented by the starting spirit is 300 ml; 300 ml is 20% of 1,500 ml.

In US customary units, the same arithmetic works with fluid ounces or gallons: use the same volume unit for the spirit, water result, and final volume. A US fluid ounce is not the same size as an imperial fluid ounce, so do not mix them. Metric users can use millilitres or litres consistently; 1 litre equals 1,000 millilitres.

What ABV Means and What the Result Assumes

ABV means alcohol by volume: the percentage of a beverage’s volume attributed to ethanol under specified measurement conditions. A 40% ABV spirit is nominally 40 parts alcohol by volume per 100 parts beverage. The dilution calculation treats that proportion as exact and assumes no alcohol is lost.

Water and ethanol mixtures can contract slightly when mixed: the final physical volume may be less than the sum of the separate liquid volumes. Temperature also affects measured volume and ABV. Therefore, the calculator is a useful planning estimate, not a guarantee that a measured finished batch will land exactly on the target. For precise production, use appropriate calibrated equipment and the relevant alcohol-strength measurement method, accounting for temperature as required.

Another common misconception is that adding a volume of water equal to the volume of spirit always halves its strength. That is true under this simple model only when the target is half the starting ABV. Starting strength, target strength, and starting volume all matter.

Common Mistakes and Limits

• Entering 0.40 instead of 40: The calculator expects percentage numbers. Enter 40 for 40% ABV.
• Choosing an impossible target: A target above the starting ABV cannot be reached by adding water. A target of zero also makes the formula undefined.
• Mixing units: Do not enter the starting volume in millilitres and interpret the result as fluid ounces. Convert first or use one unit throughout.
• Rounding too early: Keep the full calculated result until measuring. The displayed water result is rounded to one decimal place, and household measuring tools may not support that precision.
• Treating the estimate as a verified measurement: Volume contraction, temperature, and uncertainty in the starting ABV can shift the actual result.

The formula calculates dilution by adding water only. It does not account for adding another alcoholic liquid, evaporation, filtering losses, or changes in volume caused by other ingredients. If the spirit’s labeled ABV is rounded or the volume measurement is approximate, the output inherits that uncertainty.

Frequently Asked Questions

How much water do I add to lower 40% ABV to 20%?
Add an amount of water equal to the starting spirit volume under the calculator’s additive-volume assumption. For 750 ml of 40% spirit, that is 750 ml of water, giving an estimated 1,500 ml total at 20% ABV.

What is the formula for diluting spirits?
Water to add = starting volume × (starting ABV ÷ target ABV − 1). Enter both ABVs as percentage numbers, such as 40 and 20, and use consistent volume units.

Can I use this calculator for litres or fluid ounces?
Yes. The formula is unit-independent as long as the starting volume and water result use the same unit. Do not mix US and imperial fluid ounces, which are different units.

Why might the measured final ABV differ from the estimate?
The calculation assumes exact starting strength and additive volumes. Ethanol and water can contract when mixed, and temperature and measurement accuracy affect the actual result.

Can water dilution make a spirit stronger?
No. Adding water lowers or leaves unchanged the ABV. To reach a target higher than the starting ABV requires a different starting mixture or a process beyond this dilution calculation.

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in v | Starting spirit volume | ml | 750 | 0
in a | Starting ABV | % | 40 | 0
in t | Target ABV | % | 40 | 0
out water | Water to add | ml | v * (a / t - 1) | 1
out total | Final volume | ml | v + water | 1