Whisky Dilution Calculator

The Whisky Dilution Calculator estimates how much alcohol-free water to add to a measured volume of whisky to reach a lower target strength. It uses the starting volume, starting alcohol by volume (ABV) and target ABV to calculate the water addition and estimated final volume.

Calculator

[TOOL TO BUILD]

Required inputs: starting volume in millilitres (ml), starting ABV as a percentage, and target ABV as a percentage. Outputs: estimated water to add in ml and estimated final volume in ml.

How to use the whisky dilution calculator

Measure the whisky already in your glass or mixing vessel and enter that volume, rather than the capacity of the glass. Enter the starting strength shown on the bottle, then choose a target strength at or below it.

Read the water addition separately from the final volume: the first is what you add, while the second includes the whisky you started with. Measure additions in ml rather than drops, since this calculator does not assign a volume to a drop.

Calculation method

The model treats the amount of alcohol as unchanged when alcohol-free water is added. It assumes that the starting whisky volume and added water volume can simply be added together.

Use V for the starting volume, S for the starting ABV and T for the target ABV. Enter strengths as percentage numbers, for example 46 rather than 0.46, using the same convention for both strengths.

Estimated final volume = V × S ÷ T

Estimated water addition = V × (S ÷ T − 1)

These equations follow from V × S = final volume × T. Calculate with unrounded inputs, then round only the displayed results.

Worked example

For a teaching example, start with 25 ml at 46% ABV and set a target of 40% ABV. The estimated final volume is 25 × 46 ÷ 40 = 28.75 ml, so the estimated water addition is 3.75 ml.

This example demonstrates the arithmetic, not a recommended serving or an ideal tasting strength. A different target produces a different water addition; the calculator makes no judgement about flavour.

Input checks and limits

The tool should accept only a positive starting volume and strengths above zero and no greater than 100% ABV. A target equal to the starting strength returns zero water; a higher target cannot be reached by adding water and should return an explanatory message, not a negative addition.

Missing or unverified inputs must remain unknown, rather than being replaced with assumed values. This model covers whisky and alcohol-free water only, not ice melting over time or mixtures containing other drinks.

The result is an estimate under the stated volume assumption, not a measurement or certification of the mixture's ABV. The model does not correct for any volume change on mixing. The calculator gives no drinking allowance, driving prediction or assessment of whether drinking is appropriate for you.

Sources

No external sources were supplied or cited. This page was prepared by deriving the stated volume-balance equation and checking the worked example arithmetically.

Last checked: November 2026