How this calculator works
If you mix a strong spirit with water, the amount of pure alcohol stays the same while the total volume grows. That is Pearson's square in its simplest form: strong volume x strong strength = final volume x target strength. For example, turning 40 per cent spirit into 20 per cent means the final volume must be twice the strong volume, so you add an equal volume of water or weaker alcohol. The calculation assumes volumes add perfectly. In reality, ethanol and water contract slightly when mixed, so a finished bottle may measure a little less than the sum of its parts.
Formula
strong volume x strong ABV = final volume x target ABV. If the final volume is known: strong volume = final volume x target ABV / strong ABV, and water = final volume - strong volume. If the strong volume is known: final volume = strong volume x strong ABV / target ABV. The dilution ratio is water / strong volume.
Worked example
You have 500 mL of 40 per cent spirit and want 20 per cent. The final volume is 500 x 0.40 / 0.20 = 1,000 mL, so you add 500 mL of water. For 750 mL of 40 per cent spirit at a 10 per cent target, the final volume is 3,000 mL and you add 2,250 mL of water, which is 3 parts water to 1 part spirit.
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Frequently asked questions
Does the calculator account for contraction?
No. Mixing ethanol and water produces a slightly smaller volume than the simple sum, by up to a few per cent at some strengths. Professional producers use density tables to hit an exact ABV. For home use the simple volume balance is close enough, but measure the strength after mixing if accuracy matters.
Can I use this to dilute wine or beer?
Yes, the same volume balance applies, but the result may taste thin because dilution lowers flavour compounds as well as alcohol. The calculator works in ABV, so convert the labelled strength to a fraction first, for example 13 per cent as 0.13.
Why must the target be lower than the strong alcohol?
Adding water cannot increase the alcohol concentration. If the target is above the strength you have, you would need to remove water, which is distillation, not dilution. The calculator rejects that case rather than returning a negative water volume.
Is it safe to estimate alcohol strength at home?
Volume balance is only as accurate as the strengths you start with. Diluting a spirit does not remove impurities, and home distillation is illegal in Canada without a licence. Use the calculator for mixing drinks from known, legally produced spirits, not for producing spirits.