This is the classic “Pearson square” mixing rule. When alcohol and water mix the volume shrinks slightly (up to about 3–4%), so the final volume is a little smaller and the strength a little higher than the simple calculation – add the water gradually and check with an alcoholmeter.
How to use the Alcohol Dilution Calculator
- Strong alcohol strength (ABV)
- Strong alcohol volume
- Water or weak alcohol strength
- Target strength
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the strength and volume of the strong alcohol, the strength of what you are adding (0% for water) and the target strength.
Tips
- Measure gravity and temperature carefully – hydrometers are calibrated for a specific temperature.
- Keep alcohol calculations conservative: never drink and drive, and know your limits.
- Write down every measurement in a brewing log so you can repeat good batches.
- Chill drinks in an ice-water bath for the fastest, safest cooling.
Common mistakes to avoid
- Reading a hydrometer at the wrong temperature without correcting it.
- Mixing up ABV (% by volume) and ABW (% by weight).
- Over-priming bottles – too much sugar can make bottles burst.
Inputs explained
| Field | What to enter |
|---|---|
| Strong alcohol strength (ABV) | In % |
| Strong alcohol volume | Units: L, ml, US gal, fl oz, cups |
| Water or weak alcohol strength | In % |
| Target strength | In % |
What each result means
- Final volume (before contraction) = [Strong alcohol volume] + [Water ÷ weak alcohol to add]
- Contraction (approx.) = [Final volume (before contraction)]×0.035×4×([Target strength]÷100)×(1 – [Target strength]÷100)
- Target strength in US proof = [Target strength]×2
Formula
Water or weak liquid to add = V × (C_strong − C_target) ÷ (C_target − C_weak)
Final volume ≈ V + added volume (minus a little contraction)
Worked example
Inputs — Strong alcohol strength (ABV): 95 %; Strong alcohol volume: 1 L; Water or weak alcohol strength: 0 %; Target strength: 40 %.
Results — Water / weak alcohol to add = 1.375 L; Final volume (before contraction) = 2.375 L; Contraction (approx.) = 0.08 L; Target strength in US proof = 80.
Frequently asked questions
How do I dilute 95% alcohol to 40%?
Add about 1.375 litres of water to each litre of 95% alcohol.
Why is the final volume smaller than expected?
Ethanol and water molecules pack together, so the mixture contracts slightly.
Can I dilute isopropyl alcohol the same way?
Yes – e.g. 91% to 70% needs 0.3 L of water per litre.