Alcohol Dilution Calculator
Volume of the spirit you're starting with, e.g. 700mL for a standard bottle.
The strength on the bottle's label, e.g. 57.8% for overproof rum or 40% for standard spirits.
Your preferred drinking strength after adding water.
Water to add
700 mL
Final volume
1,400 mL
Standard drinks (total)
22.1
Add 700mL of water to 700mL of 40.0% spirit to bring it down to 20.0% ABV, giving you 1,400mL total containing 22.1 standard drinks.
Quick answer
Diluting alcohol means adding water until you reach your target ABV: water to add (mL) equals current volume times (current ABV divided by target ABV, minus 1). For example, 700mL of 40% spirit needs 700mL of water to reach 20% ABV, giving 1,400mL total at the new strength.
Explore more Brewing calculators
Alcohol Dilution Calculator is one of brewing and alcohol making tools on OneCalculate — see the full set for this category.
Browse BrewingHow it works
This alcohol dilution calculator uses the standard volume-based dilution formula — a practical version of the chemistry shortcut C1V1 = C2V2 — to work out how much water to add when cutting a high-proof spirit down to a lower drinking strength. Whether you think of it as a diluting alcohol calculator, an abv dilution calculator, or simply a dilution calculator for alcohol, the maths underneath is the same scaled-water calculation:
Water to add (mL) = Current volume (mL) × (Current ABV ÷ Target ABV − 1)
Final volume (mL) = Current volume (mL) + Water to add (mL)Because adding water only spreads the existing alcohol across more liquid, the total pure alcohol in the container never changes — only its concentration and the volume it’s dissolved in do. To show that in real terms, the calculator also converts the final mix into Australian standard drinks, using the official definition of 10 grams of pure alcohol per standard drink and an ethanol density of 0.789 g/mL, per the Department of Health’s standard drinks guide.
One important note before you use this as a spirit dilution calculator: in Australia, distilling spirits at home is illegal without an excise manufacturer licence from the ATO, even if it’s only for personal use — see the ATO’s page on excise and excise-equivalent goods. That’s different from homebrewing beer, cider, wine or mead, which is legal for personal use without a licence. This tool is built for two situations: licensed distillers working out cutting water for their batches, and anyone diluting an already-purchased, legally-obtained high-proof spirit — like overproof rum or cask-strength whisky — down to a more drinkable strength.
Worked example
Liam, a home bartender in Cairns, picks up a 700mL bottle of Bundaberg OP overproof rum at 57.8% ABV from his local bottle shop for a backyard party. Straight from the bottle it’s far stronger than he wants to serve, so he decides to cut it down to a standard 40% ABV — about the strength of most spirits on the shelf.
Plugging his numbers into the formula: 700 × (57.8 ÷ 40 − 1) = around 312mL of water. Adding that to his 700mL bottle gives him roughly 1,012mL of 40% rum. Because dilution doesn’t remove or add any alcohol, the bottle still holds the same total amount of pure alcohol before and after — about 31.9 standard drinks either way, just spread across more liquid at a lower strength per serve.
| ABV | Volume | Standard drinks (total) | |
|---|---|---|---|
| Before dilution | 57.8% | 700mL | 31.9 |
| After dilution | 40% | ~1,012mL | 31.9 |
Frequently asked questions
Is it legal to dilute spirits at home in Australia?
Diluting an already-purchased, legally-obtained spirit (like overproof rum or cask-strength whisky) down to drinking strength is fine. Distilling spirits yourself is a different matter — that requires an ATO excise manufacturer licence even for personal use, unlike homebrewing beer, cider, wine or mead, which is legal without one.
How do I calculate how much water to add to dilute alcohol?
Use water to add (mL) = current volume (mL) × (current ABV ÷ target ABV − 1). For 700mL of 40% spirit diluted to 20%, that's 700 × (40/20 − 1) = 700mL of water, giving 1,400mL total at 20% ABV.
What ABV should I dilute overproof spirits to?
Most overproof spirits (57.8% ABV rum is a common example) get diluted to standard bottling strength, typically 37–40% ABV, to match how they're normally sold and drunk. Enter your preferred target ABV in the calculator to see the exact water needed.
Does diluting a spirit change how many standard drinks are in the bottle?
No. Adding water lowers the concentration and increases the volume, but the total grams of pure alcohol — and so the total standard drinks in the container — stays the same. Only the number of standard drinks per serve changes, because each serve is now weaker.
Can I use this as a spirit dilution calculator for whisky or gin?
Yes. The same volume-based dilution formula applies to any spirit — cask-strength whisky, navy-strength gin, overproof rum or moonshine-style spirits — as long as you know its starting ABV and the volume you're diluting.
Why can't I dilute a spirit up to a higher ABV?
Adding water only ever lowers alcohol concentration, so you can't dilute your way to a higher ABV — that would need removing water (via distillation) or adding more alcohol. If your target ABV is set higher than your current ABV, the calculator flags it instead of returning a negative water figure.
Related calculators
Written and verified by Nirbhay Tripathi
Last updated 18 August 2026
All rates on this page are verified againstAustralian Government Department of Health — Standard drinks guide, Australian Taxation Office — excise and excise-equivalent goods (alcohol)on 18 August 2026. See our methodology for the full update calendar.