Why use this calculator
Proofing down is one of those jobs where the arithmetic looks trivial and the result still comes out wrong. The Pearson square gives the ratio in seconds, but two things trip people up. The first is that ethanol and water occupy less volume together than they did apart, so pouring in the arithmetic amount of water leaves you fractionally strong, which matters when you are filling bottles to a label claim.
The second is that people reach for the wrong equation entirely when the thing they are adding is not water, such as blending a cask strength whisky with a weaker batch or building a bottled cocktail with vermouth. This calculator handles both cases from the same formula, shows the exact final volume, and counts the finished batch in the standard drinks used where you live.
How to use it
- Enter the current strength of the spirit, taken from the bottle label or measured with an alcoholmeter at 20 °C if you distilled it yourself, because temperature shifts a hydrometer reading noticeably.
- Set the strength you want to finish at, which must sit below the spirit and above whatever you are adding.
- Enter how much of the strong spirit you are starting with in millilitres, remembering a standard bottle is 700 mL in Australia and the UK and 750 mL in the United States.
- Set the strength of what you are adding, which is zero for water and a real figure if you are blending down with a weaker spirit, wine or vermouth.
- Finally, choose the standard drink definition for your country so the finished batch is counted in the units your health guidelines use.
What each result means
- Water or diluent to add
- The straight Pearson square answer in millilitres, before any allowance for the mixture shrinking.
- Allowing for volume contraction, add about
- The slightly larger real amount needed, because ethanol and water occupy less space together than apart.
- Final volume
- What the finished batch measures. This figure is exact, since ABV is defined against the finished volume.
- Bottles to fill
- The finished batch counted in standard spirit bottles, 700 mL in Australia and the UK or 750 mL in the US, with any part bottle shown in millilitres so you have a container ready.
- Pure alcohol in the batch
- Millilitres and grams of ethanol present. Dilution spreads it out but never removes any of it.
- Standard drinks in the final batch
- How many drinks or units the whole batch represents under the national definition you selected.
- Blend ratio
- The same answer as a ratio, useful for scaling the job up or down without recalculating anything.
- Starting strength in US proof
- American proof is simply twice the ABV, so a 60 per cent spirit is 120 proof before dilution.
- Target strength in US proof
- The proof figure you are aiming for, which is what an American label would print instead of ABV.
The formula
The Pearson square gives parts of spirit = target minus diluent strength
Parts of diluent = spirit strength minus target
So the diluent volume to add = starting volume x (spirit ABV - target ABV) / (target ABV - diluent ABV)
With water at 0% this collapses to the familiar
C1V1 = C2V2 arrangement
Pure alcohol volume = volume x ABV / 100 and is conserved through the whole operation
Ethanol weighs about 0.789 g per mL at 20 °C, so standard drinks = pure alcohol mL x 0.789 divided by 10 g in Australia, 8 g per UK unit or 14 g per US standard drink
How this number is used in practice
Distillers proof down in stages rather than in one hit, usually resting the spirit for days or weeks between steps, because a sudden dilution shocks the flavour and can take months to settle. The other thing that happens on the way down is louching: a spirit that has not been chill filtered turns cloudy as it passes roughly 46 per cent, because fatty acid esters that were dissolved in alcohol fall out of solution in the more watery mixture.
It is completely harmless and is a sign the spirit kept its natural oils, which is exactly why many small distilleries bottle at 46 per cent or above rather than filter the character out. Absinthe makes a feature of the same effect, and the cloudy louche is the whole point of adding water to it.
The volume contraction is real physics rather than a rounding error. Mixing 52 volumes of ethanol with 48 volumes of water gives about 96 volumes, not 100, and the shrinkage peaks near 3.6 per cent around half and half. Because ABV is legally defined as pure ethanol volume divided by the volume of the finished product, the final volume predicted by the arithmetic is correct, while the amount of water you actually pour in has to be a few per cent more than the naive difference.
For a bottle at home the gap is a teaspoon or two and nobody would notice. For a distillery filling a run against a label claim and an excise return, the difference is the whole ballgame, which is why official alcoholometric tables exist and why the final check is always a hydrometer reading at 20 °C.
Worked examples
Cutting 700 mL of 60% spirit down to standard bottling strength with water
Add 350 mL of water for 1,050 mL at 40% ABV, or closer to 360 mL once contraction is allowed for.
Proofing 500 mL of 63.5% cask strength whisky down to 46%
Add 190.2 mL of water for 690.2 mL, keeping it above the 46% point where an unfiltered whisky starts to louche.
Building a bottled cocktail: 700 mL of 40% whisky blended down with 16% vermouth
Add 700 mL of vermouth for 1,400 mL at 28% ABV, an even one to one blend.
Homebrew new-make spirit at 80% going into a barrel at 62.5%
Add 560 mL of water to 2,000 mL for 2,560 mL at barrel entry strength.
When people use it
- Bringing a cask strength whisky down to a drinking or bottling strength
- Proofing new-make spirit to barrel entry strength after a run
- Batching cocktails ahead of a party at a known, repeatable ABV
- Blending a strong batch with a weaker one to land on a target strength
- Working out how many standard drinks a homemade liqueur or infusion actually contains
Tips
- Always add water to the spirit rather than spirit to water, and stir as you go so the mixture does not layer.
- Use filtered or distilled water. Hard tap water leaves a mineral haze and can flatten the aroma of a delicate spirit.
- Go down in stages over days for anything you care about. A single large dilution can taste thin and soapy for weeks.
- Expect cloudiness below about 46 per cent in unfiltered spirits. Louching is harmless and usually a sign of good oils.
- Check the result with an alcoholmeter at 20 °C, because a hydrometer reading taken warm will read low and mislead you.
- US proof is simply twice the ABV, so 40 per cent is 80 proof. British navy proof is 57.1 per cent, not 100 per cent.
What it assumes, and where it stops being accurate
- ABV figures are assumed to be measured at 20 °C, which is the international reference temperature for alcoholometry.
- The contraction estimate is a fitted approximation of published ethanol and water density tables, accurate to roughly half a percentage point.
- Sugar, glycerol and dissolved solids in liqueurs and infusions are ignored, and they will shift a hydrometer reading noticeably.
- Evaporation losses while measuring, pouring and resting are not modelled, and they run higher with warm spirit and open vessels.
- This is a calculation tool. Distilling spirits at home is regulated or prohibited in many places, and this is not legal advice.
Terms used on this page
- ABV
- Alcohol by volume: millilitres of pure ethanol per 100 mL of finished liquid, measured at 20 °C.
- Pearson square
- A diagonal subtraction layout that gives the parts of two liquids needed to hit a target concentration.
- Proof
- An older strength scale. In the US it is twice the ABV; in the old British system 100 proof was about 57.1 per cent ABV.
- Louching
- The cloudiness that appears when oils dissolved in strong alcohol fall out of solution as water is added.
- Volume contraction
- The shrinkage that occurs when ethanol and water mix, peaking at about 3.6 per cent near a half and half blend.
- Standard drink
- A fixed amount of pure alcohol used in health guidance: 10 g in Australia, 8 g per UK unit, 14 g in the US.
Questions people ask
- What is the Pearson square?
- A two by two layout for blending: put the strong liquid at the top left, the weak one at the bottom left and the target in the middle, then subtract diagonally. The differences give the parts of each you need. Diluting with water is just the special case where the weak liquid is zero per cent, which reduces the whole thing to volume x (current minus target) divided by target.
- How does C1V1 = C2V2 apply to alcohol?
- It says the amount of pure alcohol is conserved: starting concentration times starting volume equals final concentration times final volume. Rearranged, the final volume is starting volume times starting ABV divided by target ABV. Since ABV is defined against the finished volume, that result is exact, and the water to add is the difference between the two volumes.
- Why does my final volume come out slightly short?
- Because ethanol and water genuinely contract when mixed, by up to about 3.6 per cent near half and half. Mixing 52 volumes of ethanol with 48 of water yields roughly 96, not 100. So the arithmetic final volume is the target you should measure to, and the water you actually pour in needs to be a couple of per cent more than the simple difference.
- Why did my spirit go cloudy when I added water?
- That is louching. Fatty acid esters and oils that dissolve happily in strong alcohol come out of solution as the mixture becomes more watery, usually somewhere around 46 per cent and below, scattering light into a haze. It is harmless, it does not mean contamination, and in unfiltered whisky or absinthe it is generally taken as a sign of character rather than a fault.
- What is proof and how does it relate to ABV?
- In the United States, proof is simply twice the ABV, so 40 per cent is 80 proof and 60 per cent is 120 proof. The older British system was different: 100 degrees proof was about 57.1 per cent ABV, which is where navy strength comes from. Most of the world now labels ABV directly and the proof scale survives mainly on American labels.
- Can I use this to blend two spirits instead of adding water?
- Yes, that is exactly what the diluent strength field is for. Enter the ABV of the weaker liquid, whether that is a second batch, a wine or a vermouth, and the Pearson square handles it. The only rule is that the target has to sit between the two strengths, since no blend can land outside the range of its ingredients.
Use this with
Alcohol Dilution Calculator answers one part of the question. These pick up where it stops: BAC Calculator, Cup Converter, Percentage Calculator and Unit Price Comparison Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- National Institute on Alcohol Abuse and Alcoholism. Standard drink definitions and pure alcohol content
- NHS: alcohol units. UK unit definition and low risk drinking guidance
- NHMRC: alcohol guidelines. Australian standard drink of 10 g of pure alcohol
Alcohol Dilution Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.