Alcohol Dilution Calculator

The formula: C1V1 = C2V2

Dilution is a conservation problem. The amount of pure alcohol in the bottle does not change when you add water, so the starting concentration times the starting volume equals the final concentration times the final volume. Written out, that is C1V1 = C2V2, and it is the only equation this calculator needs.

Rearranged for the number you want, the final volume is C1V1 divided by C2. The water to add is that final volume minus the starting volume. In practice: multiply the volume by the strength, divide by the target strength, then subtract the original volume.

One worked pass makes it concrete. You have 750 mL of 40% spirit and want 30%. Multiply 750 by 40 to get 30,000, divide by 30 to get 1,000 mL, and subtract 750 to get 250 mL of water. The bottle ends up at 1,000 mL and the strength is 30%.

The formula assumes volumes add perfectly, which they nearly do. It also assumes the strength on the label is accurate, which it usually is within a fraction of a percent. For home use that is plenty. For a commercial batch you would work in mass and use the TTB gauging tables instead, because the small errors compound over a barrel.

Proof and ABV: proof is twice the percentage

American labels carry both numbers and they are not the same thing. Proof is exactly twice the alcohol by volume, a definition the TTB spells out in 27 CFR 5.1: proof is the ethyl alcohol content stated as twice the percentage by volume at 60 degrees Fahrenheit. So 80 proof is 40% ABV, and 151 proof is 75.5%.

The convention comes from an old gunpowder test, where spirit was poured on powder and lit. If it still burned, it was proved. The scale was later pinned to the volume percentage so that 100 proof sits at 50% alcohol, and the doubling has survived into the modern rule book even though the test is long gone.

British proof is a different scale and does not double, which is why an old English recipe can look wrong when you check it. If a source gives proof without a country, assume US proof unless the numbers stop making sense.

Worked example: 750 mL of 80 proof down to 60 proof

The most common job in a home kitchen is taking a standard bottle down a notch for a liqueur, a tincture or a punch base. Here is the whole calculation.

• Starting volume: 750 mL
• Starting strength: 80 proof, which is 40% ABV
• Target strength: 60 proof, which is 30% ABV
• Final volume: 750 x 40 / 30 = 1,000 mL
• Water to add: 1,000 - 750 = 250 mL, or 8.45 fluid ounces

Two things are worth checking after you mix. First, the alcohol content of the bottle did not change: it still holds 300 mL of pure alcohol, which is 10.1 fluid ounces and about 16.9 standard drinks. You added water, not dilution of the alcohol itself, so the total alcohol is the same and each serving is larger.

Second, the mixture will be slightly less than 1,000 mL. Alcohol and water pull together when they combine, and the total shrinks by a small percentage. Do not top it up to the mark with more water, because that would push the strength below target. Mix to the calculated water volume and accept the small shortfall.

If you are scaling up, do the same arithmetic in liters or gallons and the answer holds. One gallon of 40% cut to 30% takes 42.7 fluid ounces of water, for 1.33 gallons of finished spirit.

A dilution chart for a 750 mL bottle

Most home dilution starts from a 750 mL bottle at 80 proof. This chart covers the strengths people aim for, and the same ratios work for a liter or a gallon if you scale them.

The ratio column is the one to remember for the bar. A 1 to 1 dilution takes 40% spirit to 20%, which is roughly the strength of a fortified wine, and a 3 to 1 dilution lands near 10%, which is wine territory. Anything below 20% will not keep as well, because the alcohol is no longer high enough to hold off spoilage in a sweet mix.

For a liqueur, most recipes aim between 20% and 25%, which is why they call for roughly equal parts spirit and syrup. The sugar adds body and the water in the syrup does the diluting, so the two jobs happen in one pour.

Volume contraction: why you get a little less

Mix 50 mL of water with 50 mL of ethanol and you do not get 100 mL. You get a little under 97 mL. The molecules are different sizes and shapes, and the smaller water molecules fit into the gaps between the larger ethanol molecules, so the combined liquid takes up less room than the two parts did on their own.

This is not a rounding error or a bad measurement. It is a real physical effect called volume contraction, and it is why the TTB maintains gauging tables rather than trusting simple arithmetic. The TTB distilled spirits guidance works from measured density at 60 degrees Fahrenheit for exactly this reason.

For a home batch the shortfall is small, a percent or two of the total, and it does not change how much alcohol is in the bottle. It does mean two things. Your final volume will read slightly under the calculated number on the side of a marked container, and if you top up to the calculated line with extra water you will land below your target strength. Add the water you calculated, stir, and let the level settle where it settles.

The effect grows as the mixture gets stronger in alcohol, so a 95% spirit cut to 40% shows more shortfall than a 40% spirit cut to 30%. It also runs slightly warmer or cooler with temperature, which is the other reason the official tables are pinned to 60 degrees Fahrenheit.

Measuring the strength you have

The label is the weakest link in the calculation. A bottle marked 40% is allowed a small tolerance, and an opened bottle that has been sitting for years has lost alcohol to evaporation, which lowers the strength rather than raising it. For most kitchen work the label is close enough. For anything you plan to bottle and share, measure.

A hydrometer or a proofing hydrometer reads the density of the liquid and converts it to strength. Float it in a tall sample, spin it to break the surface tension, and read at eye level. Temperature matters: the TTB fixes proof at 60 degrees Fahrenheit, and a warm sample reads high. A 20 degree swing can move the reading by about 1% ABV, which is enough to matter at the edges of a recipe.

A refractometer works too and needs only a few drops, but the sugar in a liqueur throws the reading off, so use one before you add syrup and not after. Whichever tool you use, record the reading and the temperature in the same note, because a number without a temperature is only half a measurement.

Mixing order, water and flavor

Strength is arithmetic, but the drink is chemistry and taste. A few habits separate a clean dilution from a cloudy one.

• Add water to spirit, slowly. Pouring water into a strong spirit in a thin stream and stirring keeps the local concentration even and reduces the chance of a haze.
• Use filtered or distilled water. Chlorine flattens delicate aromatics, and hard water can throw a cloud when the strength drops below about 30%.
• Watch for chill haze. Anise, ouzo and some whiskies go milky when water is added because the oils come out of solution. That is normal, and it clears as the drink warms.
• Rest it. A freshly diluted spirit tastes sharp for a day or two. Give it 24 hours in a sealed jar and the edges round off.
• Chill before serving, not before mixing. Cold liquid measures slightly short, so measure at room temperature and chill afterwards.

One safety note that belongs in any kitchen guide: dilute only ethanol you bought as a beverage or a food grade spirit. Methanol and denatured alcohol are not drinkable at any strength, and no amount of water makes them safe. Keep the food grade bottle labeled and stored away from cleaning supplies.

When a batch goes wrong, the symptom usually points at one cause. The table below covers the four that come up most.

Standard drinks and servings per batch

Dilution changes the volume of a drink and not the alcohol in it, so the serving count is the honest way to describe a batch. The National Institute on Alcohol Abuse and Alcoholism defines a US standard drink as about 14 grams or 0.6 fluid ounces of pure alcohol, which is a 12 ounce beer at 5%, a 5 ounce glass of wine at 12%, or a 1.5 ounce shot of 40% spirits.

Run that against a diluted bottle. A 750 mL bottle of 80 proof holds 300 mL of pure alcohol, which is 10.1 fluid ounces, or 16.9 standard drinks. Dilute it to 60 proof and the bottle grows to 1,000 mL, but it still holds 16.9 standard drinks. The only thing that changed is the size of the pour: a 1.5 ounce serving now carries 0.45 ounces of alcohol instead of 0.6, so it counts as three quarters of a standard drink.

That is the useful number when you are making punch for a party. A gallon of 20% punch, which is what a 1 to 1 dilution of 80 proof gives you, holds about 25.6 fluid ounces of pure alcohol, or 42 standard drinks. Knowing that is a better guide to how much to pour than counting cups.

This is a mixing guide, not health advice. If you drink, keep the count in view, and do not dilute a spirit and then treat the larger volume as if it were the same strength.