How this instrument works
Diluting a spirit works the same way diluting any solution does: the total amount of alcohol present does not change when you add water, only the volume it's spread through does. That conservation principle is captured in the dilution identity C1V1 = C2V2, where concentration times volume before dilution equals concentration times volume after — because both sides simply count the same fixed quantity of pure alcohol.
Rearranged to solve for the unknown, V2 = C1V1 / C2 gives the final volume needed once you know your starting concentration, starting volume, and target concentration. That final volume is always larger than the starting volume whenever you're diluting to something weaker — the difference between the two is exactly how much water to stir or shake in.
This is the same maths a bartender uses cutting a barrel-proof whiskey down to bottling strength, a distiller uses proofing spirit off the still down to 40% for bottling, or a brewer uses thinning a high-gravity beer with water. The identity does not care what the liquid is, only that concentration and volume trade off against each other while the total solute stays fixed.
- Enter the spirit's current strength as Starting concentration (%) — the ABV on the bottle, or a hydrometer/refractometer reading off the still.
- Enter Starting volume (mL) — how much of the starting spirit you actually have.
- Enter Target concentration (%) — the strength you want to end up at; it must be lower than the starting concentration, since this only handles dilution, not concentration.
- Read Final (diluted) volume (mL) — subtract your starting volume from this figure to see exactly how much water to add.
Worked example — cutting a 750 mL bottle from 40% to 20%
Start with a 750 mL bottle at 40% ABV, and the goal is to cut it down to 20%. The dilution identity gives V2 = (40 × 750) / 20 = 30,000 / 20 = 1,500 mL as the total final volume.
Since the starting volume was already 750 mL, the water to add is 1,500 − 750 = 750 mL — exactly doubling the volume, which makes intuitive sense: halving the concentration from 40% to 20% requires doubling the total volume the same fixed amount of alcohol is spread through.
Questions
How much water do I actually add, not just the final volume?
Subtract your starting volume from Final (diluted) volume. The instrument reports the total volume once dilution is complete, because that's what the C1V1=C2V2 identity directly solves for — the amount of water is simply that total minus what you started with, so a 750 mL bottle diluted to a 1,500 mL final volume needs exactly 750 mL of water added.
Can I use this to dilute wine or beer, not just spirits?
Yes — the dilution identity applies to any liquid where you're adding water to reduce alcohol concentration and nothing else is changing the total alcohol present. It's most commonly used for spirits because their high starting concentration makes dilution a routine step, but the same maths works for cutting a high-gravity homebrew or a fortified wine down with water.
Why can't I enter a target concentration higher than my starting concentration?
Because adding water can only lower concentration, never raise it — you cannot 'concentrate' a spirit by diluting it. Raising ABV requires removing water (through distillation) or blending in a stronger spirit, neither of which this dilution identity models, so the instrument blocks a target above the starting concentration rather than returning a negative or nonsensical volume.
Does this account for the fact that alcohol and water don't mix additively by volume?
No — this uses the simple linear dilution identity, which is exactly correct for concentration measured on a mass or mole basis but only approximately correct for ABV by volume, because ethanol and water molecules pack together slightly more tightly than their separate volumes suggest (a small effect called volume contraction). For proofing spirits at home, the error is small enough that this approximation is the industry-standard shortcut.
What's the difference between ABV and proof?
In the US, proof is defined as exactly twice the percentage of alcohol by volume, so a spirit labeled 80 proof is 40% ABV. This calculator works in percentage ABV throughout — if your bottle is labeled in proof, divide by two before entering it as Starting concentration, and double the result again if you need to report your target back in proof.