Free tool

Spirit Dilution Calculator

Diluting a spirit with water lowers proof while preserving proof gallons. Enter your current volume and proof and a target proof to solve for the exact water to add, along with the final proof and volume. This alcohol dilution calculator uses the same proof-gallon math as Spirit Sight.

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The formula

proof gallons = volume x proof / 100; final proof = proof gallons x 100 / final volume

A worked example

Ten gallons of barrel-proof spirit at 118 proof is 11.8 proof gallons. Cutting to 90 proof gives a final volume of 11.8 x 100 / 90 = 13.11 gallons, so you add about 3.11 gallons of water. Proof gallons stay constant; only the water changes.

Doing this math barrel by barrel? The platform runs the same calculation on every record automatically, reconciled against the books. See it live.

Proof-gallon conservation (27 CFR 30.36). This tool uses the same calculation as the Spirit Sight platform. It is general information, not tax or legal advice.

How do you calculate dilution to a target proof?
Proof gallons stay constant when you add water. Final volume = proof gallons x 100 / target proof; water to add = final volume minus current volume.
How much water do I add to proof down a spirit?
Add water equal to the difference between your target volume and current volume. For 100 gallons at 130 proof going to 100 proof, final volume is 130 gallons, so you add 30 gallons of water.
Does adding water change proof gallons?
No. Water adds volume but no alcohol, so proof gallons are conserved while proof drops. That conservation is what lets you solve for the exact water to add.
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