Proofing-down calculator

The Proofing Down calculator tells you how much water to add to bring spirits down to exactly 100° proof. It matters because alcohol and water contract when you mix them — the water you need is more than you'd get by simply subtracting volumes. This calculator uses the published 27 CFR §30.66 "reduce to proof 100" method, which accounts for that contraction, so the answer is the one the procedure gives, not a naive volume difference.

It's free, needs no account, and computes in your browser.

Open the calculator

• On the public site: in the top navigation choose Tools, then on the Proofing Down card choose Open calculator.
• Inside the app: in the navigation choose Tools, then on the Proofing Down card choose Open calculator.

Work out the water to add

1. In the Current proof field, enter the spirits' current proof. It must be over 100 and up to 200 — this tool reduces down to exactly 100° proof, so a starting proof at or below 100 has nothing to reduce.
2. The calculator immediately shows Water per 100 wine gallons — the gallons of water to add for every 100 wine gallons of your spirits.
3. To scale that to your actual batch, enter your batch's volume in the Volume (optional) field and choose its Unit (Milliliters (mL), Liters (L), U.S. gallons, Wine gallons, Barrels (31 U.S. gal), or Fluid ounces). A second figure appears: Water to add for your volume.

If you enter a current proof of 100 or less (or over 200), the field shows Enter a proof over 100 and up to 200 (this reduces down to exactly 100° proof). and no result is shown until you give a valid starting proof.

Worked example

Reducing 50 U.S. gallons of 112° proof spirit down to 100°:

1. Current proof: 112
2. Volume (optional): 50
3. Unit: U.S. gallons

The calculator shows Water per 100 wine gallons 12.42 and Water to add for your volume 6.21. Note that 12.42 gallons per 100 is materially more than the 12 you'd get by just subtracting proofs — that difference is the alcohol–water contraction the §30.66 method accounts for.

When to reach for it

Reach for Proofing Down when you're cutting a high-proof spirit to bottling or barrel-entry strength at 100° proof and need to know the water up front. It only targets exactly 100° proof; for reducing to a different proof, or for proofing down by weight on a scale, use your gauging records and the TTB tables directly.

The calculator notes on-screen that it's a quick estimate for reference — a tool, not an official TTB filing or gauge record — and that it uses the §30.66 shortcut, which accounts for the contraction so the water needed is more than simply subtracting volumes.