Proofing-down calculator

The Proofing Down calculator tells you how much water to add to bring spirits down to a target 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. The calculator uses the published 27 CFR §30.66 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.

The page subtitle reads Find the water needed to bring spirits down to a target proof. (On the public site and in a commercial workspace it also cites 27 CFR §30.66.)

Work out the water to add

The Unit starts on your workspace's measurement system (liters on a metric workspace, U.S. gallons otherwise).

1. In the Current proof field, enter the spirits' current proof — over 0 and up to 200.
2. Check the Target proof field. It starts filled in with 100; change it to the proof you want to reach. The target must be below the current proof, because adding water can only lower proof.
3. The calculator immediately shows the water per 100 gallons. The label reads Water per 100 gallons of spirit in the app for a hobby workspace; on the public site and in a commercial workspace it reads Water per 100 wine gallons.
4. To scale that to your actual batch, choose the Unit (Milliliters (mL), Liters (L), U.S. gallons, Wine gallons, Barrels (31 U.S. gal), or Fluid ounces), then enter your batch's volume in the Volume (optional) field. Two more figures appear: Water to add and Volume after proofing down, both in the unit you chose.

Until the proofs are valid, the card says Enter a current proof and a lower target proof to see the result. A proof outside 0–200 shows Enter a proof over 0 and up to 200. under the field, and a target at or above the current proof shows Target proof must be below the current proof — adding water lowers proof, it cannot raise it.

Worked example

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

1. Unit: U.S. gallons
2. Current proof: 112
3. Target proof: leave it at 100
4. Volume (optional): 50

The calculator shows about 12.42 gallons of water per 100 gallons of spirit and Water to add of about 6.21 gallons. That is materially more than the 12 you'd get by just subtracting proofs — the difference is the alcohol–water contraction.

Warnings you may see

• A target far below your current proof, or lower than any spirit is normally proofed to, shows a "worth a second look" note — water can't bring proof back up once it's in.
• If the finished volume is far larger than what you started with, a note tells you how much spirit you'll end up holding.
• A reduction of under 2° of proof shows a note that the water figure is coarse at that resolution; gauge and add water incrementally.

How this is computed

Inside a hobby workspace the regulation is tucked into a How this is computed disclosure at the bottom of the card: choose it to read that the calculator uses the 27 CFR §30.66 Table-6 method, which accounts for alcohol–water contraction. On the public site and in a commercial workspace the same text is shown directly.

The calculator notes on-screen that it's a quick estimate for reference — a tool, not an official filing or gauge record. Use your gauging records for anything you file.