Proof Dilution Calculator

Frequently Asked Questions

How do I dilute a spirit to a target proof?

Use the dilution rule C1 × V1 = C2 × V2: final volume V2 = (starting ABV × starting volume) ÷ target ABV. Cutting 750 ml of 60% spirit to 40% gives 750 × 60 ÷ 40 = 1,125 ml, so add 375 ml of water.

How does proof relate to ABV?

In the US, proof = 2 × ABV, so 80 proof is 40% ABV and 100 proof is 50% ABV. Cutting to a target proof just means cutting to half that number in ABV.

How much water cuts cask-strength whiskey?

For 700 ml at 58% down to 46%, target volume = 700 × 58 ÷ 46 ≈ 882 ml, so add about 182 ml of water. Add gradually and taste, since some prefer less dilution.

Why does mixed-volume not add up exactly?

Water and ethanol contract slightly when mixed, so the combined volume is a touch less than the sum - usually small enough to ignore at home. This tool is informational; please drink responsibly.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

Enter the volume and ABV of your spirit and the ABV (or proof) you want to dilute to. The calculator tells you how much water to add and the final bottled volume, and shows the proof equivalent of both ABV values. It is built for distillers, blenders, and anyone proofing a cask-strength spirit down to bottling strength, in milliliters or fluid ounces.

This is an informational tool, not drinking advice. Diluting lowers strength per volume but the total alcohol is unchanged - know your limits and drink responsibly.

Formula / Method

Alcohol is conserved when you add water, so V_start × ABV_start = V_final × ABV_target. Rearranging, water to add = V × (ABV_start ÷ ABV_target − 1), and the final volume is the starting volume plus that water. US proof is exactly twice ABV, so proof = 2 × ABV - a 40% spirit is 80 proof. You can only dilute down, so the target ABV must be lower than the start.

Worked Example

To proof 750 ml of 65% ABV (130 proof) new-make spirit down to 40% ABV (80 proof): water = 750 × (65 ÷ 40 − 1) = 750 × 0.625 = 468.75 ml. Final volume ≈ 750 + 469 = 1,219 ml - roughly a 750 ml plus a 469 ml top-up. The bottle now holds the same amount of pure alcohol as before, just spread across more liquid at a lower concentration. Add water slowly and re-measure with a hydrometer to fine-tune.

Common Mistakes

Confusing proof and ABV - in the US, proof is double ABV; the UK uses a different historical scale.

Assuming volumes add perfectly - ethanol and water contract slightly when mixed, so verify final strength by measurement.

Trying to dilute "up" to a higher ABV; adding water can only lower strength.

Measuring strength while the spirit is warm - hydrometers are calibrated to a set temperature, so correct for it.

Adding all the water at once and overshooting; approach the target gradually.

Tips & FAQ

Why does cask strength need proofing? New-make and cask spirit often sit at 55–70% ABV; bottling strength is usually 40–46%. Will the spirit get cloudy? Non-chill-filtered whiskies can louche when proofed down - that is harmless and flavor-rich. What water should I use? Distilled or low-mineral water avoids haze and off-flavors. Does the math account for contraction? It assumes additive volumes; expect a small real-world difference, so confirm with a hydrometer. Treat the result as a close starting point and adjust to your measured ABV.

Proof Dilution Calculator

Frequently Asked Questions

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