Alcohol Dilution Calculator
Enter your spirit volume, starting ABV and target ABV, then read off how much water to add. The calculator uses the Pearson's square (cross method) and corrects for alcohol contraction - the small volume loss that happens when alcohol and water mix. Switch to blend mode to combine two spirits of different strengths.
How to use this calculator
Select Dilute mode to find out how much water to add to reduce a spirit's strength, or Blend mode to see what ABV results from mixing two spirits. In Dilute mode, enter your starting volume and starting ABV, then type in your target ABV. The water-to-add figure updates instantly. If you are diluting with a low-strength spirit rather than pure water, set the "Diluting agent ABV" field. Toggle "Account for alcohol contraction" to correct for the small volume shrinkage that occurs when alcohol and water mix. The chart below the results shows how ABV falls as you add more water, so you can see the full dilution curve at a glance.
The math: Pearson's square and the cross method
The underlying formula is the Pearson's square, also known as the cross method, which is the standard approach for blending two solutions of different strengths to reach a target concentration. Label the stronger liquid A (its ABV), the weaker liquid B (its ABV, usually 0 for water), and your target C. The number of parts of strong liquid you need is (C - B), and the number of parts of weak liquid is (A - C). So: Water to add = Start volume x (A - C) / (C - B). For example, to bring 1000 mL of 60% spirit down to 40%: Water = 1000 x (60 - 40) / (40 - 0) = 1000 x 20 / 40 = 500 mL. The same formula applies when the diluent is not pure water, just substitute B with its actual ABV.
Alcohol contraction: why the final volume is slightly less than expected
When you mix ethanol and water, the molecules interact and pack together more tightly than either liquid alone. A 50:50 mix by volume does not produce twice the volume of either component: it produces slightly less. The shrinkage is greatest around 50% ABV (roughly 6-7 mL per litre of mixture) and decreases toward 0% and 100%. This matters for distillers proofing spirits to a legal label strength: add the calculated water, measure the actual volume, and compare it to the expected volume to confirm accuracy. The contraction values used here are interpolated from AOAC (Association of Official Analytical Chemists) tables, which are the industry standard reference.
ABV, proof, and how to convert between them
ABV (alcohol by volume) is the universal metric standard: it is the percentage of the total liquid volume that is pure ethanol at 20 degrees C. US proof is simply ABV multiplied by 2, so 40% ABV equals 80 proof. UK proof (historical, now rarely used) was a different scale where 100 proof equalled about 57.1% ABV. Distillers in the United States must label spirits in both ABV and proof. For spirits sold in the European Union and most of the world, only ABV is used. This calculator shows US proof as a secondary output alongside ABV.
Common spirit strengths and legal definitions
| Spirit category | Typical ABV range | US proof range | Notes |
|---|---|---|---|
| Beer | 3-8% | 6-16 | Varies widely by style |
| Wine | 9-16% | 18-32 | Fortified wines up to 22% |
| Sake | 14-16% | 28-32 | Nigori sake slightly lower |
| Vermouth | 14-22% | 28-44 | Dry vermouth typically lower |
| Liqueur / cordial | 15-55% | 30-110 | Sugar content varies greatly |
| Vodka (US standard) | 40% | 80 | US minimum 40% for "vodka" |
| Gin (US standard) | 40% | 80 | US minimum 40% for "gin" |
| Rum | 37-80% | 74-160 | Overproof rum up to 95% |
| Tequila / mezcal | 35-55% | 70-110 | Mezcal often bottled higher |
| Whiskey (US bourbon) | 40-67% | 80-134 | Cask strength can exceed |
| Brandy | 35-60% | 70-120 | Cognac minimum 40% |
| Everclear (grain) | 75-95% | 150-190 | Illegal in some US states |
Reference ABV ranges for popular categories. Exact values vary by producer and country.
Frequently asked questions
What is the formula for diluting alcohol to a target ABV?
The standard formula is: Water to add = V1 x (C1 - C2) / (C2 - C3), where V1 is your starting volume, C1 is your starting ABV, C2 is your target ABV, and C3 is the ABV of your diluent (0% for plain water). This is the Pearson's square cross method. For example, 1 L of 60% spirit to 40%: 1000 x (60 - 40) / (40 - 0) = 500 mL of water.
Why is my final volume slightly less than the starting volume plus the water I added?
This is alcohol contraction. When ethanol and water mix, their molecules fit together more efficiently than when pure, so the combined volume is slightly smaller than the sum of the two parts. The effect peaks around 50% ABV at about 6-7 mL lost per litre of final mixture. The calculator's contraction correction accounts for this.
Can I use this calculator to dilute isopropyl alcohol or cleaning alcohol?
The same dilution formula (C1V1 = C2V2 and Pearson's square) works for isopropyl alcohol (IPA), ethanol-based hand sanitiser, and other solutions. To make a 70% IPA solution from 91% IPA: Water = V x (91 - 70) / (70 - 0) = V x 0.30. However, the contraction figures in this calculator are calibrated for ethanol-water mixtures; IPA contraction is slightly different.
What is US proof and how does it relate to ABV?
US proof is exactly twice the ABV percentage. A spirit that is 40% ABV is 80 proof, 50% ABV is 100 proof, and so on. The word proof dates to the 18th century, when British customs agents tested spirits by soaking gunpowder in them: if the gunpowder still ignited, the spirit was "proved" to contain enough alcohol. The modern US legal standard simply defines 100 proof as 50% ABV.
Can I dilute with a low-strength spirit instead of plain water?
Yes. Set the "Diluting agent ABV" field to the ABV of that spirit. The Pearson's square formula adjusts automatically. For example, to blend a 70% concentrate with 20% wine to reach 40%: Water equiv = V x (70 - 40) / (40 - 20) = V x 1.5.
How accurate is this calculator for commercial distilling?
For home and craft use it is very accurate. Commercial producers must comply with TTB (USA) or HMRC (UK) regulations that require calibrated hydrometers, temperature-corrected measurements at a standard 20 degrees C reference, and certified tables. The contraction values here are interpolated from AOAC tables and match published figures to within 0.1-0.2 mL/L, which is adequate for most practical purposes.