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Alcohol Dilution Calculator

Achieving the precise alcohol content in homemade spirits, tinctures, or cordials requires accurate calculations. This calculator determines how much strong alcohol and water (or weak spirit) to combine to reach your target strength. Whether you're working with 95% rectified spirit for tinctures, standard 40% vodka, or preparing disinfectant solutions from 91% isopropanol, the tool handles the mathematics and accounts for alcohol contraction—the volume loss that occurs when alcohol and water molecules bond together.

Last updated:

Creators Aleksandra Zając, MSc Nutrition
8,449 people find this calculator helpful

How to Use This Calculator

Start by identifying your strong alcohol base: rectified spirit (95%), vodka (40%), rum (37–40%), or any other spirit with a known ABV. Next, specify what you're diluting with—pure water registers as 0% alcohol, but you can also blend two spirits if your secondary liquid contains measurable alcohol.

Enter your target alcohol content. This must fall between the weak and strong concentrations. For example, if mixing 80% spirit with water, your goal might be 40% or 20%. Finally, input the volume of strong alcohol you have on hand. The calculator reveals both the amount of diluting liquid required and the total final volume.

Enable the contraction checkbox if you need precision. Alcohol contracts when mixed—a litre of spirit plus a litre of water yields slightly less than 2 litres. The degree of contraction depends on the final ABV; concentrations around 40% show the most shrinkage (roughly 30 mL per litre), while very weak or very strong solutions contract less.

Dilution Mathematics

The core principle balances the alcohol content across your mixture. These formulas calculate the exact volumes needed:

Water/weak alcohol needed = (Strong ABV − Target ABV) × Strong volume

÷ (Target ABV − Weak ABV)

Total solution = Strong volume + Water/weak alcohol volume

Volume contraction ≈ f(Total volume, Target ABV)

Final usable volume = Total solution − Contraction

  • Strong ABV — Alcohol by volume of your base spirit (e.g., 95%, 80%, 40%)
  • Target ABV — Your desired final alcohol concentration
  • Weak ABV — Alcohol content of your diluent; 0% for water
  • Strong volume — Starting amount of concentrated spirit
  • Volume contraction — Liquid lost due to molecular bonding; varies with ABV and total volume

Sweetening Homemade Spirits

Dilution and sweetness are separate considerations. After reaching your target strength, you may add sweeteners—sugar, honey, or ready-made syrup each behave differently.

  • Regular sugar: 100 g occupies roughly 60 mL of volume
  • Honey: 100 g occupies roughly 75 mL and is 30% less sweet than equivalent sugar by weight
  • Syrup: Already dissolved; add by volume with minimal volume impact

Tincture sweetness ranges are conventional starting points. Dry tinctures contain up to 50 g sugar per litre; semi-dry versions range 50–150 g/L; and sweet varieties exceed 150 g/L. Adjust to personal taste, noting that flavour perception often shifts as the spirit ages.

Understanding Alcohol Contraction

When alcohol and water combine, some alcohol molecules nestle between water molecules, reducing total volume. This contraction is real and measurable, not a loss of liquid to evaporation.

The contraction effect peaks around 40% ABV—mixing one litre of spirit and one litre of water at that target strength loses approximately 29–31 mL. Very high concentrations (90%+) and very low ones (under 10%) contract far less because the molecular packing is less efficient in either case. A mixture at 10% ABV loses only about 6 mL per litre, while 90% loses roughly 10 mL.

For casual homebrewing, contraction may be negligible. For precise tincture recipes or when scaling batches, account for it. The calculator provides exact contraction estimates based on your final ABV and total volume.

Common Pitfalls When Diluting Spirits

Avoid these mistakes when preparing your alcohol mixtures.

  1. Reversing the dilution math — Always add the weaker solution to the stronger one, never the reverse. If you need 2 litres of 40% spirit from 80% base, calculate that you need 1 litre of the 80% spirit plus 1 litre of water—don't try to add spirit to water and hope for the right concentration.
  2. Forgetting that contraction shrinks your batch — If your recipe specifies a final volume, contraction will fall short by 20–30 mL per litre, depending on ABV. Always measure the actual volume after mixing and sitting for a few hours if accuracy matters.
  3. Confusing ABV with proof or volume ratios — Alcohol by volume (ABV) as a percentage is not the same as proof or simple volume ratios. Use the percentage values consistently. Many traditional recipes list alcohol by parts (e.g., 3 parts spirit to 1 part water), which you must convert to ABV first.
  4. Neglecting secondary spirit ABV — If diluting one spirit with another lower-strength spirit rather than water, always enter the weaker spirit's true ABV. Assuming water (0%) when you're actually using a 15% wine will throw off all subsequent volumes.

Frequently Asked Questions

What is alcohol contraction and how much volume do I lose?

Alcohol contraction occurs because alcohol molecules pack more efficiently between water molecules than they do among themselves. The loss is small but measurable: roughly 6 mL per litre at 10% ABV, climbing to a maximum of about 30 mL per litre around 40% ABV, then declining again toward higher concentrations. For a 1-litre batch at 40% strength, expect to lose approximately 29 mL. This is a physical phenomenon, not evaporation, and becomes relevant for scaled recipes or high-precision applications.

Can I dilute high-strength isopropanol alcohol for disinfection?

Yes. To dilute 91% isopropanol to 70% (a common disinfectant strength), combine 770 mL of the 91% isopropanol with 230 mL of water. The calculation follows the standard dilution formula: determine the percentage-point gap (91% − 70% = 21%), multiply by your starting volume, then divide by the target percentage. Always add isopropanol to water slowly and in a well-ventilated space, as mixing generates heat. Wear gloves and avoid skin contact.

How do I dilute spirits for tincture-making?

Tinctures typically use rectified spirit at 95% ABV as the extracting base. If your target is 40% (a common final strength), measure your 95% spirit, then calculate the water needed using the dilution formula. For instance, 1 litre of 95% spirit diluted to 40% requires approximately 1.375 litres of water, yielding roughly 2.3 litres of finished tincture after contraction. Allow the mixture to rest for 24 hours before use so the contraction completes and the concentration stabilizes.

What happens if I mix two spirits of different strengths?

The same dilution rules apply, but you must account for both spirits' ABV. If blending 80% rum with 20% wine to achieve 50%, enter 80% as the strong spirit, 20% as the weak spirit, and 50% as your target. The calculator adjusts proportions accordingly. This approach is common in liqueur production, where a high-proof base spirit is softened by adding a lower-strength wine or cordial rather than plain water, introducing flavor complexity alongside volume adjustment.

Does adding sugar or honey change the final alcohol content?

Sugar and honey do not contain alcohol, so they do not dilute your spirit in the ABV sense. However, they do increase total volume. If your recipe calls for 1 litre of 40% spirit and you then add 100 g of sugar (taking up ~60 mL), your final volume is approximately 1.06 litres, but the ABV remains 40%. For strict recipe adherence, calculate and mix your alcohol dilution first, verify the final volume, then add sweeteners as a separate step.

Why is my diluted spirit different from the expected volume?

Contraction is the most common cause. Less visibly, temperature changes affect volume—warm alcohol occupies more space than cold alcohol. If you mixed at room temperature and stored in a cool place, the liquid level will drop slightly. For the most accurate results, mix your solution, let it settle at your target storage temperature for 24 hours, then measure. If precision is critical for a recipe, verify contraction values for your specific ABV using reference tables and adjust expected yield accordingly.

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