Ethanol–Water Dilution Calculator
Dilute high-proof ethanol to a target ABV and get the exact water to add — corrected for the volume contraction that a plain C₁V₁=C₂V₂ misses.
Worked examples
95% ethanol down to 70% for disinfectant
Diluting 1 L of 95% ABV ethanol to the 70% ABV that's most effective as a surface and hand disinfectant.
- Stock
- 95% ABV, 1 L
- Target
- 70% ABV
Add 452.8 mL water → 1,357 mL at 70% (naive would say only 357 mL)
High-proof neutral spirit to 40% vodka
Cutting 750 mL of 95% ABV neutral spirit down to a standard 40% ABV bottling strength.
- Stock
- 95% ABV, 750 mL
- Target
- 40% ABV
Add 1,128 mL water → 1,781 mL at 40% (contraction adds ~97 mL)
How the calculation works
ABV is defined by volume — it's the volume of pure ethanol divided by the total mixture volume — and that pure-ethanol volume doesn't change when you add water. So the final volume follows straight from the definition, with no contraction term at all:
The water you add is where contraction bites. You can't just take V₂ − V₁, because ethanol and water together occupy less space than their separate volumes. Instead the calculator works in mass, which is conserved: it reads the mixture's density and weight-fraction (ABW) at the target ABV from a measured 20°C table, gets the total and ethanol masses at the final volume, and the leftover mass is water — converted back to a volume at water's own density:
The gap between this V_water and the naive V₂ − V₁ is the contraction correction shown in the results — the extra water you'd be short by if you trusted additive volumes. Density and ABW values between table rows are linearly interpolated across the nearest two points.
Ethanol–water reference table (20°C)
Density and weight-percent (ABW) for ethanol-water mixtures by ABV, at 20°C. The calculator interpolates linearly between these rows.
| ABV (% v/v) | Density (g/mL) | ABW (% w/w) |
|---|---|---|
| 0 | 0.99820 | 0.00 |
| 5 | 0.99106 | 3.98 |
| 10 | 0.98471 | 8.01 |
| 15 | 0.97897 | 12.09 |
| 20 | 0.97356 | 16.21 |
| 25 | 0.96810 | 20.38 |
| 30 | 0.96221 | 24.61 |
| 35 | 0.95559 | 28.91 |
| 40 | 0.94805 | 33.30 |
| 45 | 0.93954 | 37.80 |
| 50 | 0.93014 | 42.43 |
| 55 | 0.91996 | 47.18 |
| 60 | 0.90911 | 52.09 |
| 65 | 0.89765 | 57.05 |
| 70 | 0.88556 | 62.39 |
| 75 | 0.87279 | 67.82 |
| 80 | 0.85927 | 73.48 |
| 85 | 0.84485 | 79.40 |
| 90 | 0.82918 | 85.66 |
| 95 | 0.81138 | 92.41 |
| 100 | 0.78924 | 100.00 |
Frequently asked questions
Why do I need this instead of C₁V₁ = C₂V₂?
Because ethanol and water don't add up. Mix a litre of ethanol with a litre of water and you get noticeably less than two litres — the molecules pack together and the volume shrinks by up to about 3.5%. C₁V₁ = C₂V₂ assumes volumes are additive, so it tells you the right final volume (ABV is defined by volume) but the wrong amount of water to add: it under-counts the water because it ignores that shrinkage. This calculator uses a measured 20°C density table to give the water volume that actually lands you on your target ABV.
What is the difference between the final volume and the water to add?
The final volume (V₂) is how much diluted product you end up with, and by the definition of ABV it follows the simple relation V₂ = V₁·C₁/C₂. The water to add is a separate, smaller-looking question with a bigger catch: because of contraction, the water you pour in is more than V₂ − V₁. The calculator reports both, and shows how far the naive 'just subtract the volumes' figure is off, so you can see the contraction correction explicitly.
Is this accurate enough to make 70% hand sanitizer or rubbing alcohol?
For mixing by volume at room temperature, yes — the density table is at 20°C and resolves ABV closely, which is well within tolerance for sanitizer, disinfectant, tinctures, or general lab dilution. Two caveats: measure at roughly 20°C (ethanol expands and contracts noticeably with temperature, shifting ABV), and confirm your starting stock's true ABV, since 'pure' ethanol is often actually ~95% (the azeotrope) or denatured to a stated proof. For regulated or medical-grade preparation, verify the finished strength with a hydrometer or alcoholmeter.
My stock says 'proof' or 'percent by weight', not ABV — how do I convert?
US proof is simply twice the ABV, so 190 proof = 95% ABV and 100 proof = 50% ABV. Percent by weight (ABW, % w/w) is different from ABV (% v/v) and the two only line up at 0% and 100%: 40% ABV is about 33% ABW, for instance. The reference table on this page lists ABV, density, and ABW side by side at 20°C, so you can read across to convert a weight-percent or proof spec into the ABV this calculator expects.
Does temperature change the result?
Yes, more than most people expect. Alcohol-water mixtures expand and contract with temperature enough that the same physical bottle reads a different ABV warm versus cold — which is why official alcoholometry is always referenced to 20°C, and why this calculator's table is too. If you're mixing well above or below room temperature, expect the actual ABV to drift from the calculated value; let the liquids equilibrate near 20°C before you measure and mix for the closest match.
Can I use this for isopropanol (IPA) or other alcohols?
Not directly — the density table here is specific to ethanol and water, and isopropanol has its own, different density-vs-concentration curve and its own contraction behaviour. The volume relationship V₂ = V₁·C₁/C₂ still holds for IPA by the definition of percent-by-volume, so the final volume is right, but the exact water-to-add figure would need an isopropanol-water density table. Treat the water volume here as ethanol-specific.