About

Mixing ethanol and water produces a volume smaller than the sum of the two components. This non-ideal behavior, first systematically measured by Mendeleev and codified in OIML international alcoholometric tables, means a naive formula (V_water = V₁ × C₁ ÷ C₂ − V₁) underestimates the water required by 3 - 5% at typical distillation strengths. Getting the dilution wrong wastes product or forces repeated correction passes. This calculator uses a 101-point density table (at 20°C) to convert volumes to masses, performs mass-balance arithmetic, then converts back accounting for contraction. It also corrects for the temperature of your hydrometer reading if you are not measuring at exactly 20°C.

Limitations: the density table assumes atmospheric pressure and pure binary ethanol-water. Fusel oils, sugars, and dissolved solids in real spirits shift density slightly. For commercial compliance, always verify with a certified laboratory densimeter. Pro tip: add water slowly, stir gently, and allow 24 hours for the mixture to equilibrate before taking a final reading.

Formulas

The dilution is computed via mass balance. First, the mass of pure ethanol in the starting solution is calculated. Then the final volume that contains the same mass at the target concentration is determined using density data.

Step 1 - Mass of ethanol:

m_ethanol = V₁ × ρ₁ × C₁100

Step 2 - Final solution volume (from mass balance):

V₂ = m_ethanolρ₂ × C₂100

Step 3 - Water to add (by mass, then volume):

m_water = V₂ × ρ₂ − V₁ × ρ₁

V_water = m_waterρ_water

Where V₁ = initial volume, C₁ = initial ABV %, C₂ = target ABV %, ρ₁ = density at C₁, ρ₂ = density at C₂, and ρ_water = 0.99823 g/mL at 20°C. The naive formula (V₁ × C₁ ÷ C₂ − V₁) is also shown for comparison to illustrate the contraction error.

Reference Data

ABV %	Density at 20°C g/mL	US Proof	Common Use
0	0.99823	0	Pure water
5	0.99103	10	Beer (lager)
10	0.98471	20	Wine (light)
15	0.97797	30	Fortified wine
20	0.96864	40	Liqueurs
25	0.95895	50	Soju, sake
30	0.94824	60	Light spirits
35	0.93625	70	Some liqueurs
37.5	0.92980	75	EU minimum spirit
40	0.92249	80	Vodka, gin, rum
43	0.91440	86	Bourbon (common)
45	0.90881	90	Cask strength Scotch
50	0.89351	100	Navy strength gin
55	0.87768	110	Cask strength whisky
60	0.86098	120	High-proof rum
65	0.84344	130	New make spirit
70	0.82489	140	Distillate cut
75	0.80530	150	Heads / tails
80	0.78461	160	High-proof distillate
85	0.80953	170	Rectified spirit
90	0.81849	180	Neutral grain spirit
95	0.80458	190	Everclear
96.5	0.80043	193	Azeotrope limit
100	0.78934	200	Absolute ethanol

Frequently Asked Questions

Ethanol molecules (CH₃CH₂OH) form hydrogen bonds with water molecules, allowing them to pack more tightly than in their pure states. At around 46% ABV the contraction reaches its maximum of approximately 3.7% by volume. This means adding 500 mL of water to 500 mL of ethanol yields roughly 964 mL, not 1000 mL. The calculator uses OIML density data to account for this non-linear contraction at every concentration.

Hydrometers and most alcoholmeters are calibrated at 20 °C (68 °F). Liquid at 25 °C appears less dense, so the hydrometer reads a higher ABV than actual. A spirit measured at 30 °C showing 40% ABV might truly be 38.5% at 20 °C. This calculator includes a temperature correction field that adjusts your apparent reading to the true ABV at 20 °C before computing dilution volumes.

Only approximately. Dissolved sugars increase solution density independently of ethanol content. A 25% ABV liqueur with 200 g/L sugar has a density close to a 15% ABV pure ethanol-water solution. The calculator assumes a pure binary ethanol-water system. For sugared products, measure ABV with a distillation-based method (ebulliometer or distilling hydrometer) rather than a simple floating hydrometer, then enter the corrected ABV.

Ethanol and water form a positive azeotrope at 95.63% ABV (78.15 °C at 1 atm). Standard distillation cannot exceed this concentration. To reach absolute (100%) ethanol, techniques such as molecular sieve desiccation, azeotropic distillation with benzene (historical, toxic), or membrane pervaporation are required. This calculator accepts inputs up to 96.5% as a practical ceiling.

Add water to alcohol. The exothermic reaction heats the mixture, and adding water to a large volume of alcohol keeps the temperature rise moderate and distributed. Adding alcohol to water can cause localized heating and turbidity (louche effect in anise spirits). Always use distilled or deionized water at room temperature to avoid introducing minerals that affect flavor.

Allow a minimum of 24 hours at stable room temperature. Ethanol-water mixing is exothermic and the solution needs time to reach thermal equilibrium and complete molecular rearrangement. In practice, craft distillers wait 48-72 hours for spirits above 60% initial ABV, as micro-bubbles and thermal gradients can persist. Stir gently after adding water, seal the container, and measure only when the liquid is at 20 °C.