Volume contraction
Also known as: negative excess volume, Mendeleev effect, kontrakcja objętości
Measure out half a litre of spirit, half a litre of water, stir — and you have about 965 ml. Nothing leaked, nothing evaporated, the measuring jug is fine. That is simply what ethanol and water do.
The reason is in the molecule. Ethanol carries an OH group on one end, the same group that holds water together through hydrogen bonds. When the two liquids meet, water slips into those bonds and squeezes between the carbon chains of the ethanol — into space that was empty before. Everything packs tighter and takes up less room.
The proper term is negative excess volume. You will also see it called the Mendeleev effect, after the measurements that worked the system out.
You do not lose percent, you lose litres
This is what people get wrong most often. There is exactly as much absolute alcohol after mixing as before — only the volume holding it has shrunk. Divide the same amount by less, and the strength comes out higher, not lower.
It only stings when you need a set amount. You want a litre at 40%, you work out the water with the simple formula, you pour — and the bottle holds about 970 ml. At roughly 41% rather than 40.
Worst exactly where people calculate most
The effect peaks between 45 and 55% by volume, where it eats about 3.6%, and disappears at the extremes — pure water and pure ethanol have nothing to contract against. The bad luck is that the peak lands squarely in vodka territory.
| Strength | Loss | From 1000 ml you get |
|---|---|---|
| 10% | 1.0% | 990 ml |
| 20% | 2.0% | 980 ml |
| 30% | 2.8% | 972 ml |
| 40% | 3.3% | 967 ml |
| 50% | 3.6% | 964 ml |
| 60% | 3.5% | 965 ml |
| 70% | 3.0% | 970 ml |
| 80% | 2.2% | 978 ml |
| 90% | 1.2% | 988 ml |
Distilleries dodge the problem simply: they weigh rather than measure volume. Mass is additive and holds no surprises — a kilo plus a kilo is always two kilos. Hence the old distilling tables given in grams.
For duty purposes the official alcoholometric tables apply, not a formula.
Our calculator shows both figures: the plain sum of the volumes, and what will actually be in the vessel. The gap surprises people on larger batches — at ten litres it is already a quarter of a litre.