Alcohol by volume (ABV)
Also known as: ABV, % vol., degrees Gay-Lussac, proof
The percentage on the label tells you how much of the volume is pure ethanol. Vodka at 40% holds 400 ml of absolute alcohol per litre. ABV stands for alcohol by volume.
That small asterisk: 20°C
In Poland and across the EU, strength is referred to 20°C, and that is not a formality. Liquids expand when warmed, and ethanol does so noticeably more eagerly than water. The same liquid measured in a summer kitchen and in a cold cellar gives two different readings — a few degrees means a few tenths of a percent.
The United States measures from 60°F, that is 15.56°C, so American tables do not map directly onto European ones.
By volume or by weight
Alongside ABV there is ABW, the percentage by weight. Ethanol is lighter than water, so the same content expressed by weight gives a smaller number: 40% ABV is about 33.3% ABW. If a table from a foreign source ever looked wrong to you, check which scale it uses before anything else.
On American bottles you will also meet proof — simply ABV doubled. 40% is 80 proof, 95% spirit is 190 proof. The old British scale worked differently and is no longer used.
To bring a spirit down to a given strength, allowing for contraction, use the calculator.