Alcoholometric tables
Also known as: OIML R 22, density tables, TTB gauging tables
Measure the density, read off the percentage — that is the whole mechanism. Alcoholometric tables link the density of an ethanol-water mixture to its strength, and duty accounting, the certification of hydrometers and the spirit trade all rest on them.
The reference document is OIML R 22, a recommendation of the International Organization of Legal Metrology. It is not a national invention: the same tables apply in Poland, Germany, France and most of the world. The United States stands apart with its TTB tables, calculated from 60°F.
Inside you will find a grid at 1% steps referred to 20°C, corrections for measuring at other temperatures, and conversions between percentage by volume and by weight.
Someone might ask why a table is needed when there is a formula. There is no formula — the density of an ethanol-water mixture is not a linear function of composition, precisely because of volume contraction. You cannot get it by averaging the densities of the components. It takes either a measured table or a serious equation of state.
The calculator on this site works from an approximation, not from the OIML tables. For diluting at home that is ample — the differences run to fractions of a percent. For tax purposes it is not, and it should not be used that way.