per mille
Dimensionlessexact by definition
Per mille is one part in one thousand, exactly 0.001, the same idea as percent with a thousand in the denominator. It is the working unit of stable isotope geochemistry, where δ¹⁸O and δ¹³C are reported per mille against a standard, of historical seawater salinity, and of gradient in some European rail and drainage practice. The factor is exact by definition.
Watch out: The proper symbol is a percent sign with a second zero in the denominator, which has no font metrics in this site's renderer and is spelled out here instead. Be careful with isotope deltas: they are already defined as a per mille deviation from a reference, so a δ value of minus 8 means minus 8 per mille and must not be multiplied by 0.001 twice.
| 1 per mille | 0.001 — |