One percent is exactly 10 per mille. Per mille means parts per thousand and is written with the symbol ‰. English uses it rarely; in Sweden and much of continental Europe it is an everyday word, mainly because of one specific application.
About the units
Per mille is one part in a thousand, standing in the same relation to the thousand as percent does to the hundred. English-language documents almost always prefer a decimal percent, but per mille is standard across the Nordic countries, Germany, the Netherlands and much of eastern Europe. Its most familiar use anywhere is blood alcohol concentration, which is expressed in per mille in most of Europe: the Swedish drink-driving limit is 0.2 ‰, meaning 0.2 grams of alcohol per kilogram of blood. American and British practice uses percent for the same quantity, so a limit of 0.08 % is 0.8 ‰ — four times the Swedish threshold.
The exact factor
The factor is exactly 10. So 0.5 % is 5 ‰, and 2 ‰ is 0.2 %. Other common uses: gradient in railway and road engineering, where a 5 ‰ grade is a rise of 5 metres per kilometre; salinity of seawater, historically about 35 ‰ though modern oceanography uses a dimensionless practical salinity scale; and fee rates in some financial contracts. Isotope ratios in geochemistry are reported as deviations in per mille from a standard, using delta notation.
Where you meet this conversion
Blood alcohol limits, which is where most people meet the symbol. Civil engineering gradients, particularly for railways where the numbers are small — a mainline rarely exceeds 25 ‰. Oceanography and geochemistry use it for salinity and isotope ratios. Financial documents in continental Europe occasionally express fees in per mille. Anyone reading a European regulation or a Nordic engineering drawing meets the symbol and needs the factor of ten to relate it to a percent.