One Scandinavian mil is exactly 10 kilometres. It is a living unit, not a historical curiosity: Swedes and Norwegians give driving distances in mil in ordinary conversation, and Swedish fuel consumption is quoted in litres per mil rather than per hundred kilometres.
About the units
The mil is a Nordic distance unit that predates metrication by centuries and varied considerably by region and era. The Swedish mil was fixed at 36,000 Swedish feet in 1699, which came to 10,688 metres, while Norwegian and Danish versions differed again. When Sweden adopted the metric system, the mil was not abolished but redefined: from 1889 it has been exactly 10 kilometres. That decision is the reason it survived. A unit that had been rounded to a convenient metric value could stay in daily use without causing arithmetic problems, and it filled a genuine gap — Sweden is long enough that road distances in kilometres run to four digits, while in mil they stay comfortably in two or three.
The exact factor
The factor is exactly 10, so the conversion is a single decimal shift. 25 mil is 250 km; 4.5 mil is 45 km. Two related units follow the same logic and cause more trouble: the square mil is 100 square kilometres, not 10, because the factor squares. And Swedish fuel economy in litres per mil converts to litres per 100 km by multiplying by ten — 0.6 l/mil is 6 l/100 km — which is the figure most of Europe uses.
Where you meet this conversion
Anyone reading Swedish or Norwegian sources meets this immediately. Road distances, driving times, commuting statistics and cycling routes are all given in mil. Swedish car reviews state consumption in litres per mil, and Swedish tax rules for mileage allowance are written per mil. Norwegian usage is the same. The pitfall for a non-Scandinavian reader is assuming the word means the English mile, which would understate every distance by more than a factor of six.