Miles into kilometres, and why the number grows
A mile is longer than a kilometre, so any distance in miles becomes a bigger number in kilometres. Sixty miles is 96.6 km; one hundred miles is 160.9 km. The rule of thumb runners use, add 60%, is close but slightly under: the true factor is 1.609344, so a 100 mile figure gains 60.9 km, not 60.
On this site the conversion has a specific job. An American mpg figure is built on miles, and to express it in a metric unit you have to convert those miles to kilometres. That is half of what the MPG to km/L converter does behind the scenes; the other half is the gallon to litre step.
The formula
kilometres = miles × 1.609344
Exact, because the mile is defined as 1,609.344 metres and has been since 1959. There is no measurement uncertainty in the factor. For a quick estimate, multiply by 1.6; for anything that matters, use the full value or the converter above.
Worked examples
The marathon is the classic one: 26.2 miles is 42.16 km, close to the official 42.195 km (the official distance is defined in metres, and 26.2 is itself a rounding). A US interstate speed limit of 70 mph is 112.7 km/h. The distance from New York to Boston, about 215 miles, is 346 km.
For fuel economy: if your trip computer says you covered 312 miles on a tank, that is 502.1 km. Divide the litres you put in by 5.021 and you have your L/100km. The manual calculation guide does the whole method with a real fill-up.
Speed limits abroad
A driver from the US or UK renting a car in Europe sees limits in km/h and a speedometer that may show both. The conversion is the same as for distance because the hour is the same on both sides: 30 mph is 48 km/h, 50 mph is 80 km/h, 70 mph is 113 km/h. Most European motorway limits of 120 or 130 km/h come out at 75 and 81 mph.
Litres, or liters, are what the fuel at the end of that drive will be sold in. The gallons to litres page covers that side of the trip.
Errors to avoid
- Dividing instead of multiplying. Miles to kilometres makes the number larger.
- Treating 26.2 miles as exactly a marathon. The race is 42.195 km; 26.2 miles is 42.165 km.
- Reading a nautical mile as a statute mile. They differ by 15%.
- Forgetting that a British mpg figure also uses the statute mile. The mile is identical in both countries; only the gallon differs.