Miles and kilometers ( km ) are both units of length usually used to measure out large traveling distances. The km is an SI unit defined as 1000 meters. A mile is an English unit set to be equal to 5280 feet.
The conversion factor between miles and meters was set to 1 mile = 1609.344 meters. Since there are 1000 meters in a kilometer, we can set the conversion factor to
1 mile = 1.609344 km
The conversion factor most often used in general approximations is
1 mile = 1.609 km
These two example problems will show how to use this conversion factor to convert miles to km and convert km to miles.
Convert miles to km Example Problem
Question: A common U.S. Interstate highway speed between cities is 75 miles per hour (mph). What is this speed in km/hr or kph?
Solution: 75 miles per hour means you will travel 75 miles in one hour. Use the conversion factor to find out how many kilometers are equal to 75 miles.
75 miles = 75 miles · conversion factor
Make sure to put the unit we want in the numerator of the conversion factor. In this case, we want kilometers.
By putting kilometers in the numerator, the miles unit will cancel out
75 miles = 120.701 km
In one hour, a person traveling at 75 mph will travel 120.701 km.
Answer: 75 mph is equal to 120.701 kph.
Convert km to miles Example Problem
Question: A common foot race is a 10k run or 10 km. How far is this in miles?
Solution: Apply the conversion factor to 10 km.
10 km = 10 km · conversion factor
This time, put miles in the numerator to cancel out the km unit.
10 km = 6.2 miles
Answer: A 10k run is a 6.2-mile race.
It’s easy to convert between miles and kilometers as long as you remember the conversion factor. A quick way to check your work is to remember that when you’re finished, you should have a larger number for the kilometers measurement than the miles measurement.