The formula
km = mi × 1.609344
Where the factor comes from — each unit by its definition, then its size in SI:
1 mi = 1760 × yd = 1,609.344 m1 km = 1000 × m = 1,000 m
Dividing the two SI sizes gives the factor: 1.609344, exact — every step above is a definition, not a measurement. The reverse factor is 0.621371192237.
Every formula on this site takes lengths in metres. Heights, distances and stopping distances quoted in feet, inches or miles have to be converted first — getting it wrong by a factor of 3.28 is the most common unit slip in homework.
Worked examples
- A sprint track straight (100 m): 0.0621371 mi = 0.1 km — 0.0621371 × 1.609344 = 0.1
- A marathon: 26.2188 mi = 42.195 km — 26.2188 × 1.609344 = 42.195
- A tall doorway (6 ft): 0.00113636 mi = 0.0018288 km — 0.00113636 × 1.609344 = 0.0018288
- A school ruler (30 cm): 0.000186411 mi = 0.0003 km — 0.000186411 × 1.609344 = 0.0003
Frequently asked questions
How many kilometres is 1 mi?
1 mi = 1.609344 km, exactly, from the unit definitions.
How do I convert kilometres back to miles?
Divide by 1.609344 — or multiply by 0.621371192237: 1 km = 0.621371192237 mi.