The formula
km/h = mph × 1.609344
Where the factor comes from — each unit by its definition, then its size in SI:
1 mph = mi / h = 0.44704 m/s1 km/h = km / h = 0.277777777778 m/s
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.
Kinematics and energy formulas need speed in metres per second. Road speeds come in km/h or mph, so convert before you square anything: kinetic energy goes with v², so a unit slip is squared too.
Worked examples
- Motorway driving: 62.1371 mph = 100 km/h — 62.1371 × 1.609344 = 100
- A brisk walk: 3.10686 mph = 5 km/h — 3.10686 × 1.609344 = 5
- US highway driving: 60 mph = 96.5606 km/h — 60 × 1.609344 = 96.5606
- A hard-thrown ball (30 m/s): 67.1081 mph = 108 km/h — 67.1081 × 1.609344 = 108
Frequently asked questions
How many kilometres per hour is 1 mph?
1 mph = 1.609344 km/h, exactly, from the unit definitions.
How do I convert kilometres per hour back to miles per hour?
Divide by 1.609344 — or multiply by 0.621371192237: 1 km/h = 0.621371192237 mph.