Kilometres per hour to miles per hour (km/h to mph)

The formula

mph = km/h × 0.6213711922

Where the factor comes from — each unit by its definition, then its size in SI:

• 1 km/h = km / h = 0.277777777778 m/s
• 1 mph = mi / h = 0.44704 m/s

Dividing the two SI sizes gives the factor: 0.621371192237 (rounded for display; the underlying ratio is fixed by the definitions). The reverse factor is 1.609344.

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: 100 km/h = 62.1371 mph — 100 × 0.6213711922 = 62.1371
• A brisk walk: 5 km/h = 3.10686 mph — 5 × 0.6213711922 = 3.10686
• US highway driving: 96.5606 km/h = 60 mph — 96.5606 × 0.6213711922 = 60
• A hard-thrown ball (30 m/s): 108 km/h = 67.1081 mph — 108 × 0.6213711922 = 67.1081

Frequently asked questions