The five speed units and where they come from
This converter holds meters per second, kilometers per hour, miles per hour, feet per second and knots against the metre per second, the SI unit. Each other unit is a distance over an hour or a second, so its factor is the distance factor divided by the time: km/h is 1000 / 3600 = 0.27778 m/s, and 1 mph is exactly 1609.344 / 3600 = 0.44704 m/s. A knot is one nautical mile (1,852 m) per hour, which is 1.852 km/h or 0.514444 m/s, and about 1.15078 mph.
Worked examples
- 60 mph × 0.44704 / 0.27778 = 96.56 km/h.
- A boat at 20 knots travels at 20 × 1.852 = 37.04 km/h, or 23.02 mph.
- 100 km/h is 62.14 mph, 27.78 m/s, 53.996 knots and 91.13 ft/s.
- To go from km/h to m/s, divide by 3.6. To go from m/s to km/h, multiply by 3.6.
Knots and Mach
Ships and aircraft use knots because a nautical mile corresponds to one minute of latitude, so a speed in knots maps to distance on a chart. "Knots per hour" is therefore a mistake: the knot already includes the "per hour". Airspeed indicators, wind forecasts at sea and METAR reports use knots, while road signs use mph or km/h.
Mach number is not a fixed speed, and the converter does not offer it. Mach 1 is the local speed of sound, which depends on air temperature (proportional to the square root of absolute temperature), not on altitude as such. It is about 340 m/s at 15 °C, around 343 m/s at 20 °C, and roughly 295 m/s in the cold upper troposphere, so Mach 0.85 is a different km/h at different altitudes. Always get the temperature before converting a Mach figure by hand.
| Speed | km/h | mph | knots |
|---|---|---|---|
| Walking | 5 | 3.11 | 2.70 |
| Common urban limit | 50 | 31.07 | 27.0 |
| US highway | 104.6 | 65 | 56.5 |
| Fast ferry | 55.56 | 34.52 | 30 |
Limits and mistakes
- Run pace is the reciprocal of speed, minutes per kilometre rather than kilometres per hour. Use the running pace calculator for that.
- Rounding to seven significant digits can hide that a speed limit conversion is rarely exact: 70 mph is 112.65 km/h, but signs round to 110 or 112.
For the distances behind these units, see the length converter.