How the speed units relate
Every speed is a distance divided by a time, so every conversion is a distance factor and a time factor multiplied together. Because the metre, the foot and the mile are all defined exactly, the factors below are exact, not rounded.
- km/h: 1,000 meters in 3,600 seconds, so 1 m/s is exactly 3.6 km/h.
- mph: the international mile is exactly 1,609.344 meters (the yard has been exactly 0.9144 m since 1959), so 1 mph is exactly 0.44704 m/s and 1.609344 km/h.
- ft/s: 1 foot is exactly 0.3048 m, so 1 mph is 22/15 ft/s, and 60 mph is exactly 88 ft/s.
- Knot: one nautical mile (exactly 1,852 meters) per hour, so 1 knot is 1.852 km/h or about 1.1508 mph.
To go from mph to km/h multiply by 1.609344, and to go the other way multiply by 0.6213711922 (which is 1 divided by 1.609344). The pages for mph to km/h and km/h to mph give the table and the mental shortcuts for each direction.
Mach and the speed of light
The speed of light in a vacuum is exactly 299,792,458 m/s. Since 1983 that number defines the meter, so it has no measurement error. In km/h it is 1,079,252,848.8.
Mach is different. It is a ratio: your speed divided by the speed of sound in the surrounding air, and the speed of sound depends on temperature. This converter uses 340.294 m/s, the speed of sound at sea level in the International Standard Atmosphere (15 °C). That makes Mach 1 about 1,225 km/h or 761 mph. In the cold air at airliner cruising height, around -56 °C in the standard atmosphere, sound travels at about 295 m/s, so Mach 1 there is only about 1,062 km/h. That is why the unit is marked approximate and why the page warns you about it. For real flight work use the temperature of the air you are in.
Which unit is used where
- mph is the unit on road signs in the United States and the United Kingdom. Most other countries post km/h.
- km/h is the everyday road unit for most of the world and is common for wind speed and cycling.
- m/s is the scientific and engineering unit, and the one meteorologists often use for wind.
- Knots are used at sea and in aviation, because a nautical mile is tied to the size of the Earth.
- ft/s shows up in US engineering, ballistics and sport, for example a pitch or a projectile.
For a quick estimate in your head, multiply km/h by 0.62 to get mph, multiply mph by 1.6 to get km/h, and multiply m/s by 3.6 to get km/h.
What this converter does not do
It converts a speed number, not a pace. Running pace such as 5:30 per kilometer is time per distance, which is the reciprocal of speed, and it is not in the list. Speedometer error and legal speed limits are also outside what a unit converter can tell you.