Speed Converter
Convert a speed between kilometres per hour, miles per hour, metres per second, knots, feet per second and the speed of light. The factors are exact, and a table shows your speed in every unit.
Result
Enter a value to convert.
The calculation runs in this browser tab. Nothing you enter is uploaded, saved or logged.
How are units of speed defined?
A speed is a distance divided by a time, so every unit of speed combines a unit of length with a unit of time. The SI unit is the metre per second. The kilometre per hour, used on road signs in most of the world, is 1,000 metres per 3,600 seconds, so 1 km/h is exactly 5/18 m/s and 1 m/s is exactly 3.6 km/h.
The mile per hour, used on road signs in the United States and the United Kingdom, is exactly 0.44704 m/s because a mile is exactly 1,609.344 metres. The knot is one nautical mile (1,852 m) per hour, used at sea and in aviation. The speed of light in a vacuum is exactly 299,792,458 m/s, which is how the metre is defined.
How to use the Speed Converter
- Enter the speed to convert. Negative values are allowed and mean the opposite direction.
- Choose the unit you are converting from and the unit you are converting to.
- Read the result, the conversion factor and its inverse. A leading ≈ means the value was rounded to the selected precision.
- Use the table to see the speed in every unit, and Copy result to copy the number alone.
Formula
- General rule
- result = value × (m/s per source unit) ÷ (m/s per target unit)
- km/h to mph
- mph = km/h ÷ 1.609344 ≈ km/h × 0.6213712
- m/s to km/h
- km/h = m/s × 3.6
- Knots to km/h
- km/h = knots × 1.852
Exact relations: 1 knot = 1.852 km/h; 60 mph = 88 ft/s; 36 km/h = 10 m/s.
Speeds near the speed of light are not converted with relativistic corrections; the speed of light is offered as a reference unit only.
Worked example
A speed limit of 100 km/h in miles per hour. These are the results the converter gives:
| Question | Result |
|---|---|
| Input: Value | 100 |
| Input: From | Kilometres per hour (km/h) |
| Input: To | Miles per hour (mph) |
| Input: Precision | 10 significant digits |
| 100 km/h equals | ≈ 62.13711922 mph |
| Conversion factor | 1 km/h = ≈ 0.6213711922 mph |
| Inverse | 1 mph = 1.609344 km/h |
| Formula | mph = km/h × ≈ 0.621371192237334 |
Assumptions and limits
- Mach is not offered because the speed of sound depends on temperature and altitude; there is no single factor.
- The speed of light is the speed in a vacuum. It is slower in air, water and glass.
- Results are shown to 6 to 15 significant digits. Non-terminating conversions such as km/h to mph are rounded and marked ≈.
- The converter changes units only; it does not compute a speed from a distance and a time. Divide the distance by the time first.
Frequently asked questions
How do I convert km/h to mph?
Divide by 1.609344, or multiply by 0.621371. So 100 km/h is about 62.14 mph, and a quick estimate is to multiply by 0.6.
How do I convert mph to km/h?
Multiply by 1.609344. A speed of 70 mph is 112.65 km/h, and a quick estimate is to multiply by 1.6.
How do I convert m/s to km/h?
Multiply by 3.6, because there are 3,600 seconds in an hour and 1,000 metres in a kilometre. So 10 m/s is 36 km/h, and 100 km/h is 27.78 m/s.
What is a knot?
A knot is one nautical mile per hour, exactly 1.852 km/h or about 1.15078 mph. It is the standard unit of speed at sea and in aviation.
How fast is the speed of light in km/h?
299,792,458 m/s is 1,079,252,848.8 km/h, about 1.08 billion kilometres per hour. The converter lists the speed of light as a unit so you can express a speed as a fraction of it.
Why is there no Mach number?
Mach 1 is the speed of sound, which changes with the air temperature and altitude (about 343 m/s at 20 °C, but about 295 m/s at 11 km altitude). A single conversion factor would be misleading.