How to use the Speed Converter
- Enter a speed, such as 100 for a highway speed in km/h.
- Pick the unit you are starting with and the one you need. Press swap to go the other way.
- Read the converted speed and the factor for one unit in each direction.
- See every speed unit at once in the list below and click to copy any of them.
Features
- Exact factors derived from the meter and the second
- Knots for sailing and aviation
- Mach number at standard sea-level conditions
- Speed of light for physics problems
- All units at once with one-click copy
- Clickable table of speed-limit conversions
Speed is distance divided by time
Every speed unit is a length unit over a time unit, so each conversion factor follows directly from the length and time definitions. One mile is exactly 1,609.344 m and one hour is 3,600 s, so 1 mph = 1,609.344 ÷ 3,600 = 0.44704 m/s exactly. Likewise, 1 km/h is 1,000 ÷ 3,600 m/s, which gives the handy rule of dividing km/h by 3.6 to get meters per second.
Where each unit is used
- km/h appears on road signs and speedometers in most of the world.
- mph is used on roads in the United States and the United Kingdom.
- m/s is the SI unit, used in physics, engineering and for wind speed in many weather services.
- Knots measure speed at sea and in the air. One knot is one nautical mile per hour, exactly 1,852 m per hour. The name comes from the old chip log: sailors let a knotted rope run out behind the ship and counted how many knots passed in a timed interval.
- Mach compares a speed with the local speed of sound.
Mach is not a fixed unit
The speed of sound depends on air temperature. In the International Standard Atmosphere at sea level and 15 °C it is about 340.3 m/s (1,225 km/h or 761 mph), and that is the value this converter uses. At an airliner’s cruising altitude, where the air is around −56 °C, sound travels at only about 295 m/s, so Mach 1 up there is roughly 13% slower in absolute terms. Treat Mach conversions as approximate unless you know the actual temperature.
The speed of light, by contrast, is exact: 299,792,458 m/s, because the meter is defined from it.
Worked examples
Highway speed: 100 km/h ÷ 1.609344 = 62.14 mph.
US interstate: 70 mph × 1.609344 = 112.65 km/h.
Sailing: 25 knots × 1.852 = 46.3 km/h, or 28.77 mph.
Sprinting: running 100 m in 10 s is 10 m/s × 3.6 = 36 km/h.
Speed-limit reference
| km/h | mph | m/s |
|---|---|---|
| 30 | 18.64 | 8.33 |
| 50 | 31.07 | 13.89 |
| 80 | 49.71 | 22.22 |
| 100 | 62.14 | 27.78 |
| 120 | 74.56 | 33.33 |
Tips
For a quick mental estimate, km/h × 5/8 is close to mph (100 km/h ≈ 62.5 mph), and mph × 1.6 is close to km/h. Runners can turn speed into pace by dividing 60 by the speed: 10 km/h is a 6:00 minute-per-kilometer pace. To work out fuel for a trip at a given speed and distance, try the Fuel Cost Calculator.
Frequently asked questions
How do I convert km/h to mph?
Divide by 1.609344, or multiply by about 0.621371. For example, 80 km/h is about 49.7 mph.
How do I convert m/s to km/h?
Multiply by 3.6. One meter per second is 3,600 meters per hour, which is 3.6 km/h.
What is a knot in km/h and mph?
One knot is exactly 1.852 km/h, which is about 1.1508 mph. It equals one nautical mile per hour.
How fast is Mach 1?
At sea level in standard 15 °C air, Mach 1 is about 340.3 m/s, 1,225 km/h or 761 mph. It is slower in colder air, such as at high altitude.
Are these speed conversions exact?
Yes for km/h, mph, m/s, ft/s, knots and the speed of light, which all come from exact definitions. Mach depends on air temperature and is approximate.