To convert km/h to mph, divide by 1.609344 (or multiply by 0.621371), so 100 km/h is 62.14 mph. To convert mph to km/h, multiply by 1.609344. Our free speed converter handles km/h, mph, meters per second, knots, feet per second and Mach in one place.
Every speed unit is a distance divided by a time, so speed conversion is really length conversion with a clock attached. Once you know how many meters per second each unit equals, any conversion takes one multiplication and one division.
Speed vs velocity
Speed is how fast something moves. Velocity is speed plus a direction, such as 20 m/s due north. Converting units changes only the number and the unit, never the direction, so the same factors apply to both. A car doing 100 km/h east has a velocity of 62.14 mph east.
Speed units and where they are used
| Unit | Symbol | Meters per second | Typical use |
|---|---|---|---|
| Meters per second | m/s | 1 | Science, engineering, some wind forecasts |
| Kilometers per hour | km/h | 0.277778 (1 ÷ 3.6) | Road speeds in most countries |
| Miles per hour | mph | 0.44704 | Road speeds in the US and UK |
| Knot | kn | 0.514444 | Aviation, shipping, marine weather |
| Feet per second | ft/s | 0.3048 | US engineering and physics problems |
| Mach | M | about 343 (varies) | Aircraft speed relative to the speed of sound |
A knot is one nautical mile per hour, and a nautical mile is exactly 1,852 meters. Navigators use it because a nautical mile was originally based on one minute of latitude, which makes distances easy to read off a chart.
Speed conversion formulas
result = value × (m/s per "from" unit) ÷ (m/s per "to" unit)
km/h → mph: mph = km/h ÷ 1.609344
mph → km/h: km/h = mph × 1.609344
km/h → m/s: m/s = km/h ÷ 3.6
m/s → km/h: km/h = m/s × 3.6
knots → km/h: km/h = knots × 1.852
knots → mph: mph = knots × 1.150779Worked example: km/h to mph
Convert 100 km/h to mph:
- Convert to meters per second: 100 ÷ 3.6 = 27.78 m/s.
- Divide by the size of 1 mph in m/s: 27.78 ÷ 0.44704 = 62.14 mph.
The one-step version gives the same answer: 100 ÷ 1.609344 = 62.14 mph.
Worked example: mph to km/h
Convert 65 mph to km/h: 65 × 1.609344 = 104.61 km/h.
Worked example: knots for wind and boats
A 25-knot wind is 25 × 1.852 = 46.3 km/h, 25 × 1.150779 = 28.77 mph, or 25 × 0.514444 = 12.86 m/s. A knot is about 15% faster than a mile per hour, so the knot figure is always the smaller number.
Running pace from speed
Runners think in pace (minutes per kilometer or mile) rather than speed. The converter gives speeds, and pace is one division away:
pace (min per km) = 60 ÷ speed in km/h
pace (min per mile) = 60 ÷ speed in mphAt 12 km/h, pace is 60 ÷ 12 = 5:00 per km. Converting 12 km/h gives 7.46 mph, so the pace is 60 ÷ 7.46 = 8.04 minutes, or about 8:03 per mile.
What Mach 1 means
A Mach number is speed divided by the speed of sound in the surrounding air, and that speed depends mostly on air temperature. The converter uses 343 m/s for Mach 1 (about 1,235 km/h or 767 mph), the speed of sound in dry air at around 20 °C (68 °F). In colder air sound travels slower: near −56 °C, typical of airliner cruising altitudes, it is about 295 m/s.
A common approximation for the speed of sound in air is 331.3 + 0.606 × T m/s, where T is the air temperature in Celsius. If you have the temperature in Fahrenheit, convert it first with the temperature converter; our article on temperature scales explains the formulas. Treat Mach results from the converter as a warm, low-altitude estimate.
How to use the Speed Converter
- Open the Speed Converter. It starts at 100 km/h to mph, which shows 62.14.
- Enter your speed in the Value field. Decimals are fine; negative values are rejected.
- Choose the starting unit in the From list: meters per second, kilometers per hour, miles per hour, knots, Mach or feet per second.
- Choose the target unit in the To list.
- Read the Result, rounded to two decimal places. It updates as you type.
- Use the swap button between the lists to reverse the conversion, for example from mph back to km/h.
Common speed conversion mistakes
- Treating knots as miles per hour. One knot is 1.151 mph, so reading a 30-knot wind as 30 mph understates it by about 15%.
- Averaging speeds instead of distances. Drive 60 km at 60 km/h, then 60 km at 30 km/h, and your average is not 45 km/h. You covered 120 km in 3 hours, so it is 40 km/h. Average speed is total distance divided by total time.
- Rounding 1.609344 to 1.6. Fine for a quick estimate, but at 120 km/h it gives 75 mph instead of 74.56 mph.
- Assuming Mach is a fixed speed. Mach 1 changes with air temperature, so a single km/h value for "the speed of sound" only holds for one set of conditions.
- Mixing up kph and km/h. Both mean the same thing; km/h is the standard symbol.
Because speed is distance over time, the length converter is the natural companion when a trip is planned in one unit and measured in another. Our guide to length units lists the exact distance factors behind every speed conversion here.
Frequently asked questions
How do I convert km/h to mph?
Divide the speed in km/h by 1.609344, or multiply it by 0.621371. For example, 80 km/h is 49.71 mph.
How do I convert m/s to km/h?
Multiply meters per second by 3.6. A speed of 10 m/s is 36 km/h. To go from km/h to m/s, divide by 3.6.
How fast is one knot?
One knot is one nautical mile per hour: exactly 1.852 km/h, about 1.151 mph, or 0.514 m/s.
How fast is Mach 1?
Mach 1 is the speed of sound, about 343 m/s (1,235 km/h or 767 mph) in dry air at 20 °C. It is lower in colder air, so the figure changes with altitude and weather.
What is the difference between speed and velocity?
Speed is how fast an object moves; velocity is speed in a stated direction. Unit conversion is identical for both, because changing units does not change the direction.
Why do pilots and sailors use knots?
A knot is one nautical mile per hour, and the nautical mile was based on one minute of latitude. That ties speed and distance directly to the latitude scale on navigation charts.