Home / Converters / Speed Converter
Speed Converter
Convert a speed between road, marine, aviation and scientific units. Enter the number, choose the unit you have and the one you need, and read the same speed in every other unit below.
The metre per second as the common base
Speed is distance divided by time, so each unit here combines a length unit and a time unit. The converter uses metres per second as its hub and applies these exact relationships:
- 1 km/h = 1/3.6 m/s, because 1,000 m ÷ 3,600 s = 0.2778 m/s
- 1 mph = 0.44704 m/s exactly, from 1,609.344 m ÷ 3,600 s
- 1 knot = 1,852 m ÷ 3,600 s ≈ 0.5144 m/s (one nautical mile per hour)
- 1 ft/s = 0.3048 m/s exactly
- The speed of light in a vacuum is exactly 299,792,458 m/s
Since all of these are exact, the only rounding comes from the digits shown.
What the Mach figure means
Mach is not a fixed speed. A Mach number is a ratio: the speed of an object divided by the local speed of sound. Sound travels faster in warm air and slower in cold air, so Mach 1 changes with temperature and therefore with altitude.
This converter states its reference openly: Mach 1 is taken as 340.29 m/s, the speed of sound in dry air at 15 °C and sea-level pressure in the International Standard Atmosphere. That equals about 1,225 km/h or 761 mph. At 20 °C the speed of sound is nearer 343 m/s, and in the cold air of the lower stratosphere it falls to roughly 295 m/s. For an aircraft at altitude, use the true local speed of sound rather than this sea-level value.
Knots, mph and common conversions
| Speed | km/h | mph |
|---|---|---|
| 1 m/s | 3.6 | 2.2369 |
| 1 knot | 1.852 | 1.1508 |
| 30 mph | 48.28 | 30 |
| 60 mph | 96.56 | 60 |
| 100 km/h | 100 | 62.14 |
Ships, aircraft and weather services commonly report knots. Wind forecasts in knots can be read as roughly 1.15 mph or 1.85 km/h per knot.
Mistakes and shortcuts
- Dividing instead of multiplying by 3.6. To go from m/s to km/h you multiply by 3.6; to go from km/h to m/s you divide.
- Mixing statute and nautical miles. Knots are tied to the nautical mile, so 20 knots is about 23 mph, not 20 mph.
- Quoting an average as a limit. A trip that covers 150 km in 2 hours averages 75 km/h, but the instantaneous speed varied along the way.
For quick estimates, multiply km/h by 0.6 to get mph, and multiply mph by 1.6 to get km/h. A speed of 88 ft/s is exactly 60 mph, a handy anchor for physics problems that mix feet and miles.
Worked example
A road limit is posted as 100 km/h. What is that in miles per hour?
Inputs
| Value | 100 |
|---|---|
| From | Kilometres per hour (km/h) |
| To | Miles per hour (mph) |
Result
100 km/h = 62.137119 mph
| Metres per second (m/s) | 27.777778 |
|---|---|
| Knots (kn) | 53.99568 |
| Feet per second (ft/s) | 91.134442 |
| Mach (ISA sea level) (Mach) | 0.08162972 |
| Speed of light (c) | 9.265669e-8 |
- Factors are exact definitions where one exists; results are shown to about six significant digits.
Accuracy note: Factors use the standard definitions stated on this page. Results are rounded for display; for engineering or legal work use the full-precision factors.
Frequently asked questions
How do I convert km/h to mph?
Divide kilometres per hour by 1.609344, or multiply by about 0.6214. For example, 100 km/h is about 62.14 mph.
How do I convert m/s to km/h?
Multiply by 3.6. A speed of 10 metres per second is 36 kilometres per hour.
How fast is one knot?
One knot is one nautical mile per hour, which is exactly 1.852 km/h or about 1.1508 mph.
How fast is Mach 1?
It depends on air temperature. At 15 °C and sea level in the standard atmosphere it is 340.29 m/s, about 1,225 km/h. This converter uses that value as its reference.
Is the speed of light exactly 299,792,458 m/s?
Yes. Since 1983 the metre has been defined from this value, so the speed of light in a vacuum is exact by definition.
Related calculators
More conversion references
For speed conversion tables beyond this tool, see PakMath speed conversion tables. PakMath is a sister project of Adzita.