Speed Converter
0.62137119 mph
- 0.277778
- m/s
- 1
- km/h
- 0.621371
- mph
- 0.911344
- ft/s
- 0.539957
- kn
- 0.000816273
- Ma
- 9.2657e-10
- c
The speed converter handles seven units including the two that trip people up: the knot, which is one nautical mile per hour rather than anything to do with statute miles, and Mach, which is not a fixed speed at all but a ratio to the local speed of sound.
How it works
Everything routes through metres per second. Most factors are exact by definition; the two exceptions are worth knowing about.
- km/h to m/s is division by exactly 3.6, since a kilometre is 1,000 m and an hour is 3,600 s.
- mph uses the exact mile of 1,609.344 m, giving 0.44704 m/s exactly.
- A knot is one nautical mile per hour: 1,852 m divided by 3,600 s, also exact.
- Mach is quoted at sea level and 15 °C, where sound travels at 340.3 m/s. At altitude it is considerably slower.
- The speed of light is exactly 299,792,458 m/s. The metre is defined from it, not the other way round.
Examples
A motorway speed limit
Value
112
From
Kilometre per hour
To
Mile per hour
Result
69.593205 mph
112 km/h is the limit in much of Europe and converts to just under 70 mph, which is why the two are often treated as equivalent.
Knots to km/h
Value
1
From
Knot
To
Kilometre per hour
Result
1.852 km/h
Exactly 1.852, because a nautical mile is defined as exactly 1,852 metres. The tidy number is the definition, not a coincidence.
Frequently asked questions
Why is a knot not the same as a mile per hour?
Because it uses the nautical mile of 1,852 m rather than the statute mile of 1,609.344 m. A nautical mile is one minute of latitude, which makes navigation arithmetic straightforward at sea. One knot is about 1.15 mph.
Is Mach 1 a fixed speed?
No, and this is the common misconception. Mach is a ratio to the local speed of sound, which depends on air temperature. At sea level and 15 °C that is 340.3 m/s, but at cruising altitude the air is much colder and Mach 1 is closer to 295 m/s. The figure here is the sea-level standard.
Why is the speed of light an exact number?
Because the metre is defined from it. Since 1983 a metre has been the distance light travels in 1/299,792,458 of a second, which makes that figure exact by definition rather than measured.
What is the quick mental conversion between km/h and mph?
Multiply km/h by 0.6 for a rough mph figure, or by 5/8 for a closer one. 112 km/h × 0.625 gives 70, against the true 69.59, close enough for road signs.