How to use the speed converter
- Enter the speed in Value.
- Choose what it is measured in under From, for example km/h on a road sign.
- Choose the unit you want under To and read the result.
- The table below lists the speed in all seven units side by side.
- Hit the swap button to convert in the opposite direction.
What it does and when to use it
Seven speed units are supported: metres per second, kilometres per hour, miles per hour, knots, feet per second, Mach, and the speed of light. Drivers use it to read speed limits abroad, sailors and pilots to switch between knots and km/h, students to turn m/s from a physics problem into something they can picture, and sports fans to compare a serve or pitch speed quoted in mph with one in km/h.
If you are planning a trip, the length converter turns the distance into miles or kilometres, and the time converter helps when a journey time is given in minutes and you want hours.
How it works
Each unit is stored as metres per second:
- 1 km/h = 1,000 m ÷ 3,600 s = 1/3.6 m/s
- 1 mph = 1,609.344 m ÷ 3,600 s = 0.44704 m/s
- 1 knot = 1,852 m ÷ 3,600 s ≈ 0.5144 m/s
- 1 ft/s = 0.3048 m/s
- Mach 1 = 340.294 m/s (the speed of sound in the ICAO standard atmosphere at sea level, 15 °C)
- speed of light = 299,792,458 m/s, exact by the SI definition of the metre
The tool turns your value into m/s and then into the target unit. Every factor except Mach is exact.
Worked examples
The converter produced each of these results.
A motorway limit. 100 km/h = 62.1371192237 mph. A 120 km/h limit is about 74.6 mph.
A boat’s cruising speed. 30 knots = 55.56 km/h, exactly, since a knot is defined from the 1,852 m nautical mile.
The sound barrier. Mach 1 = 1,225.0584 km/h at sea level in standard conditions.
Limits and tips
- Mach is a fixed reference here, not a real reading at altitude. Aircraft instruments work out Mach from the actual air temperature, so a jet at cruising height reaches Mach 1 at a lower true speed than shown here. The tool reminds you of this with a note under each result.
- Speed of light values are tiny for everyday speeds and switch to scientific notation.
- Wind scales such as Beaufort are not included.
- Fuel use depends on speed, but converting mpg and L/100 km is a different calculation; the fuel converter handles it.
Frequently asked questions
How do I convert km/h to mph in my head?
What is a knot?
How fast is Mach 1?
How do I turn m/s into km/h?
Can I convert running pace like 5:30 per km?
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