How the conversion works
Every speed is a distance over a time, so speed units convert with the same exact factors as the distances behind them. A mile is exactly 1.609344 km, so a mile per hour is exactly 1.609344 km/h. A nautical mile is exactly 1,852 m, so a knot is exactly 1.852 km/h. And because an hour is 3,600 seconds, a metre per second is exactly 3.6 km/h.
The tool converts your value into metres per second first, then back out into every other unit, rounding only at the moment of display. The one non-exact unit is Mach: the tool uses the sea-level speed of sound in air at 15°C, which is 340.29 m/s (about 761.2 mph or 1,225 km/h). The speed of sound drops as air gets colder and thinner, so at airliner cruising altitude Mach 1 is nearer 660 mph. Treat the Mach row as a sea-level approximation, not a fixed constant.
Worked examples
Quick reference
- 30 mph = 48.28 km/h, and 50 km/h (the common urban limit abroad) = 31.07 mph
- 1 m/s = 3.6 km/h = 2.237 mph, handy for wind gauges and physics homework
- 20 knots = 37.04 km/h = 23.02 mph, a brisk sailing breeze
- A gale starts at 34 knots in the shipping forecast, which is about 39 mph
- A cruising airliner at 900 km/h is doing 559 mph, or about 486 knots
- Mach 1 at sea level ≈ 761.2 mph = 1,225 km/h = 340.29 m/s
Why so many speed units survive
Britain drives in mph while most of the world signs its roads in km/h, which is where the bulk of these conversions come from: hire cars abroad, imported cars with km/h dials, sat navs set to the wrong unit, and speed camera stories from European holidays. Weather adds another layer, because UK forecasts give wind in mph but marine and aviation forecasts use knots, and the shipping forecast's gale warnings are defined in knots too.
Metres per second is the scientist's unit, and the one you meet in physics questions, wind turbine specs and athletics analysis. It is the easiest to reason with: multiply by 3.6 for km/h and you are back in road units. Knots survive at sea and in the air because the nautical mile matches one minute of latitude, which makes chart navigation cleaner, and Mach survives because what matters to an aircraft near the speed of sound is the ratio, not the raw number.
For distances rather than speeds, our miles to km converter has running and driving references built in, and if you are converting speed to work out fuel for a journey, the mpg converter and fuel cost calculator carry on where this page stops.
Frequently asked questions
How do I convert mph to km/h?
Multiply by 1.609344, the exact number of kilometres in a mile. So the UK motorway limit of 70 mph is 112.65 km/h. Going the other way, divide by 1.609344: a 100 km/h limit abroad is 62.14 mph.
What is a knot?
One nautical mile per hour. The nautical mile is exactly 1,852 metres, so a knot is 1.852 km/h, or about 1.15 mph. Knots are the standard for ships, aircraft and wind speeds in marine forecasts, because the nautical mile matches one minute of latitude.
How fast is Mach 1?
About 761 mph or 1,225 km/h at sea level in air at 15 degrees Celsius, which is the figure this tool uses. The speed of sound falls as air gets colder, so at airliner cruising altitude Mach 1 is closer to 660 mph. Treat any Mach conversion as approximate unless you know the air conditions.
What is a fast running speed in mph?
A 30 minute 5K works out at 10 km/h, which is 6.2 mph. An elite marathon runner holds around 21 km/h (13 mph) for the whole race, and Usain Bolt's 9.58 second 100 m averaged 37.6 km/h, about 23.4 mph.
Why are UK speed limits in mph when most countries use km/h?
Britain planned to metricate road signs in the 1970s but never did, so speed limits, distance signs and car odometers stayed in miles. Most of the rest of the world signs in km/h, which is why hire cars abroad and imported dash cams so often need this conversion.