Speed Conversions — mph, km/h, Knots and Mach Explained

Speed is measured differently depending on whether you're driving, flying or sailing. Here's why different industries use different units and how to convert between them.

The mile per hour (mph) is primarily used in the United States and United Kingdom for road speeds. The kilometre per hour (km/h) is used everywhere else for road and general speed measurement. Converting between the two is straightforward: multiply mph by 1.60934 to get km/h, or divide km/h by 1.60934 to get mph. The approximate rule of thumb — 60mph ≈ 100km/h — is convenient and accurate to within 4%.

Knots are used in aviation and maritime navigation. One knot is one nautical mile per hour, and a nautical mile is defined as one minute of arc along a meridian of the Earth — approximately 1,852 metres. This makes knots particularly useful for navigation because the relationship between speed, distance and position on a chart is direct. One knot = 1.852 km/h = 1.15078 mph.

Mach numbers measure speed relative to the speed of sound, which varies with altitude and temperature. At sea level and 15°C, the speed of sound is approximately 340 metres per second (761 mph, 1,225 km/h). Mach 1 is the speed of sound, Mach 2 is twice the speed of sound, and so on. Concorde cruised at Mach 2.04 — about 2,170 km/h at its cruising altitude. Commercial aircraft typically cruise at Mach 0.78-0.85.

Metres per second (m/s) is the SI unit for speed and the preferred unit in scientific contexts. It relates cleanly to other SI units — a force of one newton accelerates one kilogram by one metre per second squared — making it essential in physics and engineering.