Speed Converter (km/h, mph, m/s, knot, Mach)

How speed conversion works

All units pivot through meters per second internally. 1 km/h = 1/3.6 m/s, 1 mph = 0.44704 m/s (exactly), 1 knot = 1852/3600 m/s. These factors are derived from exact length and time definitions, so the conversion is numerically exact.

Mach is special: it's a ratio to the local speed of sound, which depends on temperature and altitude. We use 343 m/s (sea level, 20°C) for the conversion. Real-world Mach numbers in aviation differ slightly because cruise altitudes are colder.

Where each unit appears

Vehicles: km/h in most of the world, mph in the US and UK. Speed limits, speedometers, and vehicle specs use these.

Aviation: knots (1 nautical mile per hour ≈ 1.852 km/h) for ground speed and indicated airspeed. Mach for jets at high altitude.

Marine: knots for ship and boat speed, related to nautical miles for navigation.

Physics and engineering: m/s as the SI base unit. Wind speed in meteorology often uses m/s in scientific contexts and km/h or mph in consumer weather apps.

Useful reference points

Walking: ~5 km/h ≈ 3 mph ≈ 1.4 m/s. Running: 10 km/h ≈ 6 mph for a moderate jog. Highway driving: 100 km/h ≈ 62 mph ≈ 28 m/s. Cruising airliner: 850 km/h ≈ 530 mph ≈ Mach 0.78. Speed of sound: 343 m/s ≈ 1235 km/h ≈ Mach 1.0. Speed of light: 299,792,458 m/s exactly.