Last updated: 2026-04-30

Speed units

Speed is the rate of change of position — distance per unit time, measured in SI as meters per second (m/s). Different industries use different units by convention: road transport uses km/h or mph; maritime and aviation navigation uses knots; scientific and engineering work uses m/s; aeronautics scales against the speed of sound (Mach); and physics references the speed of light (c). Running pace (min/km or min/mi) is an inverse unit — a separate calculator handles those.

This converter handles the major members of the family, including Mach number (relative to the speed of sound) and c (speed of light) for order-of-magnitude reference.

Conversion Reference

All conversions pivot on meters per second. Imperial speed units inherit exactness from the 1959 inch.

When Each Unit Shows Up

km/h is the universal road-speed unit outside the US, UK, and a few others. mph is the US/UK road default. The two scales are close enough to be confused: 100 km/h ≈ 62 mph, 60 mph ≈ 97 km/h.

Knots are nautical and aeronautical: ships, aircraft, weather forecasts (wind speed in marine and aviation contexts), and tropical-cyclone categories all use knots because one knot is one nautical mile per hour, and one nautical mile is one minute of latitude — so on a chart, the conversion to distance over time is geometrically clean.

m/s is the engineer's and physicist's working unit. Wind speed in scientific weather reports, conveyor-belt speeds, and any equation involving energy ($KE = \frac{1}{2}mv^2$) all expect m/s.

Mach matters only when something is fast enough that the speed of sound itself is the relevant reference: jet aircraft, supersonic flight, projectile aerodynamics. Note that "Mach 1" is not a fixed speed — it depends on temperature and air density, which depend on altitude. The 340.3 m/s figure used here is for dry air at 15 °C at sea level. At 11 km altitude the speed of sound is closer to 295 m/s, so a "Mach 2" cruise at altitude is slower than Mach 2 at the surface.

Speed of light rarely shows up in everyday work but is useful for sanity-checks: light travels Earth's circumference in about 0.13 seconds, the Earth–Moon distance in 1.3 seconds, and the Earth–Sun distance in about 8 minutes 20 seconds.