Speed Converter

Last updated: 2026-04-30

Speed Is the Most Context-Dependent Conversion

Length and mass are read off measuring devices. Speed is read off completely different devices depending on what's moving — and each industry has settled on its own unit. Cars use km/h or mph; ships and aircraft use knots; runners think in min/km or min/mi (a separate calculator handles those); engineers use m/s; physicists scale by the speed of light or the speed of sound. Same dimension, very different conventions.

This converter handles the major members of the family, including Mach number (relative to the speed of sound) and c (speed of light) for cases where you need an 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.