Speed Converter

All speeds convert through metres per second. The metric-to-imperial link is exact because both the mile and the hour are exactly defined: 1 mph is exactly 0.44704 m/s, and 1 km/h is exactly 1/3.6 m/s. The familiar rule of thumb that mph is roughly 1.6 times km/h is accurate to about 0.3%, which is fine for mental arithmetic and not fine for anything logged.

The knot is defined as one nautical mile per hour, or exactly 1.852 km/h. It survives in navigation because it maps directly onto latitude: a vessel making 10 knots covers 10 minutes of latitude an hour, which turns dead reckoning into simple arithmetic on a chart. Aviation kept it for the same reason, and because international flight planning needed a single unit that was not tied to either metric or imperial road conventions.

Mach number is a ratio, not a fixed speed. It compares an object's speed to the local speed of sound, which depends almost entirely on air temperature: about 340 m/s at 15 °C at sea level, but only around 295 m/s in the stratosphere. The figure shown here uses the ISA sea-level value, so an aircraft cruising at Mach 0.85 in the flight levels is travelling considerably slower in absolute terms than the same Mach number implies at ground level.