Speed Converter
m/s, km/h, mph, knots, ft/s — with the sea-level Mach number thrown in.
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The engineering
Aviation and marine work run knots (one nautical mile per hour, so speed and chart distance share a unit), road transport splits mph/km h⁻¹ by country, ballistics and US hydraulics keep ft/s, and physics wants m/s. The Mach row uses the ISA sea-level speed of sound, 340.294 m/s — a *reference*, not a constant: at cruise altitude the speed of sound is about 295 m/s, so the same true airspeed is a higher Mach number up high. Treat the row as sea-level context, not flight instrumentation.
The classic mistakes: reading knots as mph (15% slow — enough to blow a fuel calculation on a long leg), km/h vs m/s in dynamics homework (factor 3.6, the most-forgotten constant in engineering), and quoting Mach without an altitude. The handy exact one: 60 mph is exactly 88 ft/s, which is why old US traffic engineering is full of 88s.
Where this math comes from
The knot is a real knot: sailors since the 16th century paid out a line knotted at fixed intervals behind the ship and counted knots per sandglass — William Bourne described the chip log in 1574. The unit outlived the rope because the nautical mile is one arcminute of latitude, making speed, time, and chart position one arithmetic system; the mile itself was fixed internationally at 1852 m at the 1929 Monaco Hydrographic Conference.
Ernst Mach photographed shock waves off supersonic projectiles in 1887, showing the speed of sound is a physical wall with geometry attached; Swiss aerodynamicist Jakob Ackeret proposed naming the velocity ratio the 'Mach number' in 1929. Chuck Yeager put an aircraft through that wall in 1947, and the ratio has been on airspeed indicators ever since — always relative to the local, altitude-dependent speed of sound.
- 1574William BourneChip log described — speed counted in literal knots.
- 1887Ernst MachPhotographs shock waves; the sonic ratio gets its physics.
- 1929Monaco Hydrographic Conference / Jakob AckeretNautical mile fixed at 1852 m; 'Mach number' named.
- 1947Chuck YeagerFirst level supersonic flight — Mach 1 crossed.
See the full timeline of the math behind every calculator →
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