mph ↔ m/s Physics Velocity Converter

Before 1960 there was no single agreed-upon physics velocity unit. CGS used cm/s, MKS used m/s, and engineers used ft/s, mph, or km/h depending on country. The 11th General Conference on Weights and Measures in 1960 adopted the International System of Units (SI) with the metre per second as the coherent derived unit of velocity. Within a decade every physics curriculum worldwide had switched.

The metre itself was redefined in 1983 in terms of the speed of light - one metre is the distance light travels in 1/299,792,458 of a second. That makes c exactly 299,792,458 m/s by definition, not by measurement. A 2019 SI revision pinned the kilogram to Planck's constant, completing a redefinition of all SI base units to fundamental constants. There is no national prototype metre or kilogram in a vault any more.

The mile per hour persists for cultural reasons. The 1959 international yard-pound agreement fixed the yard at exactly 0.9144 m, which makes the mile exactly 1609.344 m and the mph exactly 0.44704 m/s. So every mph reading is just a particular fixed multiple of an SI m/s value. The mile is now an SI-defined unit pretending to be imperial.

The International Space Station orbits at 7.66 km/s, completing 16 orbits per day at 400 km altitude. Time on the ISS runs slightly slower than on Earth due to both special relativistic time dilation (faster clock = slower time) and general relativistic gravitational potential (higher altitude = faster time). The two effects partially cancel, but special relativity wins; astronauts return having aged about 0.007 seconds less per 6 months.

Modern metrology measures velocity to extraordinary precision. The LIGO gravitational wave detector observes light travel times in 4 km arms with attometre sensitivity - one ten-thousandth the size of a proton. That implies clock and length stability at the level of v/c equal to 10 to the -22. Newton would not have believed it possible.

Relativistic precision metrology now drives the definition of every SI unit. Optical-lattice atomic clocks at NIST and PTB Berlin tick at 429,228,004,229,873.0 Hz for strontium-87, accurate to a few parts in 10 to the -18. That is one second of drift per 30 billion years. These clocks define the second and let us measure velocity against c with the same precision.

Velocity, the simplest concept in mechanics - distance over time - has the deepest connection to physics through c, the universal speed limit. Whether a sprinter runs at 10 m/s or a photon at 299,792,458 m/s, both speeds live on the same SI scale. The converter above lets you sweep across nine orders of magnitude on a single screen.