Kilometer per Hour to Mile per Second

Most of the world uses km/h, including all of the EU, Australia, Canada, China, and India. The United States, Myanmar, and Liberia are the primary countries still using miles per hour for road signs. The UK is a notable exception — it uses mph on roads despite being otherwise metric in daily life, a situation that has persisted since the 1970s metrication program stalled.

Typical commercial jets (Boeing 737, Airbus A320) cruise at 800–900 km/h at altitude, roughly Mach 0.78–0.85. The Concorde flew at 2,179 km/h (Mach 2.04). Airspeed is officially measured in knots (1 knot ≈ 1.852 km/h), so flight data systems show 432–485 knots, not km/h, even in metric countries.

The land speed record is 1,227.985 km/h (763.035 mph), set by Andy Green in the jet-powered ThrustSSC in 1997 in the Black Rock Desert, Nevada — breaking the sound barrier on land. The fastest production car is the SSC Tuatara, which achieved 455 km/h in 2020. Most track supercars top out around 320–350 km/h.

The peregrine falcon holds the overall record at roughly 390 km/h in a hunting dive (stoop). On land, the cheetah tops out at about 112 km/h in short bursts. The black marlin is the fastest fish at approximately 130 km/h. Among insects, the Australian tiger beetle runs at 9 km/h — slow-sounding until you realize that, for its body size, it is moving so fast its eyes cannot keep up and it has to stop repeatedly to re-locate prey.

It depends on the country. The UK Met Office and Australian BOM use km/h for public forecasts. European services often use km/h or m/s depending on audience — scientific literature uses m/s. In the US, wind speed is given in mph or knots. The World Meteorological Organization uses m/s as the standard for international data exchange.

The speed of light in a vacuum is exactly 299,792,458 m/s, which converts to approximately 186,282.397 mi/s. Before the SI system was globalised, US and British scientific texts routinely used this figure. The value 186,000 mi/s appears as a rounded approximation in older textbooks. Today, physicists universally use c = 3 × 10⁸ m/s or the exact SI value.

The solar wind — a stream of charged particles ejected from the Sun's corona — travels at roughly 250–500 mi/s (400–800 km/s) depending on whether it is the slow or fast component. During coronal mass ejections, bursts can exceed 1,200 mi/s (2,000 km/s). At 500 mi/s the solar wind crosses the 93-million-mile Earth–Sun gap in about 2 days, which is why space weather forecasters can give roughly 48 hours' notice before a geomagnetic storm hits.

Yes — several. NASA's Parker Solar Probe reached about 430,000 mph (119 mi/s or 192 km/s) at perihelion, making it the fastest human-made object ever. High-powered railgun projectiles tested by the US Navy reached around 2.5 km/s (1.5 mi/s). Intercontinental ballistic missiles (ICBMs) reach approximately 4 mi/s during their boost phase.

Exactly one second — that's the definition. At 1 mi/s you would cross the Atlantic Ocean (about 3,400 miles) in under an hour, reach the Moon (238,855 miles) in about 2.8 days, and reach the Sun (93 million miles) in roughly 12 days. The speed is conceptually useful for calibrating astronomical distances in imperial terms.

Rarely. The main residual use is in some US military ballistics documents and historical astrophysics references. The International Astronomical Union and NASA primarily use km/s for planetary and solar system speeds, and AU/year or c (fraction of light speed) for interstellar scales. Mi/s survives mostly in popular science writing aimed at American audiences unfamiliar with metric.