Speed Unit Converter
Convert between any pair of speed units — meters per second, kilometers per hour, miles per hour, knots, feet per second, Mach, and the speed of light — using conversion factors verified against NIST and BIPM references. Type a value and the result updates instantly.
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What is speed, and why are there so many units?
Speed is the rate at which an object covers distance — distance divided by time. In SI units, the coherent unit is the meter per second (m/s): one meter of distance covered in one second. It is the unit physicists prefer because every other mechanical formula — kinetic energy, momentum, drag — drops out cleanly when speed is in m/s.
Yet most people never quote speeds in m/s. Drivers read km/h or mph from their dashboards, sailors and pilots talk in knots, athletes time themselves in m/s only on the track, and astronomers reach for the speed of light. The reason is human, not scientific: each transport mode picked a unit that produces comfortable everyday numbers. A highway speed of 100 km/h, 62 mph, 54 knots, or 27.8 m/s is the same physical thing — but only the first two roll off the tongue.
The speed units, explained
Meter per second (m/s) — the SI baseline
1 m/s is one meter of distance covered in one second. It is the coherent SI unit derived from the base units of length and time. Sprint physics, fluid dynamics, ballistics, and physics homework all use m/s. A brisk walk is about 1.4 m/s; world-class 100 m sprinters peak above 12 m/s; the speed of sound at sea level is roughly 340.29 m/s.
Kilometer per hour (km/h) — the global road standard
1 km/h is exactly 1000/3600 m/s ≈ 0.2778 m/s. Almost every country other than the United States and the United Kingdom posts road speed limits in km/h, and most car manufacturers default to km/h on the speedometer. Common limits: 50 km/h in town, 90–100 km/h on rural roads, 110–130 km/h on motorways.
Mile per hour (mph) — the imperial road standard
1 mph is exactly 0.44704 m/s by definition (1 international mile = 1609.344 m, divided by 3600). The United States, the United Kingdom, and a handful of other jurisdictions still post mph limits. Conversion shortcut: km/h ≈ mph × 1.609; mph ≈ km/h × 0.621. The 60 mph mark equals 96.56 km/h — close to but not the same as 100 km/h.
Knot (kn) — aviation and maritime
1 knot is 1 nautical mile per hour = 1852/3600 m/s ≈ 0.5144 m/s. The nautical mile equals one minute of arc along a meridian, so one knot maps cleanly onto navigation charts. Aircraft airspeeds (V₁, V₂, cruise) and ship speeds are quoted in knots worldwide. A typical airliner cruises at 450–500 knots true airspeed.
Foot per second (ft/s) — engineering and ballistics
1 ft/s is exactly 0.3048 m/s. Civil engineers use ft/s for open-channel flow velocity in the United States, ballisticians measure muzzle velocity in feet per second (a 9 mm pistol round leaves the barrel near 1100 ft/s = 335 m/s), and aviation rates of climb appear in feet per minute (60 ft/min = 1 ft/s).
Mach (M) — multiples of the speed of sound
Mach number is dimensionless: Mach = vehicle speed ÷ local speed of sound. Because the speed of sound varies with temperature, the value of "Mach 1" depends on altitude. At ICAO standard sea level (15 °C), Mach 1 ≈ 340.29 m/s = 1225 km/h = 661.5 knots. At 11 km altitude (where airliners cruise), Mach 1 drops to about 295 m/s. This converter uses the sea-level value.
Speed of light fraction (c) — relativistic physics
The exact, defined speed of light in vacuum is 299 792 458 m/s. Astronomers and particle physicists use fractions of c. A photon travels at c, the Sun is 8.3 light-minutes away (≈1.5 × 10¹¹ m at c), and high-energy electrons in particle accelerators reach 0.999999c.
Real-world applications and which unit to expect
- Road speed limits: Outside the US/UK, road signs always show km/h: 50 km/h urban, 100 km/h rural, 130 km/h on a German Autobahn (advisory), 80 km/h on Australian rural roads. US/UK signs read mph: 25 mph residential, 65–70 mph US interstate, 70 mph UK motorway.
- Aircraft speed: Airliners at cruise altitude show three speeds: indicated airspeed (knots), true airspeed (knots), and Mach number (e.g. M0.78). Below 25 000 ft pilots fly knots; above, Mach number takes over because the same true airspeed corresponds to different indicated airspeeds at high altitude.
- Ship and boat speed: Ships always quote speed in knots — a container ship cruises at 18–22 knots, a tugboat at 10–13 knots, a racing yacht at 8–15 knots. The unit literally comes from sailors counting knots on a logline payed out over a measured time.
- Sports performance: Track sprints are timed and converted to m/s for analysis: Usain Bolt's 100 m world record (9.58 s) implies 10.44 m/s average and a peak of 12.27 m/s. Cycling and swimming use km/h or m/s. Football kick speeds reach 30 m/s (108 km/h).
- Wind speed: Aviation and marine reports use knots (10 kn ≈ 18.5 km/h). Land-based weather forecasts in metric countries use km/h; in the US, mph. The Beaufort scale defines 'gale' as 34–47 knots = 63–87 km/h = 39–54 mph.
- Ballistics: Muzzle velocity is reported in ft/s in the US (rifle .223: ≈3200 ft/s ≈ 975 m/s) and m/s elsewhere. Subsonic suppressed loads stay below 343 m/s = Mach 1 at sea level so they do not produce a sonic crack.
- Astronomy and particle physics: Solar system velocities are reported in km/s (Earth orbits the Sun at 29.78 km/s). Cosmological recession velocities use km/s. Particle accelerator output reaches fractions of c — the LHC accelerates protons to 0.999999991c.
How much is 1 unit of each in meters per second?
| Unit | Value in m/s |
|---|---|
| 1 m/s (Meter per second) | 1 Pa |
| 1 km/h (Kilometer per hour) | 0.2777777777777778 Pa |
| 1 mph (Mile per hour) | 0.44704 Pa |
| 1 kn (Knot) | 0.5144444444444445 Pa |
| 1 ft/s (Foot per second) | 0.3048 Pa |
| 1 M (Mach (sea level, 15°C)) | 340.29 Pa |
| 1 c (Speed of light fraction) | 299792458 Pa |
Frequently asked questions about speed units
How do I convert between any two speed units?
Multiply by the source unit's m/s-factor and divide by the target's. Example: convert 60 mph to km/h → 60 × 0.44704 ÷ 0.2777778 = 96.56 km/h. The converter above does this in real time.
What is the relationship between km/h and mph?
1 mile = 1609.344 m exactly, so 1 mph = 1.609344 km/h and 1 km/h = 0.621371 mph. Useful round figures: 60 mph = 96.56 km/h, 100 km/h = 62.14 mph, 100 mph = 160.93 km/h. Speed limit signs in km/h-countries land on round metric numbers; mph-countries use 25, 35, 55, 65, 70, 75.
Why do aircraft and ships use knots?
One knot equals one nautical mile per hour, and one nautical mile equals one minute of arc along a meridian. That makes navigation maths simple: a vessel travelling 60 knots covers 60 nautical miles per hour, which equals 1 degree of latitude. The unit predates SI by centuries and remains the international standard for aviation and maritime traffic.
What does Mach 1 mean exactly?
Mach 1 is the local speed of sound — the speed at which pressure waves propagate through the surrounding air. Because the speed of sound depends on temperature (and only weakly on pressure), Mach 1 is not a single number. At ICAO standard sea level (15 °C), Mach 1 ≈ 340.29 m/s = 1225 km/h = 761 mph = 661.5 knots. At 11 km altitude (−56.5 °C), Mach 1 drops to about 295 m/s. This converter uses the sea-level value; for aviation calculations, use the altitude-specific value.
Is the speed of light really exactly 299 792 458 m/s?
Yes — exactly. Since 1983 the meter has been defined in terms of the speed of light: one meter is the distance light travels in vacuum in 1/299 792 458 second. So c is no longer measured but defined, with zero uncertainty by definition. Light slows down in glass, water, or other media, but in vacuum it is fixed.
How precise are these conversion factors?
All factors used by this converter are exact or sourced from NIST SP 811 and the BIPM SI Brochure. 1 mph = 0.44704 m/s, 1 ft/s = 0.3048 m/s, 1 knot = 1852/3600 m/s, and 1 c = 299 792 458 m/s are all exact by definition. The Mach factor (340.29 m/s) reflects the ICAO standard sea-level speed of sound; if you need it at altitude, look up the altitude-specific value.
What is a typical highway speed in different units?
100 km/h equals 62.14 mph, 27.78 m/s, 54 knots, 91.13 ft/s, Mach 0.0816 (sea level), or 9.27 × 10⁻⁸ c. 70 mph equals 112.65 km/h, 31.29 m/s, 60.8 knots.
How do I read indicated airspeed vs. true airspeed?
Indicated airspeed (IAS, in knots) is what the cockpit gauge shows — based on dynamic pressure, which depends on air density. True airspeed (TAS) is your actual speed through the air. At altitude air is thin, so TAS is higher than IAS for a given gauge reading. Pilots use a flight computer or rule of thumb (TAS ≈ IAS × 1.02 per 1000 ft) to convert.
Why is wind reported in different units in different countries?
Aviation and marine wind reports always use knots, regardless of country. Civilian land forecasts use km/h in metric countries and mph in the US/UK. National weather services have legacy choices: NOAA uses mph for surface gusts, the UK Met Office mph for public, knots for aviation.
Can I link to a specific conversion?
Yes. The URL updates as you change units and values, so you can copy the address bar after any conversion. Example: ?from=mph&to=km_h&x=60.
References
Dedicated converter pages
CONVERSION18