Speed Converter

Speed is reported in mph in the US and UK, km/h in most of the world, knots in aviation and boating, and m/s in physics. Converting is mostly straightforward, but two pitfalls catch people: the difference between statute miles and nautical miles, and the confusing habit of calling airplane speeds 'Mach 2' without ever converting to a real number.

The knot is one nautical mile per hour, where a nautical mile is one minute of latitude (1,852 m). This ties the unit directly to navigation: an aircraft traveling at 60 knots covers one degree of latitude per hour. The name comes from the 17th-century practice of measuring ship speed by counting knots on a line paid out behind the ship over a fixed time — hence 'knots,' not 'knots per hour.'

The mile per hour comes from the statute mile (5,280 feet), standardized by Queen Elizabeth I in 1593. The kilometer per hour came along with the metric system in 1795. The meter per second is the SI unit but feels too small for traffic — 30 m/s is highway speed, 343 m/s is the speed of sound at sea level.

mph to km/h: multiply by 1.609. km/h to mph: multiply by 0.621, or divide by 1.609. Knots to mph: multiply by 1.151. m/s to km/h: multiply by 3.6 (because there are 3.6 thousand seconds in an hour, divided by 1000 m/km). m/s to mph: multiply by 2.237.

For the Fibonacci trick: 3 mph ≈ 5 km/h, 5 mph ≈ 8 km/h, 8 mph ≈ 13 km/h, 13 mph ≈ 21 km/h, 21 mph ≈ 34 km/h, 34 mph ≈ 55 km/h, 55 mph ≈ 89 km/h. Each is a Fibonacci pair. Handy for reading European speed-limit signs while driving.

• Meters per second
• Kilometers per hour
• Miles per hour
• Knots
• Feet per second
• Mach (at sea level)

• 'Knots per hour.' A knot is already per-hour. 'Knots per hour' would be an acceleration.
• Confusing statute mph and nautical mph. Pilots say 'my ground speed was 500 knots,' which is 575 statute mph. A '500 mph' plane is slower than a '500 knot' plane by 15%.
• Mach numbers. Mach 1 is not a fixed speed — it's the local speed of sound, which varies with temperature. Mach 1 at sea level is 343 m/s (1,125 ft/s, 767 mph); at 36,000 ft it's about 295 m/s (660 mph). Fighter jets talk about 'Mach 2' knowing it's altitude-dependent.
• Unit shorthand. 'KPH' is not a standard abbreviation — the correct form is 'km/h.' The US often writes 'mph' (lowercase) while the UK writes 'MPH.' Road signs in Europe sometimes drop the unit entirely, which is fine inside a country but confusing at border crossings.
• Average vs. instantaneous speed. A '30-minute commute of 15 miles' averages 30 mph but can spike to 70 on highway segments. Speed limits are instantaneous limits, not averages.

• Walking pace: 1.4 m/s (5 km/h, 3.1 mph).
• Jogging: 2.8 m/s (10 km/h, 6.2 mph).
• Cycling, casual: 5.5 m/s (20 km/h, 12.4 mph).
• Usain Bolt's 100-m record pace: 10.44 m/s (37.6 km/h, 23.4 mph) — peak speed ~12.4 m/s.
• Urban speed limit (residential): typically 30 mph / 50 km/h.
• Highway speed limit: 70 mph / 113 km/h (US) or 130 km/h / 81 mph (German autobahn, where limited).
• High-speed rail (TGV): 90 m/s (320 km/h, 200 mph).
• Commercial jet cruise: 250 m/s (900 km/h, 560 mph, Mach ~0.85 at altitude).
• Speed of sound at sea level: 343 m/s (1,235 km/h, 767 mph).
• Earth's orbital speed around the Sun: ~30,000 m/s (67,000 mph).

• When driving abroad, check whether your rental has a km/h or mph speedometer. Most modern cars switch via a menu.
• For running pace, the running world uses min/km or min/mi, not speed. 5 min/km = 12 km/h = 7.5 mph.
• For ocean sailing, know that current and wind are almost always quoted in knots, but weather forecasts for land use km/h or mph. Converting is a daily chore.

Related: Acceleration Converter · Fuel Economy Converter · Time Converter.