Speed Converter: km/h, m/s, mph, Knots and Mach

The speed converter changes a value between seven units: metre per second (m/s), kilometre per hour (km/h), mile per hour (mph), knot (kn), foot per second (ft/s), Mach (Ma) and the speed of light (c). Typing on either card updates the other card and the All Units list.

Updated 2026-09-30Reviewed by CalcX Editorial Team5 sources5 min read

What the speed converter does

Every entry but Mach is a fixed factor: 1 km/h = 1/3.6 m/s, 1 mph = 0.44704 m/s, 1 kn = 1 nmi/h = 1852/3600 m/s, 1 ft/s = 0.3048 m/s and 1 c = 299,792,458 m/s. Mach is a ratio to the local speed of sound, so this list pins 1 Ma = 343 m/s, the familiar figure for dry air at 20 °C; its readouts are a standard-air convenience, not flight data.

Miles and nautical miles are defined in exact metres on the length converter; hours and seconds are on the time converter; speeds inside longer calculations belong to the scientific calculator.

How to use it

Pick the unit you have on the left card and the one you want on the right; both dropdowns list the same seven units.
Type on either side and the other converts as you type; the middle button swaps the units and carries the value across.
The focused field also takes expressions: 100+40 shows a quiet = 140 hint and converts the running value; Enter or = settles it.
An unfinished 3 + shows = ? and keeps the last settled value.
The All Units list repeats the speed in every unit; click a row to move it to the focused side. The keypad button opens a standard calculator.

Worked examples

Every readout below comes from this converter.

Walking pace. 1.4 m/s — a brisk walk — is 5.04 km/h, 3.1317108089 mph and 2.7213822894 kn.

Two road limits. 100 km/h converts to 62.1371192237 mph and 27.7777777778 m/s; the British 70 mph limit is 112.65408 km/h. The 3.6 bridge is exact: 1 km = 1,000 m and 1 h = 3,600 s.

Airliner cruise. 250 m/s reads 900 km/h, 559.234073014 mph and 485.96112311 kn. The Mach row says 0.7288629738: this tool divides by its pinned 343 m/s. At 11 km, where the local speed of sound is about 295 m/s, that same 250 m/s is near Mach 0.85.

Knots are not miles per hour. 100 kn is 185.2 km/h, 115.077944802 mph and 51.4444444444 m/s. One nautical mile is 1,852 m and one statute mile 1,609.344 m; their ratio of 1.150779448 is the whole difference.

Where the units come from

The metre per second is the SI derived unit of speed, built from the metre and the second; km/h is the same ratio scaled by 1,000/3,600. The speed of light has been fixed at 299,792,458 m/s exactly since 1983, which is why 1 c is a definition, not a measurement.

Imperial speeds come from the 1959 international yard and pound agreement: 1 ft = 0.3048 m and 1 mi = 1,609.344 m exactly, giving 1 ft/s = 0.3048 m/s and 1 mph = 0.44704 m/s. The nautical mile — 1,852 m since 1929 — gives the knot: 1 kn = 1852/3600 m/s. The knot remains the working unit of marine and air navigation (ICAO permits it in aviation), with kn as its ISO symbol.

Mach is named after the physicist Ernst Mach: a speed divided by the local speed of sound, which in air depends on temperature — about 343 m/s at 20 °C, 340.3 m/s at ISA sea level (15 °C) and 295 m/s in the −56.5 °C air at 11,000 m.

Reading results and limits

Display rule.: Results carry up to 12 significant digits. Between 10⁻¹⁰ and 10¹⁵ they print with comma grouping and at most 10 decimal places; outside that band, exponential form with six decimal places (2.997925e+23). The stored value has more digits than the screen.
Rounding at the light-speed row.: The light-speed row is display rounding, not definition: 0.3048 ÷ 299,792,458 is about 1.0167 × 10⁻⁹ c. Just below the threshold, 10⁻¹⁰ m/s still reads 0.0000000001 m/s; negative inputs convert normally, so −20 m/s is -72 km/h.
Speeds only.: It does not convert distance or time: lengths and nautical miles live on the length converter and durations on the time converter.

Representative readouts:

Input (left card)	Row shown	Readout
1 m/s	Mile per hour	2.2369362921 mph
1 kn	Metre per second	0.5144444444 m/s
1 c	Metre per second	299,792,458 m/s
1 ft/s	Speed of light	0.000000001 c
10⁻¹¹ m/s	Metre per second	1.000000e-11 m/s

Common mistakes

A knot is not a mile per hour. Both sit one row apart and read “per hour”, but 100 kn is 115.077944802 mph — about 15% longer. Navigation uses knots; roads use km/h or mph.
km/h and m/s are not interchangeable numbers. Multiply or divide by 3.6: 36 km/h is exactly 10 m/s; 100 km/h is 27.7777777778 m/s. The factor is exact, not approximate.
Mach is a ratio, not a unit. This tool pins 1 Ma = 343 m/s, so 2 Ma reads 2,469.6 km/h; the real ratio follows air temperature — about 340.3 m/s at ISA sea level, about 295 m/s at 11,000 m.
“Knots per hour” is wrong. A knot already means one nautical mile per hour; “per hour” twice would describe acceleration.
The speed of light is a speed. One c is 1,079,252,848.8 km/h; a light-year is a distance, not a time, and belongs on the length converter.

Where it is used

Road limits. 100 km/h is 62.1371192237 mph and 36 km/h is exactly 10 m/s, so a limit and a speedometer agree in either system.
Navigation and aviation. 100 kn is 185.2 km/h; knots stay the working unit at sea and in the air.
Flight and wind figures. A 250 m/s cruise reads 900 km/h and a pinned Mach 0.7288629738, with the standard-air caveat attached.

Sources

BIPM, The International System of Units (SI Brochure), 9th edition — the metre per second, and the fixed speed of light 299,792,458 m/s. bipm.org （访问日期：2026-09-30）
NIST Special Publication 811, Appendix B.8 — exact factors for foot per second (0.3048 m/s), mile per hour (0.44704 m/s), kilometre per hour and the knot. nist.gov （访问日期：2026-09-30）
Wikipedia, Knot (unit) — one knot = one nautical mile per hour = 1,852 m/h, and the knot in aviation, where ICAO permits it. en.wikipedia.org （访问日期：2026-09-30）
Wikipedia, Mach number — Mach as a ratio to the local speed of sound; 340.3 m/s at ISA sea level, 295.0 m/s at 11,000 m. en.wikipedia.org （访问日期：2026-09-30）
Wikipedia, Speed of sound — about 343 m/s in air at 20 °C and its temperature dependence. en.wikipedia.org （访问日期：2026-09-30）