Speed Unit Converter

Speed Conversion: Knots, the "Mai" Mix-up, and Why Mach Is Not a Fixed Number

Basic conversion relationships

The most useful mental anchor: m/s × 3.6 = km/h, and divide by 3.6 to go back. Driving at 100 km/h is about 27.8 m/s — and that matters for braking distance and impact energy, because physics formulas use m/s.

What a "knot" is, and why sea and air use it

1 knot = 1 nautical mile per hour = 1.852 km/h. The nautical mile is defined as 1852 m, and 1852 m happens to equal one minute of latitude on the meridian.

That definition simplifies charts and flight plans: one minute of latitude = one nautical mile, so a divider measures the latitude difference and reads distance directly; the resulting speed is naturally "knots". Aviation also speaks of ground speed (actual speed accounting for wind), still in knots, while air speed (speed relative to the airflow) sets lift and stall margins — they are not the same, which is why ground speed drops noticeably when flying into a headwind.

The "mai" wording is wrong

In Chinese colloquial use, "doing 80 mai" usually means 80 km/h, but "mai" comes from the English "mile" and strictly means 80 mph (about 128.7 km/h). The two differ by 60%.

In any formal setting do not use "mai" for speed, because you cannot tell whether the listener means km/h or mph. Write "80 km/h" or "80 mph" with no ambiguity. China's road limit signs are all in km/h.

Mach is not a fixed speed

Mach number is the ratio of speed to the local speed of sound, and the speed of sound varies with temperature (through medium elasticity and density):

a = √(γRT), dominated by temperature

So "what is Mach 1 in km/h" has no single answer — you must state altitude and temperature. The colder, thinner air aloft lowers the sound speed by about 13%, which is why supersonic airliners cruise at Mach number "more easily" up high: they are not faster, sound is just slower. Mach is also relative to air speed, not ground speed, so a plane into a headwind can show high Mach yet advance slowly.

Common errors in conversion and intuition

• Averaging speeds arithmetically. 60 km/h out and 120 km/h back averages not 90 but the harmonic mean 2/(1/60 + 1/120) = 80 km/h, because the distances are equal but the times differ, and time is the denominator.
• Taking instantaneous speed as average. A navigator's "arrive in 2 hours" uses a stop-inclusive average, unrelated to the instantaneous speed on the dial.
• Braking distance scales with the square of speed. Double the speed and braking distance grows to roughly 4× (ignoring traction change). So 60 vs 120 km/h is not 2× but about 4× the stopping distance — the premise for seeing speed in m/s.
• Mixed limit units. Where mph and km/h coexist (US, UK, Canada), cross-border driving demands care; the dial may be dual-scale.
• Wind units vary too. Meteorology uses m/s, km/h, knots, and mph; marine forecasts use knots, mainland China often m/s with the Beaufort scale. Confirm the unit before reading different sources.

The tool covers m/s, km/h, mph, ft/s, knots, and Mach (Mach computed against the sea-level standard sound speed, with that noted). Conversions happen entirely inside the page.