Speed Converter
Convert between speed units like km/h, m/s, mph, knots, and ft/s.
Speed conversion and the units that survive by convention
Speed units are more fragmented than any other measurement family, because aviation, shipping, meteorology, and road transport each standardised separately and none of them switched. Knowing the pivot factors makes the whole set manageable.
Formula
target = source x (source factor to m/s / target factor to m/s)
All speeds convert through metres per second. The most useful shortcut is that 1 m/s is exactly 3.6 km/h.
The conversions worth memorising
Three factors cover almost every practical case, and two of them are exact.
- 1 m/s = 3.6 km/h exactly. This follows directly from 3,600 seconds in an hour and 1,000 metres in a kilometre.
- 1 knot = 1.852 km/h exactly, because a nautical mile is defined as exactly 1,852 metres.
- 1 mph = 1.609344 km/h exactly, from the 1959 definition of the mile.
- A rough mental conversion for mph to km/h is to add 60%: 60 mph is about 96 km/h against a true 96.6. For km/h to mph, multiply by 0.6: 100 km/h is about 60 mph against a true 62.1.
Why aviation and shipping still use knots
A knot is one nautical mile per hour, and a nautical mile is one minute of latitude. This makes chart navigation arithmetic trivial: a vessel making 12 knots for 30 minutes has covered 6 nautical miles, which is 6 minutes of latitude on the chart.
The unit takes its name from the chip log, a rope knotted at fixed intervals and paid out behind a ship for a measured time. Counting the knots that ran out gave the speed directly.
ICAO permits the knot for aviation as a non-SI unit accepted for continued use, alongside the foot for altitude and the nautical mile for distance. The cost and safety risk of a global transition has repeatedly outweighed the benefits of standardisation.
Mach, the speed of sound, and the c-fraction
Mach number is a ratio, not a fixed speed. It is the object's speed divided by the local speed of sound, which depends on air temperature - roughly 340 m/s at sea level and 15 C, falling to about 295 m/s at cruising altitude.
This means Mach 1 at 11,000 m is around 1,062 km/h, while Mach 1 at sea level is about 1,225 km/h. An aircraft can be supersonic at altitude and subsonic at the same true airspeed lower down.
The speed of light in a vacuum is exactly 299,792,458 m/s, a defined rather than measured value since 1983. It is the reason the metre is what it is.
Worked example
An aircraft reports 450 knots. In km/h: 450 x 1.852 = 833.4. In m/s: 833.4 / 3.6 = 231.5. In mph: 833.4 / 1.609344 = 517.9.
At a cruising altitude of 11,000 m where the local speed of sound is roughly 295 m/s, the Mach number is 231.5 / 295 = 0.785.
At sea level, that same true airspeed would give 231.5 / 340 = 0.681 Mach. The aircraft has not changed speed; the reference has. This is why aircraft are limited by Mach number at altitude and by indicated airspeed lower down.
Speed units in metres per second
| Unit | In m/s | Typically used for |
|---|---|---|
| km/h | 0.277778 | Road transport outside the US and UK |
| mph | 0.44704 | Road transport in the US and UK |
| knot | 0.514444 | Aviation, shipping, meteorology |
| ft/s | 0.3048 | US engineering, ballistics |
| Mach (sea level, 15 C) | 340.3 | Aerodynamics; varies with temperature |
| speed of light | 299,792,458 | Exact by definition |
Frequently Asked Questions
Related calculators
References
- BIPM - non-SI units accepted for use with the SI
- ICAO Annex 5 - units of measurement in air and ground operations
Last reviewed: 2026-08-07. This page is informational. For legal, medical, tax, or financial decisions, confirm the result with a qualified professional.