Speed Conversions
Speed
Speed is the scalar magnitude of velocity — the rate at which an object covers distance over time. In the SI system, the base unit for speed is metres per second (m/s), though kilometres per hour (km/h) and miles per hour (mph) dominate everyday usage in transport and meteorology. The relationship between these units — 1 m/s = 3.6 km/h = 2.237 mph — is fundamental in physics education and appears frequently in mechanics problems.
The speed of sound and the speed of light represent two of the most physically significant velocity benchmarks. The speed of sound in dry air at 20 °C is approximately 343 m/s (1,235 km/h or 767 mph), referred to as Mach 1. This value changes with temperature and altitude, decreasing as air thins and cools — Mach 1 at 35,000 ft is roughly 295 m/s. The speed of light in a vacuum, denoted c, is exactly 299,792,458 m/s and is a fundamental constant of nature, forming the basis of Einstein's Special Theory of Relativity.
In aviation and maritime navigation, speed is expressed in knots — one knot being equal to one nautical mile per hour (1.852 km/h). This unit persists because nautical miles are tied to the geometry of the Earth: one nautical mile equals one arcminute of latitude, making navigation calculations more intuitive. Understanding speed unit conversions is therefore essential not only in physics, but across transport, meteorology, and sport science.
Popular Comparisons
| km/h | mph | m/s | kn | Mach | |
|---|---|---|---|---|---|
| Walking pace | 5 | 3.1 | 1.39 | 2.7 | 0.004 |
| City speed limit | 50 | 31.1 | 13.9 | 27 | 0.041 |
| Highway limit | 120 | 74.6 | 33.3 | 64.8 | 0.098 |
| Commercial plane | 900 | 559 | 250 | 486 | 0.735 |
| Speed of sound | 1,235 | 767 | 343 | 667 | 1 |
| Speed of light | 1,079,252,849 | 670,616,629 | 299,792,458 | 582,749,918 | 874,030 |
Frequently Asked Questions (FAQ)
How do I convert miles per hour to kilometres per hour?
Multiply mph by 1.60934 to get km/h. For example, 60 mph equals 96.56 km/h. A useful mental shortcut: multiply by 1.6 for a close approximation. This conversion is most commonly needed when travelling between countries that use different speed limit systems, such as driving from the US into Canada.
What is a knot and why is it used in aviation and sailing?
A knot is one nautical mile per hour, equal to 1.852 km/h or 1.151 mph. It is used in aviation and maritime navigation because one nautical mile corresponds to one arcminute of latitude on Earth's surface — making it inherently tied to the geometry of the globe and simplifying navigation calculations. One degree of latitude equals exactly 60 nautical miles.