What is a Speed Unit Converter?
The speed unit converter switches between km/h, m/s, mph, knots, and Mach.
How it's calculated
km/h ÷ 3.6 = m/s, 1 mph = 1.609 km/h, 1 knot = 1.852 km/h, Mach 1 ≈ 1,225 km/h (at sea level, 15°C)
Units that are easy to mix up
| Units | Why it's confusing | How to tell them apart |
|---|---|---|
| km/h vs knot | Both measure speed | 1 knot = 1.852 km/h; used specifically for sea and air navigation |
| mph vs km/h | Different national standards | 1 mph = 1.609 km/h; US/UK use mph, most countries use km/h |
| Mach is not fixed | Assumed to be a constant speed | Speed of sound varies with temperature, so Mach 1 differs by altitude |
Mach isn't a fixed speed
Mach 1 refers to the local speed of sound, which changes with temperature. At sea level (15°C) it's about 1,225 km/h; at 11km altitude (-56°C) it's about 1,062 km/h. The same Mach number means a different actual speed depending on altitude — an airliner cruising at Mach 0.85 is actually traveling around 900 km/h relative to the ground. The speed of sound depends only on temperature, not pressure, because it's determined by how fast air molecules themselves are moving.
Good to know
- A sonic boom isn't a one-time event at the moment of crossing Mach 1 — it's continuously generated the entire time an aircraft flies supersonic. On the ground, you only hear it as the cone-shaped shockwave passes overhead.
- This is one reason the Concorde was retired — supersonic flight over land was banned due to sonic boom noise, restricting it to transatlantic routes only.
- The speed of light, 299,792,458 m/s, isn't a coincidentally precise number — the meter itself was later redefined based on that speed.
- Earth's rotational speed at the equator is about 1,670 km/h — imperceptible because the atmosphere rotates along with the surface.
- Human top speed is about 45 km/h (Usain Bolt); a cheetah reaches 110 km/h, and a peregrine falcon exceeds 380 km/h in a hunting dive.
Frequently asked questions
Q. How many m/s is 100 km/h?
About 27.8 m/s. Divide by 3.6.
Q. Is Mach 1 always the same speed?
No. The speed of sound varies with temperature, so Mach 1 corresponds to a slower actual speed at higher altitude.
Q. Why are knots used at sea?
Because 1 knot equals 1 nautical mile per hour, and a nautical mile equals one minute of latitude, making navigation calculations convenient.