Enter a value in any speed unit and instantly see the equivalent in m/s, km/h, mph, knots, Mach and the speed of light.
| Unit | Value |
|---|
Speed is measured differently depending on where you are and what you're doing — road signs in mph or km/h, aviation and shipping in knots, aircraft performance in Mach numbers, and physics in fractions of the speed of light. This tool lets you enter a value once and see it converted to every common speed unit at the same time.
Values below zero or above the speed of light are rejected, since neither is physically meaningful for a speed magnitude.
Every speed unit converts through meters per second (m/s), the SI unit of speed. To convert from unit A to unit B:
All speed units here are linear multiples of m/s, so every conversion is straightforward multiplication or division.
| Unit | Symbol | Value in m/s |
|---|---|---|
| Meter per Second | m/s | 1 |
| Kilometer per Hour | km/h | 0.277778 |
| Mile per Hour | mph | 0.44704 |
| Foot per Second | ft/s | 0.3048 |
| Knot | kn | 0.514444 |
| Mach (approx.) | Ma | 343 |
| Speed of Light | c | 299,792,458 |
Note: mph, ft/s and knots are exact (derived from exact length/time definitions). The speed of light is exact by definition of the meter. Mach 1 is an approximation — the actual speed of sound depends on air temperature, pressure and humidity. 343 m/s is the commonly cited value for dry air at 20°C at sea level; at cruising altitude, where air is much colder, Mach 1 is closer to 295 m/s.
Example 1 — km/h to mph
A highway speed limit of 120 km/h: 120 ÷ 3.6 = 33.333 m/s, ÷ 0.44704 m/s/mph ≈ 74.6 mph.
Example 2 — Knots to km/h
A ship cruising at 20 knots: 20 × 0.514444 m/s/kn × 3.6 ≈ 37.04 km/h.
Example 3 — Mach to km/h
A jet at Mach 2: 2 × 343 m/s × 3.6 ≈ 2,469.6 km/h (using the sea-level approximation).
Example 4 — Fraction of the Speed of Light
A spacecraft at 30,000 km/h: 30,000 ÷ 3.6 = 8,333.3 m/s, ÷ 299,792,458 m/s ≈ 0.0000278 c — a good illustration of just how enormous the speed of light is.