Speed Unit Converter
Convert speed between m/s, km/h, mph, knots, ft/s, Mach, and more.
このツールについて
Enter a speed value in any unit and instantly see conversions across seven units: meters per second (m/s), kilometres per hour (km/h), miles per hour (mph), nautical knots, feet per second (ft/s), Mach number (at sea level, 20 °C), and as a fraction of the speed of light (c). Fun contextual comparisons put the number in perspective — how many times faster than a walking human, an Olympic sprinter, the speed of sound, or even light.
使い方
- 1 Type a speed value in the input field.
- 2 Select the source unit from the dropdown (m/s, km/h, mph, knots, ft/s, Mach, or c).
- 3 All converted values update instantly.
- 4 Read the fun comparisons section below to put the speed in context.
Why every speed conversion goes through metres per second
Speed is just distance divided by time, but the world measures it in incompatible units: a sailor thinks in knots, a pilot in Mach, a driver in km/h or mph, and a physicist in m/s. This converter solves the mismatch the same way a careful engineer would — it picks one canonical unit, the metre per second (m/s), and routes everything through it. Each unit carries a fixed factor that says how many m/s it equals. Your input is multiplied by its factor to get m/s, then divided by every other unit's factor to get that unit's value. Because there is a single pivot, all seven outputs stay perfectly consistent with each other.
The actual conversion factors
| Unit | Equals (in m/s) |
|---|---|
| m/s | 1 |
| km/h | 1 ÷ 3.6 ≈ 0.27778 |
| mph | 0.44704 |
| knot | 0.514444 (1 nautical mile/hour) |
| ft/s | 0.3048 |
| Mach | 343.2 (speed of sound, sea level, 20 °C) |
| c (light) | 299,792,458 |
Two of these are conventions rather than fixed physical constants. Mach 1 is fixed here at 343.2 m/s, the speed of sound in dry air at sea level and 20 °C — but real-world Mach falls with altitude as the air gets colder, which is why a jet at cruising height reaches Mach 1 at a lower true airspeed. The speed of light, by contrast, is exact by definition (299,792,458 m/s), so the "fraction of c" reading is as precise as your input.
A worked example: 60 mph
Enter 60 and choose mph. The tool computes 60 × 0.44704 = 26.8224 m/s. From that single pivot it produces every other reading: 26.8224 ÷ 0.27778 ≈ 96.56 km/h; ÷ 0.514444 ≈ 52.14 knots; ÷ 0.3048 = 88 ft/s exactly; ÷ 343.2 ≈ 0.0781 Mach; and ÷ 299,792,458 ≈ 8.95 × 10⁻⁸ c. Notice that 60 mph is exactly 88 ft/s — a tidy coincidence drivers' education classes love, because it means a car at 60 mph covers a football field in roughly one second.
Reading the "fun comparisons"
Below the table, the tool ratios your speed against six reference speeds — a walking human (1.4 m/s), an Olympic sprinter (10.44 m/s), a cheetah (33.5 m/s), an F1 car (100 m/s), the speed of sound (343.2 m/s), and light (299,792,458 m/s). It shows how many times faster or slower you are. At 60 mph (26.82 m/s) you are about 19× faster than a walking pace, 2.57× faster than a sprinter, but only 0.8× the speed of a cheetah at full tilt — so the comparison row marks the cheetah as faster than you. These ratios are genuinely informative for sanity-checking a number: if a "speed" you typed comes out as a tiny fraction of walking pace, you probably entered the wrong unit.
Common mistakes to avoid
- Confusing knots with km/h. A knot is a nautical mile per hour (1.852 km/h), not a statute mile per hour. Reporting a boat's "20 knots" as "20 mph" understates it by about 15%.
- Treating Mach as a fixed speed. Mach is a ratio to the local speed of sound, which changes with temperature. The 343.2 m/s figure here is the sea-level reference, not a universal constant.
- Mixing up ft/s and mph. They differ by a factor of about 1.467 (since 1 mph = 1.467 ft/s). Engineers working in feet per second and drivers thinking in mph are off by nearly half otherwise.
- Forgetting km/h ≠ km/s. A dropped "h" turns a highway speed into an orbital one.
How the numbers are formatted
Speeds span an enormous range — from a strolling 1.4 m/s to light's 299,792,458 m/s — so a fixed number of decimal places would be useless at one end or the other. The converter adapts: ordinary values are shown to about seven significant figures, large numbers gain thousands separators, and anything beyond a trillion or below a ten-thousandth switches to scientific notation (for example, the speed-of-light fraction of a highway speed shows as something like 8.95 × 10⁻⁸). This keeps a 50 km/h reading readable as 50 while still expressing 5,000 mph as a clean, comma-grouped figure rather than a wall of digits.
Practical uses
Pilots and sailors convert between knots and km/h for cross-checking instruments and charts; runners translate a treadmill's mph into a pace; physics and engineering students normalise everything to m/s before plugging into formulas; and the speed-of-light fraction is a quick way to appreciate just how slow even a spacecraft is on a cosmic scale — a probe travelling 60,000 km/h is still moving at only about 0.0056% of light speed. Everything here is plain arithmetic that runs instantly in your browser; nothing you type is sent anywhere.