Core answer: 1 m/s = 3.6 km/h exactly; so km/h ÷ 3.6 = m/s. 100 km/h = 27.8 m/s — in one second of inattention at highway speed you travel 28 meters. 1 knot = 1.852 km/h; 1 mph = 1.609 km/h. Speed of sound at sea level ≈ 340 m/s ≈ 1,225 km/h = Mach 1.
The conversion spine
| From | To km/h | Note |
|---|---|---|
| 1 m/s | 3.6 | physics unit |
| 1 mph | 1.609 | US/UK roads |
| 1 knot | 1.852 | aviation/shipping |
| 1 km/min | 60 | rare |
| Mach 1 (sea level, 15 °C) | 1,225 | sound |
Pace ↔ speed (runners): pace (min/km) = 60 ÷ speed (km/h). 5:00/km = 12 km/h; 4:00/km = 15 km/h; a 6:00/km jog = 10 km/h.
Worked examples
Example 1 — Braking intuition. At 120 km/h you cover 33.3 m/s; a 1.5 s reaction + ~50 m braking means you need well over 90 m of clear road — why tailgating at highway speed is lethal.
Example 2 — US road trip. A "65 mph" limit = 104.6 km/h; your rental's speedo shows both, but the outer ring is usually mph.
Example 3 — Flight time. Cruising at 850 km/h ≈ 460 knots; a 1,200 km route takes ~1.5 h of pure cruise, ~2.5 h gate-to-gate.
Example 4 — Typhoon news. A "45 m/s" typhoon = 162 km/h wind = Category-5-equivalent gusts. m/s × 3.6 is the reflex to build.
Example 5 — Running pace. Finishing 10 km in 50 minutes = 5:00/km = 12 km/h; a treadmill set to 12 km/h reproduces it exactly.
Common mistakes and myths
- Dividing km/h by 3 instead of 3.6 — 90 km/h is 25 m/s, not 30; the 20% error matters in physics problems.
- Confusing knots with mph — knots are 15% faster than mph (1.852 vs 1.609); aviation weather in knots, car speedos in mph.
- Assuming Mach is a fixed speed — Mach 1 falls with altitude's cold (~1,062 km/h at cruise altitude); "Mach 2" jets fly slower in km/h than sea-level math suggests.
- Average speed ≠ average of speeds — 60 km/h out and 40 km/h back over equal DISTANCE averages 48 km/h (harmonic mean), not 50.
- Believing speedometers read true — most read 3–6% high by design (regulations forbid reading low); GPS is the honest number.