Speed unit conversion
Road signs, car speedometers and aviation or marine instruments often use different units — a quick reference keeps them straight.
Unit converter
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| From | To | Multiply by |
|---|---|---|
| 1 km/h | mph | 0.621371 |
| 1 mph | km/h | 1.609344 |
| 1 m/s | km/h | 3.6 |
| 1 km/h | m/s | 0.277778 |
| 1 knot (kn) | km/h | 1.852 |
| 1 knot (kn) | mph | 1.15078 |
| 1 mph | m/s | 0.44704 |
| 1 Mach (approx sea level) | m/s | ≈340 |
Notes
- Mach depends on temperature and altitude; the row is a rough sea-level hint only.
- 1 knot = 1 nautical mile per hour.
km/h and mph: two speedometer worlds
Most of the world marks speed limits and car speedometers in kilometers per hour, while the United States (and the UK for road signs) uses miles per hour. The two are linked by one exact factor from the table: 1 mph = 1.609344 km/h. Every other row in the sheet — knots, m/s, Mach — ultimately hangs off this definition of the mile, so a value converted here is exact, not an approximation.
Fast mental conversions
To convert mph to km/h, multiply by 1.6 and you will be within half a percent: 30 mph ≈ 48 km/h, 60 mph ≈ 96 km/h, 100 mph = 160.9 km/h. Going the other way, multiply km/h by 0.62 — or take 60% of the figure — so 100 km/h ≈ 62 mph and 50 km/h ≈ 31 mph. The table gives the precise values (1 km/h = 0.621371 mph), but the 60% rule is accurate enough for reading foreign speedometers and rental-car dashboards.
Knots and Mach: speed at sea and in the air
Aviation and marine navigation measure speed in knots: one knot is one nautical mile (1852 m) per hour, exactly 1.852 km/h — the conversion row at the top of the table. Mach numbers are different in kind: Mach 1 is the speed of sound, roughly 1225 km/h at sea level, and it falls with temperature and altitude, which is why the table labels it approx sea level. A plane cruising at Mach 0.8 is therefore near 980 km/h.
Everyday reference points
An easy walking pace is about 5 km/h (3 mph). City traffic limits cluster around 50 km/h — 30 mph in British cities. Motorways commonly allow 120–130 km/h, which is 75–80 mph, while US interstate limits near 65–70 mph equal 105–113 km/h. Anchoring a few of these pairs from the table makes any conversion instantly sanity-checkable: if a converted value feels faster than the road allows, you have swapped the direction.
Frequently asked questions
- How many mph is 100 km/h?
- 100 km/h is about 62.1 mph (multiply km/h by 0.621371). This is a common highway speed.
- How do you convert m/s to km/h?
- Multiply by 3.6: for example, 10 m/s equals 36 km/h. To go back, divide km/h by 3.6.
- How fast is a knot in km/h?
- One knot is 1.852 km/h, because it equals one nautical mile (1,852 m) per hour. It is used in aviation and marine navigation.