How it works
The four speed units you'll actually meet
This speed converter handles the four units that show up in real life: mph (miles per hour, UK and US road speeds), km/h (kilometres per hour, most of the rest of the world), m/s (metres per second, the SI scientific unit used in physics), and knots (nautical miles per hour, used at sea and in aviation).
The exact conversion factors below are the only numbers you need — every other speed conversion is built from them.
The exact conversion factors
- 1 mph = 1.609344 km/h (exact, from the international mile)
- 1 km/h = 0.621371 mph
- 1 mph = 0.44704 m/s
- 1 m/s = 3.6 km/h (a useful shortcut: multiply m/s by 3.6 to get km/h)
- 1 knot = 1.852 km/h = 1.15078 mph (a nautical mile is 1,852 m by international definition)
Three worked examples
1. UK motorway speed to km/h
The UK motorway limit is 70 mph. In km/h: 70 × 1.609344 ≈ 112.7 km/h. If you're driving into mainland Europe where signs are in km/h, this is the figure to have in your head.
2. European speed limit to mph
A French autoroute limit of 130 km/h in mph: 130 × 0.621371 ≈ 80.8 mph — noticeably higher than the UK's 70 mph motorway limit.
3. Physics problem: m/s to km/h
A dropped object reaches 20 m/s just before impact. In km/h: 20 × 3.6 = 72 km/h — the ×3.6 shortcut works because there are 3,600 seconds in an hour and 1,000 metres in a kilometre (3600/1000 = 3.6).
Quick reference: common speeds converted
Round-number reference points worth having memorised.
| mph | km/h | m/s |
|---|---|---|
| 20 mph | 32.2 km/h | 8.9 m/s |
| 30 mph | 48.3 km/h | 13.4 m/s |
| 50 mph | 80.5 km/h | 22.4 m/s |
| 60 mph | 96.6 km/h | 26.8 m/s |
| 70 mph | 112.7 km/h | 31.3 m/s |
Common mistakes
- Rounding 1.609344 to 1.6 too early in multi-step calculations — fine for a quick mental estimate, but compounds into a real error over long conversions or repeated calculations.
- Confusing knots with mph. A knot (1.15 mph) is not a mph and not a km/h — it's specifically nautical miles per hour, and mixing it into a road-speed calculation gives a wrong answer that looks plausible.
- Using the wrong direction of the factor. Multiplying by 1.609344 converts mph → km/h; going the other way needs the reciprocal (0.621371), not the same number.
- Forgetting UK road signs are in mph, not km/h — a common trip-up for visitors used to km/h-only countries doing satnav or rental-car speed checks.
Related conversions
Speed conversions often come up alongside these:
- **Length converter** — miles, kilometres, metres and feet, the units speed is built from.
- **Fuel cost calculator** — once you know your speed and distance, work out the trip cost.
- **Weight converter** — kilograms, pounds and stones, for the other unit conversions that come up on the same trip (luggage limits, towing weights).