Key facts
- 1 mph is exactly 0.44704 m/s, because the international mile is exactly 1,609.344 m.
- 1 m/s is exactly 3.6 km/h, and 1 km/h is 0.277778 m/s.
- 1 knot is exactly 1.852 km/h: one nautical mile of 1,852 m covered in one hour.
- 100 km/h is 62.14 mph, and 70 mph is 112.65 km/h.
- Knots are used at sea, in aviation and for wind; km/h and mph are used on roads.
The exact relationships
Every speed unit here is a fixed distance divided by a fixed time, so the ratios between them are exact numbers rather than estimates. The mile and the yard come from the 1959 international agreement, which fixed 1 mile at exactly 1,609.344 m. Divided by 3,600 seconds, that gives 1 mph = 0.44704 m/s, and no rounding anywhere.
The metric relationships are exact because both units are decimal. One metre per second is 1,000 m in 3,600 s, which is exactly 3.6 km/h, so you multiply by 3.6 to go from m/s to km/h and divide by 3.6 to go back.
The knot is the odd one out because its distance is not a statute mile. It is fixed to the nautical mile of 1,852 m, agreed internationally in 1929, so 1 kn = 1,852 ÷ 3,600 = 0.514444 m/s, which is exactly 1.852 km/h. One knot works out at 1.15078 mph.
| Unit | Symbol | km/h | m/s |
|---|---|---|---|
| Kilometres per hour | km/h | 1 | 0.277778 |
| Miles per hour | mph | 1.609344 | 0.44704 |
| Metres per second | m/s | 3.6 | 1 |
| Knots | kn | 1.852 | 0.514444 |
Doing the arithmetic by hand
Each conversion needs one multiplication or one division. Miles per hour and kilometres per hour are related by 1.609344, the factor behind the mph to km/h and km/h to mph converters.
Miles per hour to kilometres per hour
Multiply by 1.609344. A 70 mph limit gives 70 × 1.609344 = 112.65408 km/h, so read it as 112.65 km/h. Round to whole numbers for road signage and to one decimal place for anything that needs to be precise.
Kilometres per hour to miles per hour
Divide by 1.609344, or multiply by 0.621371. A 100 km/h road speed is 100 ÷ 1.609344 = 62.1371 mph, which is 62.14 mph. 160 km/h is 99.42 mph.
Metres per second
Multiply by 3.6 to reach km/h, and by 2.23694 to reach mph. The speed of sound in air is about 343 m/s at 20 °C, which is 343 × 3.6 = 1,234.8 km/h, or 767.27 mph.
Knots
Multiply by 1.852 to reach km/h and by 1.15078 to reach mph. A 30 knot approach is 55.56 km/h, or 34.52 mph. Ten knots, a slow harbour entry, is exactly 18.52 km/h.
Common road speeds
These are the speeds you meet on European and Australasian signage, with the mph equivalents and the knot figures for comparison.
| km/h | mph | knots |
|---|---|---|
| 30 | 18.64 | 16.20 |
| 50 | 31.07 | 27.00 |
| 60 | 37.28 | 32.40 |
| 80 | 49.71 | 43.20 |
| 100 | 62.14 | 54.00 |
| 120 | 74.56 | 64.79 |
What a speed figure actually measures
Road speeds are ground speeds: distance covered over ground, divided by time. Nothing else changes them, so a km/h figure on a sign and a mph figure for the same limit describe the same physical motion.
Air speeds are different. The reading on an aircraft's airspeed indicator is indicated airspeed, a figure derived from the pressure the air is exerting on the pitot tube. It is not the speed over the ground. True airspeed, the speed through the air mass, rises as altitude increases because the air thins, and ground speed adds or subtracts the wind component along the flight path.
Converting an indicated airspeed from knots to miles per hour changes the unit and nothing else. 180 knots is 207.14 mph, and it remains an indicated figure of 180 knots on the dial, not 207 mph of true airspeed. The UnitUtils speed converter and the pair pages work the same way: they apply the exact unit ratio and adjust the digits, not the meaning of the number.
Shortcuts and where they drift
Three one-number approximations cover most road arithmetic, and each is biased in a known direction.
- km/h × 5/8 for mph. Use 0.625. At 100 km/h this gives 62.5 mph against an exact 62.14, so it reads about 0.6 per cent high. Fine for a rough check, wrong for a sign.
- mph × 1.6 for km/h. At 70 mph this gives 112 km/h against an exact 112.65, about 0.6 per cent low.
- knots × 1.15 for mph. At 30 knots this gives 34.5 mph against an exact 34.52. This one is close enough to use as if it were exact.
Errors like these scale with the number, so a shortcut that is acceptable at 30 knots is not acceptable at 600 knots. When the figure matters, use the full factor.
Definitions, precision and imperial units
Every factor on this page traces back to a legal or SI definition rather than to a round figure chosen for convenience, which is why the mile-to-kilometre ratio comes out as 1.609344 and not 1.61. The definitions section of the methodology sets out where each constant comes from.
Results are shown to six significant figures by default, so 100 km/h appears as 62.1371 mph and can be expanded further if you need more digits. Trailing digits beyond that are display choices, not extra accuracy in the underlying factor.
For the wider picture on why the US and UK versions of a unit often differ, see the guide to US and imperial units. Speed is unaffected: the US and the UK use the same statute mile, so a figure in mph means the same in both countries.