The factor is exact, because the international mile is defined as exactly 1,609.344 metres.
| MPH | KM/H | Where you see it |
|---|---|---|
| 20 mph | 32.2 km/h | UK residential zones |
| 25 mph | 40.2 km/h | US residential |
| 30 mph | 48.3 km/h | UK built-up areas |
| 40 mph | 64.4 km/h | |
| 50 mph | 80.5 km/h | |
| 55 mph | 88.5 km/h | |
| 60 mph | 96.6 km/h | UK single carriageway |
| 65 mph | 104.6 km/h | common US interstate |
| 70 mph | 112.7 km/h | UK motorway |
| 75 mph | 120.7 km/h | some US interstates |
| 80 mph | 128.7 km/h | |
| 100 mph | 160.9 km/h |
The mental shortcut
mph to km/h: add 60%. Take the number, add half of it, then add a tenth. 60 mph → 60 + 30 + 6 = 96 km/h, true value 96.6.
km/h to mph: subtract about 40%. Halve it and add a bit back — 100 km/h → roughly 62 mph, true value 62.1.
There is also the Fibonacci trick, which is a genuine curiosity: consecutive Fibonacci numbers approximate the mile-kilometre ratio. 5 mi ≈ 8 km, 8 mi ≈ 13 km, 13 mi ≈ 21 km. It works because the ratio 1.609 is close to the golden ratio 1.618.
Which countries use which
Almost the whole world posts limits in km/h. The significant exceptions are the United States and the United Kingdom, both of which use mph. Also worth knowing: Ireland switched to km/h in 2005, so a drive from Northern Ireland into the Republic changes units mid-journey.
A car hired in one country keeps its home speedometer. Driving a UK car in France means reading mph against km/h signs, and 130 km/h looks reassuringly like a number you might mistake for mph. It is 81 mph. Most modern dashboards can switch units in the settings menu — worth doing before you set off rather than at 130.
Knots, and why they are not just another unit
1 knot = 1 nautical mile per hour = 1.852 km/h exactly. The nautical mile is not an arbitrary length — it is one minute of latitude, which makes navigation arithmetic on a chart trivial. A degree of latitude is 60 nautical miles, everywhere.
That is why aviation and shipping kept it while everything else went metric: it is geometrically useful in a way kilometres are not.
Mach
Mach is a ratio, not a fixed speed — it compares an object's speed to the local speed of sound, which depends on temperature. At sea level around 15°C, Mach 1 is roughly 1,225 km/h or 761 mph. In the thin, cold air at cruising altitude it drops to about 1,062 km/h. So a jet can hold a constant Mach number while its speed over the ground changes.
Speed conversion mph, km/h, m/s, knots, ft/s and Mach.
Open the converter