Miles per Hour to Kilometers per Hour

Speed limits, car specifications and weather reports switch systems at borders, and the mile per hour is the one that keeps turning up in British and American sources. An hour is an hour on both sides, so the factor is exactly the one that relates miles to kilometers: 1.609344.

How it is calculated

km/h = mph × 1.609344

Because only the distance unit changes, this is the same exact factor as for miles to kilometers. The common shortcut of adding half again is out by 7 % — at 70 mph that is eight kilometers per hour, which is the difference between keeping a licence and losing one.

Source: NIST SP 811 (2008), Appendix B.8 — mile per hour (mi / h) to kilometer per hour (km / h) = 1.609 344 E+00

Common values

Questions people ask

What is 70 mph in kilometers per hour?

112.65 km/h. That is the British motorway limit, which is why hire cars from the continent show 113 on the dial when keeping pace with traffic there.

Is there a shortcut I can do in my head?

Multiply by 1.6 and you are within half a percent, which is fine for reading road signs. The trick of adding half the number is much worse — it gives 1.5 instead of 1.609, so it under-reads by 7 %.

Why do some cars show both?

Because they are sold across borders. A dial marked in both is a single mechanism with two scales, so the conversion is baked into the printing rather than computed — the needle points at the same speed either way.

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