The mph to km/h formula
Multiply the speed in miles per hour by 1.609344. A speed of 60 mph becomes 60 × 1.609344 = 96.56064 km/h.
The factor is not a measurement, it is a definition. A mile is 5,280 feet, a foot is exactly 0.3048 meters, and so a mile is exactly 1,609.344 meters. Since 1 July 1959 the yard has been defined as exactly 0.9144 meters by agreement between the United States, the United Kingdom and other countries, which fixed the mile too. A mile per hour is 1,609.344 meters per 3,600 seconds and a kilometer per hour is 1,000 meters per 3,600 seconds, so the time cancels and 1,609.344 ÷ 1,000 = 1.609344 is what is left. To get meters per second instead, multiply mph by 0.44704.
Speed limits on both sides of the sign
Road signs in the United States and the United Kingdom use mph, so drivers who travel elsewhere meet km/h signs and the other way round. A few anchors help:
- 30 mph (a common built-up limit in the UK) is 48.3 km/h.
- 55 mph is 88.5 km/h and 65 mph is 104.6 km/h, both familiar US limits.
- 70 mph (the UK motorway limit) is 112.7 km/h.
- 75 mph is 120.7 km/h, and 80 mph is 128.7 km/h.
In km/h countries a town limit of 50 km/h is 31.07 mph, which is why it feels a little faster than a 30 mph limit. The table under the converter lists 5 to 80 mph in steps of 5, then 90 and 100 mph.
Converting in your head
Multiplying by 1.6 is close enough for a speed limit: it is only 0.58% too low. Add a little if you want to be nearer, for example 60 mph is 96 by the shortcut and 96.56 exactly.
A neat trick uses the Fibonacci numbers 5, 8, 13, 21, 34, 55 and 89. Each is about 1.618 times the one before, which is close to 1.609. So 5 mph is about 8 km/h, 13 mph is about 21 km/h and 55 mph is about 89 km/h (exactly 88.5). It only works at those particular values, but they cover most road speeds.
Where the dial and the number can disagree
This converts a number, so it assumes the speed you typed is the true speed. In many countries a speedometer is allowed to read a little higher than your real speed, so the dial, a GPS app and a speed camera can all show slightly different figures. If a limit matters, trust the posted limit and a GPS reading rather than a conversion of the dial.