mph to km/h

Multiply by 1.609344. To go back, multiply km/h by 0.621371.

The formula

km/h = mph × 1.609344

One international mile is exactly 1,609.344 metres — a definition fixed in 1959 when the US, UK, Canada, Australia, New Zealand and South Africa agreed to standardise the inch at exactly 25.4 mm. Before that, the US and UK miles differed very slightly.

Doing it in your head

Add half, then a tenth. This approximates 1.6 and is accurate to about 0.6 percent:

• 60 mph → 60 + 30 + 6 = 96 km/h (true 96.6)
• 70 mph → 70 + 35 + 7 = 112 km/h (true 112.7)
• 30 mph → 30 + 15 + 3 = 48 km/h (true 48.3)

The rough version — multiply by 1.5 — is quicker but under-reads by 7 percent, which is too much to be useful near a speed limit.

Driving abroad

The conversions that actually matter when you hire a car:

Two traps for British and American drivers on the continent:

1. A 130 sign is not 130 mph. It is 130 km/h, or 81 mph.
2. Your hire car dashboard may be metric. Check which ring of numbers is the

outer one before you set off.

Self Speed switches units with one tap, so you can read your true speed in whichever system the signs are using.

Why three countries still use miles

The United Kingdom, the United States and Liberia are the only countries where road signs are posted in miles per hour. The reasons are political rather than technical.

The US formally adopted the metric system as its preferred system in the Metric Conversion Act of 1975 — but the act made conversion voluntary, and public resistance to changing road signs killed the road programme within four years. American science, medicine and the military all went metric; the roads did not.

The UK began converting in 1965 and got most of the way there. Distances on official documents, fuel, food and construction are all metric. Road signs were scheduled for conversion in 1973, then repeatedly deferred on grounds of cost, and the programme was formally abandoned. The result is a country that buys petrol in litres to drive miles.

Liberia's usage descends from close historical ties to the United States.

Common speeds, converted

Frequently asked

What is 60 mph in km/h?

96.6 km/h. This is the UK national speed limit for single carriageways, which is why continental drivers in Britain often see 97 on a metric dashboard and assume they have misread a sign.

Is 100 mph fast?

161 km/h. It is above the motorway limit of essentially every country in the world. Germany's unrestricted autobahn sections are the main exception, and even there the advisory limit is 130 km/h, or 81 mph.

How do I convert quickly without a calculator?

Add half, then add a tenth. For 70 mph: 70 + 35 + 7 = 112, against a true 112.7. It is accurate to under 1 percent, which is good enough for reading a road sign.

Does my phone convert automatically?

Self Speed does. Tap the unit you want and every figure on the page — speed, maximum, average and distance — switches with it, including your saved trip history.