km/h to mph

Multiply by 0.621371. To go the other way, multiply mph by 1.609344.

The formula

mph = km/h × 0.621371

The number comes from the definition of the mile. One international mile is exactly 1,609.344 metres, fixed by international agreement in 1959. So one kilometre is 1 ÷ 1.609344 = 0.621371 miles, and the ratio carries directly into speeds because both units share the same hour.

Doing it in your head

The reliable trick is multiply by 6 and drop a digit, which approximates 0.6 rather than 0.621 — so it under-reads by about 3 percent.

• 100 km/h → 100 × 6 = 600 → 60 mph (true answer 62.1)
• 50 km/h → 300 → 30 mph (true 31.1)

For better accuracy, add back a twentieth: 60 + 3 = 63, against a true 62.1.

A neater trick uses the Fibonacci sequence, because consecutive Fibonacci numbers approach the golden ratio 1.618, which is remarkably close to 1.609:

• 5 km/h ≈ 3 mph, 8 km/h ≈ 5 mph, 13 km/h ≈ 8 mph, 21 km/h ≈ 13 mph

These are accurate to under 1 percent.

Speed limits in each system

Which countries use which

Only three countries use miles per hour on road signs as standard: the United Kingdom, the United States and Liberia. Everywhere else uses km/h.

The UK is a hybrid worth watching out for — road signs and car speedometers are in mph, but fuel is sold in litres and distances in official documents are often metric.

Self Speed switches between km/h, mph, m/s and knots with one tap, so you never have to convert anything yourself.

Where the kilometre came from

The metre was defined in 1793 by the French Academy of Sciences as one ten-millionth of the distance from the North Pole to the equator, measured along the meridian through Paris. The survey took seven years and, as it turned out, contained a small error — the Earth is slightly flatter than the surveyors assumed — so the original metre was about 0.2 mm short.

Rather than redo it, the length was kept and progressively redefined: first against a platinum bar, then against a wavelength of krypton-86 light, and since 1983 in terms of the speed of light, which is now defined as exactly 299,792,458 metres per second. The metre is whatever distance light covers in 1/299,792,458 of a second.

The mile has an older and messier lineage. It descends from the Roman mille passus, a thousand paces of two steps each, roughly 1,480 m. English statute law in 1593 fixed the mile at eight furlongs — a furlong being the length of a ploughed furrow — giving 5,280 feet, which is why the number is so unmemorable.

Conversions in sport

Athletics runs almost entirely in metric, but a few imperial distances survive and the comparisons are a common source of confusion:

The gap between 1,500 m and the mile is 109 m, which is why the two records are kept separately and why the "four-minute mile" has no metric equivalent.

Frequently asked

Is 100 km/h the same as 60 mph?

Close but not equal. 100 km/h is 62.1 mph. The rough equivalence is useful for estimating, but at motorway speeds a 2 mph error is enough to matter near a limit.

What is 1 km/h in mph?

0.621 mph. Conversely 1 mph is 1.609 km/h.

Why do some speedometers show both?

Cars sold in markets that border a country using the other system usually carry both scales. Canadian and Mexican cars often show mph inside the km/h ring for drivers crossing into the United States, and UK-market cars show km/h for drivers taking the ferry to France.