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MPH to KPH Converter

1 mph = 1.609344 km/h exactly. Type in either box.

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Add sixty per cent

This is the conversion every driver needs on holiday. A 70 mph motorway limit at home becomes 113 km/h abroad, and the signs give you no help.

1 mph = 1.609344 km/h exactly

The factor is the mile-to-kilometre factor unchanged, because the "per hour" is on both sides and cancels. A mile is exactly 1,609.344 metres by the 1959 international agreement, so this figure is exact.

For mental work, add sixty per cent. 70 mph plus 60 % is 112, against a true 112.65. The shortcut runs about half a per cent low right across the useful range.

Miles per hour

The road speed unit in the United States and the United Kingdom, and still common in aviation for ground speed. One mile per hour is exactly 0.44704 m/s, because the mile is defined exactly in metres.

Against kilometres per hour the factor is 1.609344 — the same number as miles to kilometres, since the hour cancels on both sides.

Kilometres per hour

The everyday speed unit almost everywhere outside the United States and the United Kingdom. Road signs, car speedometers and weather reports all use it.

The relationship to metres per second is exactly 3.6, because there are 3,600 seconds in an hour and 1,000 metres in a kilometre. Dividing by 3.6 is the one speed conversion with no rounding in it at all.

The working, step by step

Every line updates with whatever you type above. The point is the method, which is the same for any pair of units in this family.

The same quantity, two different numbers

One physical amount, read off two scales. The tick marks do not line up, and the gap between them is the conversion factor made visible.

How much is that?

A number is easier to judge next to something familiar.

Your turn

Reading a method and being able to use it are different things. Answer within half a per cent and it counts as correct.

Score: 0 of 0

How to use this converter

  1. Type into either box. The other side updates as you go; there is no button to press.
  2. Swap the direction with the button between the boxes, carrying your value across.
  3. Set the decimals if the default is more or less precision than you need.
  4. Copy the answer with its unit, ready to paste.
  5. Use the shortcut buttons for the values that come up most often.
  6. Share the result. The address bar tracks what you type, so the link you copy opens on the same number.

Common uses and tips

Where speed conversions come up

  • Driving abroad. A 100 km/h limit is 62 mph; a 70 mph limit is 113 km/h. Hire cars usually show both, but the signs show only one.
  • Weather. Wind speed comes in km/h, mph, knots or metres per second depending on the service. Storm warnings are almost always in knots at sea and mph or km/h on land.
  • Running and cycling. Apps report pace in minutes per kilometre or per mile and speed in km/h or mph, often on the same screen.
  • Physics and engineering. Anything going into a formula needs metres per second, because that is what the SI equations expect.
  • Aviation and sailing, where knots are standard because a nautical mile is one minute of latitude.

Numbers worth carrying

  • km/h to m/s: divide by 3.6. Exactly, with no rounding — 3,600 seconds in an hour, 1,000 metres in a kilometre.
  • mph to km/h: multiply by 1.609344. Adding 60 % is a good mental version: 70 mph + 60 % = 112, against a true 112.7.
  • knots to mph: add about 15 %. One knot is 1.15078 mph.
  • The speed of sound is about 343 m/s, or 1,235 km/h, or 767 mph at room temperature. It falls as the air gets colder, which is why an aircraft's Mach number changes with altitude at constant airspeed.

Mistakes worth avoiding

  • Speed and pace are reciprocals. Doubling your speed halves your pace. Converting one as though it were the other gets the arithmetic backwards.
  • A knot is already "per hour". There is no such thing as "knots per hour" for speed — the hour is inside the unit.
  • Rounding matters at speed limits. 30 mph is 48.3 km/h, and a limit posted as 50 km/h is 31 mph, which is not the same thing.

Values people look up most

Each one opens the converter with the number already in it.

Quick reference

Miles per hourKilometres per hour
5 mph8.04672 km/h
10 mph16.09344 km/h
20 mph32.18688 km/h
30 mph48.28032 km/h
40 mph64.37376 km/h
50 mph80.4672 km/h
60 mph96.56064 km/h
70 mph112.65408 km/h
80 mph128.74752 km/h
100 mph160.9344 km/h
120 mph193.12128 km/h
200 mph321.8688 km/h

Questions people ask

What is the formula for mph to km/h?

Multiply by 1.609344.

What is 70 mph in km/h?

112.65 km/h. British motorway speed limits translate to roughly 113 km/h.

Why is the factor the same as miles to kilometres?

Because the hour appears on both sides of the conversion and cancels out. Only the distance unit changes.