The km/h to mph formula
Divide the speed in kilometers per hour by 1.609344, or multiply it by 0.6213711922, which is the same thing. 100 km/h is 100 ÷ 1.609344 = 62.1371 mph.
The factor 0.6213711922 is a rounded view of an exact fraction, 15625/25146, because a kilometer is not a whole number of miles. The converter keeps the exact fraction and only rounds the answer, so converting 100 km/h to mph and back gives 100 again.
Reading European signs from the driver's seat
If your car and your head work in mph but the signs are in km/h, these are the numbers to remember:
- 30 km/h is 18.6 mph, a common limit near schools and in residential zones.
- 50 km/h is 31.1 mph, a typical town limit.
- 80 km/h is 49.7 mph and 90 km/h is 55.9 mph, common on country roads.
- 100 km/h is 62.1 mph, 120 km/h is 74.6 mph and 130 km/h is 80.8 mph, common on motorways.
Posted limits differ by country and road, so treat these as conversions of numbers, not as a statement of what the limit is where you are. The table below the converter covers 10 to 140 km/h in steps of 10.
A shortcut that is hard to forget
Five eighths is 0.625, and 1 ÷ 1.609344 is 0.6214, so multiplying by 5 and dividing by 8 is within 0.6% of the exact answer. To convert 80 km/h, take 80 × 5 = 400 and divide by 8 to get 50 mph. The exact answer is 49.71. That is precise enough for reading road signs.
Another rule is to multiply by 0.62, which is within 0.22% of the exact factor. Most people find the five eighths rule easier to do in their head.
Other places km/h appears
Cycling and running machines, wind forecasts, train and flight speeds, and sports such as sprinting all quote km/h. A cyclist riding at 25 km/h is doing 15.5 mph, and a jet at 900 km/h is doing about 559 mph. The informal abbreviation kph means the same as km/h, although km/h is the official SI symbol.