Mph to kph converter

Formula

How to convert mile per hour to kilometer per hour (mph → km/h)

One mile per hour equals exactly 1.609344 kilometers per hour. This is an exact definition, not an approximation, so the factor never changes.

Formula: kilometers per hour = miles per hour × 1.609344

To go the other way, multiply by 0.6213711922: miles per hour = kilometers per hour × 0.6213711922.

For example, 30 mph × 1.609344 = 48.2803 km/h.

Background

What is a mile per hour?

Miles per hour (mph) is a unit of speed equal to one mile of distance covered in one hour. One mph equals exactly 1.609344 kilometers per hour.

Miles per hour became the standard speed unit on roads in the English-speaking world as cars spread in the early twentieth century, and it remains in use in the US and UK.

Everyday scale: A relaxed walk is about 3 mph, a typical city speed limit is 25 to 30 mph, and highway driving is usually 55 to 75 mph.

System: imperial / US customary.

Background

What is a kilometer per hour?

Kilometers per hour (km/h) is a unit of speed equal to one kilometer of distance covered in one hour. One km/h equals about 0.621371 miles per hour.

Kilometers per hour is the standard speed unit for road traffic in most of the world, shown on speedometers and speed-limit signs outside the US and UK.

Everyday scale: A relaxed walk is about 5 km/h, a common urban speed limit is 50 km/h, and highway driving is usually 100 to 120 km/h.

System: metric (SI).

Accuracy

Precision and common mistakes

The mile per hour to kilometer per hour factor is exact by definition, so any error comes from rounding too early. Keep full precision through your calculation and round only the final answer to the precision your task needs.

A common mistake is rounding the conversion factor to two or three digits before multiplying, which can shift the result noticeably on large values. Multiply first, then round. Always keep the unit label with the number so the meaning is never lost.