MPH to KM/H Calculator

Background.

Multiply by 1.609344 and the conversion is finished — and that factor is exact, not a measurement. Since 1959, when the United States, Britain and several Commonwealth countries agreed on an international yard of exactly 0.9144 meters, the mile of 1,760 yards has been exactly 1,609.344 meters. Both units count distance per hour, so the speed conversion inherits the same factor: 65 mph is 104.60736 km/h, with nothing left over.

The question usually comes up at a border. The United States and the United Kingdom post their speed limits in miles per hour; most of the rest of the world uses kilometers per hour. A driver used to cruising at 65 mph who sees a 100 km/h sign needs to know that 100 km/h is only 62.14 mph. Going the other way, the 130 km/h limit on French autoroutes is 80.78 mph.

For quick work in your head, multiply by 1.6, or add 60%: 65 + 39 = 104 km/h, about 0.6% under the exact answer — close enough for driving, not for published figures.

What is mph to km/h calculator?

Miles per hour and kilometers per hour both measure speed as distance covered in one hour; they differ only in the unit of distance. One international mile is exactly 1.609344 km, so 1 mph is exactly 1.609344 km/h and 1 km/h is about 0.621371 mph. In SI base units, 1 mph is exactly 0.44704 meters per second. Because neither scale has an offset — zero is zero in both — the conversion is a single multiplication.

How to use this calculator.

1. 01Enter the speed in miles per hour, from 0 up to 300 — a speed limit, a speedometer reading or a vehicle specification.
2. 02Read the result in kilometers per hour. It is exact to every digit shown, because the factor is defined rather than measured.
3. 03To go from km/h back to mph, divide the km/h figure by 1.609344, or multiply it by 0.621371.

The formula.

km/h = mph × 1.609344. The factor comes from the definition of the mile: 1,760 yards × 0.9144 m per yard = 1,609.344 m, and dividing by 1,000 gives 1.609344 km. The hour appears on both sides and cancels, so no time conversion is involved. Rounding the factor to 1.6 makes every answer 0.584% too low; that is 0.61 km/h at 65 mph, and it grows in proportion to the speed, so it matters for precise data. The inverse factor, 1 ÷ 1.609344 = 0.621371, is not exact as a decimal, which is why dividing by 1.609344 is the cleaner way back.

A worked example.

Frequently asked questions.

References& sources.

1. [1]National Institute of Standards and Technology (2008). NIST Special Publication 811 — Guide for the Use of the International System of Units (SI), Appendix B: Conversion Factors.
2. [2]National Bureau of Standards (1959). Refinement of Values for the Yard and the Pound — fixed the international yard at exactly 0.9144 meters, effective 1 July 1959, the basis for the exact mile-to-kilometer factor.