What Is The Speed Of Light In Miles Per Hour?

This unit conversion example problem demonstrates how to convert the speed of light in meters per second to miles per hour.

Problem

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Meter Definition and Unit Conversions

By Anne Marie Helmenstine, Ph.D.

The speed of light in a vacuum is 2.998 x 10⁸ m/sec. What is this speed in miles per hour?

Solution

To convert this measurement, we need to convert meters to miles and seconds to hours. To do this, we need the following relationships:
1000 meter = 1 kilometer
1 kilometers = 0.621 mile
60 seconds = 1 minute
60 minutes = 1 hour
We can now set up the equation using these relationships so the units cancel out leaving only the desired miles/hour.
speed_MPH = 2.998 x 10⁸ m/sec x (1 km/1000 m) x (0.621 mi/1 km) x (60 sec/1 min) x (60 min/1 hr)
Note all the units cancelled out, leaving only miles/hr:
speed_MPH = (2.998 x 10⁸ x 1/1000 x 0.621 x 60 x 60) miles/hr
speed_MPH = 6.702 x 10⁸ miles/hr

Answer

The speed of light in miles per hour is 6.702 x 10⁸ miles/hr.