Speed Of Light to Miles Per Hour (c to mph)

1 speed of light equals 670,616,629.4 miles per hour (rounded).

1 speed of light = 670,616,629.4 miles per hour

How to convert speed of light to miles per hour

To convert speed of light to miles per hour, multiply the number of speed of light by 670,616,629.4. The factor is rounded to 10 significant digits. To go the other way, multiply miles per hour by 1.491164931×10⁻⁹ (rounded).

miles per hour = speed of light × 670,616,629.4

Example: 10 speed of light = 6,706,166,294 miles per hour.

About these units

Speed Of Light (c)

ScientificCurrent unit

Exactly 299,792,458 m/s, the speed of light in a vacuum and the fundamental constant that defines the meter.

Light was once thought to arrive instantly. In 1676 Ole Rømer timed eclipses of Jupiter's moon Io and concluded that light takes about 22 minutes to cross Earth's orbit. In 1729 James Bradley used the aberration of starlight to estimate about 8 minutes 12 seconds from Sun to Earth. Later experimenters used a toothed wheel (Fizeau), a rotating mirror (Foucault), and in 1930 to 1935 Michelson, Pease and Pearson used a mile-long vacuum chamber. The letter c is itself uncertain: it may stand for constant or the Latin celeritas, swiftness, and Einstein switched to it in 1907.

Since 1983 the speed of light has been fixed at exactly 299,792,458 m/s, and the meter is defined as the distance light travels in a vacuum in 1/299,792,458 of a second. Measurement therefore became a matter of defining the constant, not of improving experiments. For scale, light takes about 1.3 seconds to travel from the Earth to the Moon, about 8.3 minutes from the Sun, and roughly one nanosecond to go a foot. James Clerk Maxwell showed in 1865 that light is an electromagnetic wave.

1 speed of light = 299,792,458 meters per second (exactly).

Sources: Wikipedia: Speed of light; NIST: the meter and the SI redefinition

Mile Per Hour (mph)

US & ImperialCurrent unit

Exactly 0.44704 m/s. The unit for road speeds in the US, UK and a few other countries.

Early British motoring law is the best window into this unit. The 1861 Locomotive Act set a limit of 10 mph for light locomotives. The 1865 Red Flag Act cut it to 4 mph in the country and 2 mph in towns, with a person walking ahead with a flag. The Locomotives on Highways Act of 1896 raised it to 14 mph, and the Motor Car Act of 1903 to 20 mph. In 1931 all limits for cars were abolished, and 30 mph in built-up areas returned in 1935.

The mile per hour combines the mile, set at 5,280 feet in Elizabeth I's reign, with the hour, and it equals exactly 0.44704 m/s or 1.609344 km/h. The United Kingdom and United States use it on roads, along with several smaller countries, mostly territories or former territories of those two, and the US, Canada and Ireland use it for rail. Sports such as cricket, tennis and baseball quote ball speeds in it. Aviation and shipping prefer knots, about 0.869 of one mph each.

1 mile per hour = 0.44704 meters per second (exactly).

Sources: Wikipedia: Miles per hour; Wikipedia: Speed limits in the United Kingdom; Wikipedia: Mile

Related conversions

See all speed units.