Speed of Light in Miles Per Second, MPH and Other Units
The speed of light in a vacuum is exactly 299,792,458 metres per second. Expressed in imperial units that is about 186,282 miles per second, or roughly 670,616,629 miles per hour. The figure is exact only in metres: since 1983 the metre itself has been defined from c, so the value cannot be “improved” by better measurement.
The familiar “186,000 miles per second” from older textbooks is the same number rounded. For homework, 3.00 × 10⁸ m/s (or 1.86 × 10⁵ mi/s) is normally accurate enough; use the full 299,792,458 m/s when precision matters.
How to Convert the Speed of Light to Any Unit
Start from c = 299,792,458 m/s and divide by the size of your target unit in metres. For miles per second, divide by 1,609.344; for miles per hour, multiply that result by 3,600; for kilometres per hour, multiply c in km/s by 3,600, which gives 1,079,252,849 km/h. The table above does all of these at once.
A handy rule of thumb: light travels about one foot per nanosecond (0.98 ft, or 30 cm). Computer engineers use it constantly, because it sets how far a signal can travel in one clock tick.
How Long Does Light Take to Travel?
Divide the distance by c. A few reference points, in a vacuum:
| Distance | Light travel time |
|---|---|
| 1 foot | 1.02 ns |
| Once around the Earth (40,075 km) | 0.134 s |
| Earth to Moon (mean, 384,400 km) | 1.28 s |
| Earth to Sun (1 AU, 92.96 million miles) | 8 min 19 s |
| Nearest star, Proxima Centauri | about 4.25 years |
Speed of Light in Water, Glass and Air
Light slows down inside matter. In a material with refractive index n, its speed is v = c / n. Using typical values for visible light:
| Medium | n | Speed (miles/s) |
|---|---|---|
| Vacuum | 1 | 186,282 |
| Air (sea level) | 1.000293 | 186,228 |
| Water | 1.333 | 139,747 |
| Typical glass | 1.5 | 124,188 |
| Diamond | 2.417 | 77,072 |
Refractive index varies slightly with wavelength and temperature, so treat these as typical values. For the physics of why light bends when its speed changes, see the Refractive Index Calculator and the full speed of light article.