Speed of Light Calculator
Calculate how long light takes to cross a distance, how far it travels in a given time, or its speed in any medium. Choose vacuum or a material below, pick your units, enter two of the three quantities, and the third is solved instantly. The panel also shows the speed as a fraction of c and compares travel time in the medium against vacuum.
Formula
Worked example
Light from the Sun (1 AU = 149,597,871 km) travels in vacuum at c = 299,792 km/s, so travel time t = 149,597,871 / 299,792 = 499.0 s = 8 min 19 s. In water (n = 1.333), the speed drops to 224,901 km/s and the same journey would take 665.2 s.
What is the speed of light?
The speed of light in vacuum, c, is exactly 299,792,458 metres per second by definition of the SI metre since 1983. It is the fastest speed at which information or energy can travel in the universe, and it serves as a conversion factor between space and time in Einstein's theory of special relativity. In everyday units it is approximately 300,000 km/s, 186,282 miles per second, or about 670 million miles per hour.
How light slows in transparent media
Light slows when it enters a transparent material because its electric and magnetic fields interact with the atoms of the medium, causing brief absorption and re-emission events at each atom. The ratio of the vacuum speed to the phase velocity in the material is the refractive index n: v = c / n. Air is so thin that n is only 1.0003, reducing the speed by a tiny fraction. Water (n = 1.333) slows light to about 225,000 km/s. Optical glass (n = 1.5) gives roughly 200,000 km/s. Diamond has one of the highest natural values, n = 2.417, cutting the speed below 125,000 km/s - which also explains why diamonds scatter visible light into such vivid spectral colours.
Three ways to use this calculator
Travel time mode answers: "How long does it take light to reach Mars?" Enter 78.3 million km in the distance field and leave the medium on vacuum; the answer is about 261 seconds. Distance covered mode answers: "How far does a fibre-optic signal travel in 1 millisecond?" Select optical fibre (n = 1.467), enter 1 ms, and the result is about 204 km. Speed mode works backward: given a measured distance and time, it derives the effective speed and the implied refractive index, useful for experimental verification of a material's optical properties.
Astronomical distances and light travel times
Because interstellar and intergalactic distances are so vast, astronomers measure them in light-seconds, light-minutes, light-years, and parsecs (1 pc = 3.26 ly). Earth-Moon is 1.28 light-seconds (one-way). Earth-Sun is 8 min 19 s (499 s). Neptune at closest approach is about 4 light-hours away. The nearest star, Proxima Centauri, is 4.24 light-years distant. The Andromeda galaxy is 2.537 million light-years away. Choosing light-year or AU as your distance unit in this calculator makes these numbers easy to enter.
Speed of light in common media
| Medium | Refractive index (n) | Speed (km/s) | Speed (% of c) |
|---|---|---|---|
| Vacuum | 1.0000 | 299,792 | 100.000% |
| Air | 1.0003 | 299,702 | 99.970% |
| Water | 1.333 | 224,901 | 75.016% |
| Optical fibre | 1.467 | 204,355 | 68.145% |
| Glass | 1.500 | 199,862 | 66.667% |
| Diamond | 2.417 | 124,048 | 41.374% |
Phase velocity of light in common media, derived from their refractive indices at visible wavelengths.
Frequently asked questions
What is the exact speed of light in vacuum?
The speed of light in vacuum is exactly 299,792,458 metres per second. This value is exact, not a measurement, because the SI metre is defined by fixing c at that number. In more practical units: approximately 299,792 km/s, 186,282 miles per second, or 670,616,629 mph.
Does light always travel at the same speed?
Light always travels at c in vacuum. In any transparent medium it slows to c / n, where n is the refractive index (always 1 or greater). The speed also varies slightly with wavelength inside materials, a property called dispersion - this is why a prism splits white light into a rainbow. The speed of c is the ultimate speed limit for matter and information; nothing with mass can reach it.
How long does it take light to travel from the Sun to Earth?
The average Earth-Sun distance is 1 AU = 149,597,871 km. At c = 299,792 km/s, travel time is 149,597,871 / 299,792 = 499.0 seconds, about 8 minutes and 19 seconds. This means we see the Sun as it was roughly 8.3 minutes ago.
What is the refractive index and how does it affect light speed?
The refractive index n of a material is the ratio of the speed of light in vacuum to the speed in that material: n = c / v. Vacuum has n = 1 by definition. A higher n means slower light. Water has n = 1.333 (light at 75% of c), glass has n = 1.5 (67% of c), and diamond has n = 2.417 (41% of c). The refractive index is also the quantity that governs how much light bends when it crosses a boundary between two materials (Snell's law).
How far does light travel in one nanosecond?
In vacuum, one nanosecond (1 ns = 1e-9 s) corresponds to 299,792,458 x 1e-9 = 0.2998 metres, or about 30 cm (roughly one foot). This is why in high-speed electronics, a single nanosecond delay matters - light only travels about 30 cm in that time, so signal propagation across a circuit board is measurable in nanoseconds.
What is a light-year?
A light-year is the distance light travels in one Julian year (365.25 days = 31,557,600 seconds). At c = 299,792,458 m/s, that gives 9.461 x 10^15 metres, or about 9.461 trillion kilometres (5.879 trillion miles). Light-years are used for stellar and galactic distances. The nearest star is 4.24 ly away; the Milky Way is about 100,000 ly across.
Why does optical fibre use a refractive index above 1?
Optical fibre cores are made of silica glass with n around 1.467. Light travels through the core by total internal reflection off the cladding layer (which has a slightly lower n), so it zig-zags along the fibre rather than escaping. The effective propagation speed is c / n = about 204,355 km/s, which means a signal takes about 4.9 microseconds to travel 1 km of fibre. For 1,000 km of transatlantic cable, one-way delay is approximately 4.9 ms from propagation alone, before accounting for amplifiers and switching.