Speed of Light Calculator

Enter a time value and choose your time unit (picoseconds to hours), then pick a medium (vacuum, air, water, glass, or optical fibre) and a distance unit — the calculator returns the distance light travels in that time, plus the effective speed of light in that medium. You can also flip the calculation to find travel time from a known distance. Also try the find Refractive Index (n) with Index of Refraction Calculator.

Solve For *

Only used when Medium is set to 'Custom Refractive Index'.

Required only when solving for Time.

Results

Distance Travelled by Light

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Travel Time

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Speed of Light in Medium

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Refractive Index (n)

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Equivalent Earth Circumferences

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Results Table

What is the speed of light, and how fast is it?

The speed of light in a vacuum is exactly 299,792,458 meters per second (approximately 300,000 km/s or 186,282 miles per second). This is the universal speed limit — no matter or information can travel faster. In one second, light can travel roughly 7.5 times around the entire Earth. See also our calculate Focal Length (f), Lens Power (D) & Lens Magnification (M) — Lens Maker Equation.

How do I calculate the distance light travels in a given time?

Use the formula: distance = speed of light × time. For example, in 1 second in a vacuum, light travels 299,792,458 metres. If the medium is not a vacuum, multiply the time by the effective speed c/n, where n is the refractive index of the medium.

Does the speed of light change in different media?

Yes. Light slows down when travelling through any medium other than a vacuum. The effective speed is v = c / n, where n is the refractive index of the material. For water (n ≈ 1.333) light travels at about 225,000 km/s, and in glass (n ≈ 1.5) at about 200,000 km/s.

What is the speed of light in mph when in a vacuum?

In a vacuum, light travels at approximately 670,616,629 miles per hour (mph), or roughly 186,282 miles per second. This is about 874,030 times faster than the speed of sound in air. You might also find our Earth Curvature Calculator useful.

Is the speed of light always constant?

The speed of light in a vacuum (c) is a universal physical constant and does not change. However, when light passes through a medium such as water, glass, or air, it slows down by a factor equal to the medium's refractive index (n). The value c itself remains unchanged.

How far does light travel in 1 minute?

In 1 minute (60 seconds) in a vacuum, light travels approximately 17,987,547,480 metres, or roughly 17,987,547 kilometres (about 11,176,944 miles). This distance is sometimes called a 'light-minute' and is used in astronomy.

What is a refractive index and how does it affect light speed?

The refractive index (n) measures how much a medium slows down light compared to a vacuum. A higher n means light travels more slowly. Vacuum has n = 1 (no slowing), water has n ≈ 1.333, and glass typically has n ≈ 1.5. You can enter a custom n value in this calculator for any material.

What astronomical distance units are used for light travel?

Common astronomical units based on light travel include the light-second (~299,792 km), light-minute (~17.99 million km), light-hour (~1.08 billion km), and light-year (~9.46 trillion km). An Astronomical Unit (AU) is the average Earth–Sun distance (~149.6 million km), and light takes about 8 minutes 20 seconds to travel from the Sun to Earth.