PhysicsCalcs
Optics

Speed of Light Calculator

Calculate light speed in any medium, travel time, and distances across the universe.

Interactive calculator

Speed of Light Calculator

Calculate distance, time, and speed of light in vacuum or any medium. Includes astronomical distance presets and refractive index support.

Try an example

Your result will appear here.

Choose a calculation mode, fill in the known values, and click Calculate.

Quick Guide

  • Choose: distance from time, time from distance, speed in medium, or n from speed.
  • Set n = 1 for vacuum, or select a medium preset.
  • Results include astronomical unit conversions.

Table of Contents

Key Takeaways

  • c = 299,792,458 m/s exactly (SI definition since 1983).
  • Nothing with mass can travel at or faster than c.
  • Light in a medium travels at v = c/n, where n is the refractive index.
  • Earth-Moon: ~1.28 s, Earth-Sun: ~8.3 min, Proxima Centauri: 4.24 years.
  • 1 light-year ≈ 9.461 × 10¹⁵ m ≈ 63,241 AU.

Speed of Light

The speed of light in vacuum, c = 299,792,458 m/s, is the universal speed limit. It is one of the most fundamental constants in physics, linking space and time in Einstein’s special relativity and energy and mass through E = mc².

Formulas

v=cn,c=299,792,458 m/sv = \frac{c}{n}, \quad c = 299{,}792{,}458 \text{ m/s}
d=v×t,t=dvd = v \times t, \quad t = \frac{d}{v}

Speed in Different Media

MediumnSpeed (km/s)% of c
Vacuum1.000299,792100%
Air1.0003299,70399.97%
Water1.333224,90175.0%
Glass1.50199,86266.7%
Diamond2.417124,03441.4%

Astronomical Distances

JourneyDistanceLight travel time
Earth → Moon384,400 km1.28 s
Earth → Sun1 AU8 min 19 s
Earth → Mars (min)54.6 M km3 min 2 s
Sun → Proxima Centauri4.24 ly4.24 years
Milky Way diameter~100,000 ly100,000 years

How to Use

  1. Select a calculation mode.
  2. Enter distance/time and refractive index (1 for vacuum).
  3. Click Calculate for results with astronomical conversions.

Examples

Earth → Sun

t = 1 AU / c = 1.496 × 10¹¹ / 2.998 × 10⁸ = 499 s ≈ 8 min 19 s

Light in water

v = 299,792,458 / 1.333 = 224,901 km/s (75% of c)

FAQ

Why is the speed of light exactly 299,792,458 m/s?›

Since 1983, the metre is defined as the distance light travels in vacuum in 1/299,792,458 of a second. This makes c exact by definition. The speed itself was measured experimentally before this redefinition.

Can anything travel faster than light?›

No physical object or information can travel faster than c. However, some phenomena appear superluminal: phase velocity in dispersive media, galaxy recession due to cosmic expansion, and quantum entanglement correlations (which don't carry information).

Why does light slow down in materials?›

Light interacts with atoms in a material. Photons are absorbed and re-emitted by electrons, causing a net delay. The refractive index n describes this slowing: v = c/n. In water (n ≈ 1.33), light travels at about 75% of c.

What is a light-year?›

A light-year is a unit of distance, not time. It's the distance light travels in one Julian year (365.25 days) in vacuum: approximately 9.461 × 10¹⁵ metres or about 5.879 trillion miles.

Sources

  1. NIST – Speed of Light
  2. IAU – Astronomical Constants
  3. Wikipedia – Speed of Light
Manish Kumar

Author & technical reviewer

Manish Kumar

PhysicsCalcs tools are reviewed with an educational focus: clear formulas, transparent assumptions, and practical context for students and science learners.

Learn more about Manish