The speed of light in a vacuum is exactly 299,792,458299,792,458299,792,458 metres per second, often approximated as 3×1083\times 10^83×108 m/s.

Quick Scoop

  • Exact value (SI units): 299,792,458299,792,458299,792,458 metres per second.
  • Rounded value for most uses: 3×1083\times 10^83×108 m/s.
  • In kilometres per second: about 300,000 km/s.
  • In miles per second: about 186,000 mi/s.

This value is treated as a universal constant in physics and is usually written as ccc. Since 1983, the metre itself is defined using this speed: one metre is the distance light travels in vacuum in 1/299,792,4581/299,792,4581/299,792,458 of a second, which makes the speed of light “exact by definition” in the SI system.

Tiny bit of story

For centuries people assumed light was instantaneous, but in the late 1600s Ole Rømer used observations of Jupiter’s moon Io to show that light has a finite speed. Over time, increasingly precise experiments led to today’s definition, where instead of “measuring” the speed of light, we define our unit of length so that this speed has a fixed value.

In modern physics, asking “what is the speed of light through a vacuum?” is almost like asking “how long is a metre?”—the two are now locked together by definition.

Key facts table (HTML)

html

<table>
  <thead>
    <tr>
      <th>Quantity</th>
      <th>Value</th>
      <th>Units</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Exact speed of light in vacuum</td>
      <td>299,792,458</td>
      <td>metres per second (m/s)[web:3]</td>
    </tr>
    <tr>
      <td>Common rounded value</td>
      <td>3 × 10^8</td>
      <td>m/s[web:1][web:9]</td>
    </tr>
    <tr>
      <td>Approximate in km/s</td>
      <td>≈ 300,000</td>
      <td>kilometres per second[web:7][web:9]</td>
    </tr>
    <tr>
      <td>Approximate in mi/s</td>
      <td>≈ 186,000</td>
      <td>miles per second[web:7][web:8]</td>
    </tr>
    <tr>
      <td>Symbol</td>
      <td>c</td>
      <td>Standard physics notation[web:3][web:8]</td>
    </tr>
  </tbody>
</table>

TL;DR: The speed of light through a vacuum is exactly 299,792,458299,792,458299,792,458 m/s, usually quoted as 3×1083\times 10^83×108 m/s.

Information gathered from public forums or data available on the internet and portrayed here.