---
title: "When was the metre last redefined?"
description: "Defined as the distance light travels in 1/299,792,458 of a second, 1983. What happened, why it has not happened since, and whether it plausibly could again."
url: https://atoneplace.net/the-last-time/metre-redefined/
site: "At One Place"
---

# When was the metre last redefined?

**When was the metre last redefined?: 1983**

Defined as the distance light travels in 1/299,792,458 of a second

The consequence is that the speed of light is no longer measured — it is fixed by definition at exactly 299,792,458 metres per second, and any improvement in measurement now refines the metre rather than the speed. Before 1983 the metre was defined by a krypton-86 emission wavelength, and before that by a metal bar. The bar version, from 1889, was itself an approximation of an eighteenth-century calculation of one ten-millionth of the distance from the pole to the equator, and it was slightly wrong.

- **What:** Defined as the distance light travels in 1/299,792,458 of a second
- **When:** 1983
- **Category:** Science and measurement
- **Could this change?:** Not in any foreseeable way. The event is closed.

## Science and measurement

| Question | When |
| --- | --- |
| When were the SI base units last redefined? | 20 May 2019 |
| When was the last leap second added? | 31 December 2016 |
| When was an element last added to the periodic table? | 28 November 2016 |
| When did the number of planets last change? | 24 August 2006 |

> This page gives a date and never a "years ago". These pages are built as static files, so a computed interval would freeze at build time and quietly become wrong — which on a reference site is worse than making the reader do the subtraction.

## Common questions

### When was the metre last redefined?

1983 — Defined as the distance light travels in 1/299,792,458 of a second.

### What happened?

The consequence is that the speed of light is no longer measured — it is fixed by definition at exactly 299,792,458 metres per second, and any improvement in measurement now refines the metre rather than the speed. Before 1983 the metre was defined by a krypton-86 emission wavelength, and before that by a metal bar. The bar version, from 1889, was itself an approximation of an eighteenth-century calculation of one ten-millionth of the distance from the pole to the equator, and it was slightly wrong.

## Related pages

- [When were the SI base units last redefined?](https://atoneplace.net/the-last-time/si-base-units/)
- [When was the last leap second added?](https://atoneplace.net/the-last-time/leap-second/)
- [When was an element last added to the periodic table?](https://atoneplace.net/the-last-time/new-element/)
- [When did the number of planets last change?](https://atoneplace.net/the-last-time/planet-count/)
- [When did the number of planets last change?](https://atoneplace.net/the-last-time/planet-count/)
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- [Everything, most recent first](https://atoneplace.net/the-last-time/)

## Sources

- Guide for the Use of the International System of Units (SP 811) — NIST (https://www.nist.gov/pml/special-publication-811) [Public domain (US Government)]