---
title: "When were the SI base units last redefined?"
description: "The kilogram, ampere, kelvin and mole were all redefined at once, 20 May 2019. What happened, why it has not happened since, and whether it plausibly could again."
url: https://atoneplace.net/the-last-time/si-base-units/
site: "At One Place"
---

# When were the SI base units last redefined?

**When were the SI base units last redefined?: 20 May 2019**

The kilogram, ampere, kelvin and mole were all redefined at once

The kilogram was the last unit defined by a physical object — a platinum-iridium cylinder in a vault near Paris, which had been losing mass relative to its own official copies for a century, in a way nobody could explain and nobody could correct, because by definition it weighed exactly a kilogram. The 2019 redefinition fixed the Planck constant instead, so the kilogram is now defined by physics rather than by an artefact. No SI base unit now depends on a manufactured object.

- **What:** The kilogram, ampere, kelvin and mole were all redefined at once
- **When:** 20 May 2019
- **Category:** Science and measurement
- **Could this change?:** Not in any foreseeable way. The event is closed.

## Science and measurement

| Question | When |
| --- | --- |
| 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 |
| When was the metre last redefined? | 1983 |

> 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 were the SI base units last redefined?

20 May 2019 — The kilogram, ampere, kelvin and mole were all redefined at once.

### What happened?

The kilogram was the last unit defined by a physical object — a platinum-iridium cylinder in a vault near Paris, which had been losing mass relative to its own official copies for a century, in a way nobody could explain and nobody could correct, because by definition it weighed exactly a kilogram. The 2019 redefinition fixed the Planck constant instead, so the kilogram is now defined by physics rather than by an artefact. No SI base unit now depends on a manufactured object.

## Related pages

- [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 was the metre last redefined?](https://atoneplace.net/the-last-time/metre-redefined/)
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- [When was the last leap second added?](https://atoneplace.net/the-last-time/leap-second/)
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- [Everything, most recent first](https://atoneplace.net/the-last-time/)

## Sources

- The International System of Units (SI Brochure) — Bureau International des Poids et Mesures (https://www.bipm.org/en/publications/si-brochure)