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? 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
- When was the last Boeing 747 delivered? Also 31 January 2023
- When did Microsoft Office last change its default font? Also 2023
- When did the London Underground last gain a line? Also 24 May 2022
- When was the last leap second added? Also 31 December 2016
- When was an element last added to the periodic table? Also 28 November 2016
- When was the last VCR manufactured? Also July 2016
- Everything, most recent first 36 last occurrences
Sources
- The International System of Units (SI Brochure) — Bureau International des Poids et Mesures