Chemistry / Unit 1 Chem 0092 short lesson, about 20 seconds
The Kilogram
Captions available
The idea
The kilogram, SI's unit of mass, is defined since 2019 by a fixed value of the Planck constant, not an object.
In practice
Before 2019 the kilogram was tied to a platinum-iridium cylinder in France; today a lab with the right apparatus realizes it directly.
Where people slip
Some references still call it "a metal cylinder's mass" — accurate before 2019, now outdated. (NIST/BIPM, Aug 20, 2026.)
Your place on the trail
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Builds on
Unlocks
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