# Has the kilogram changed?

In late 2018 the kilogram's definition switched from a physical object (the International Prototype Kilogram) to Planck's Constant; a grand move in my opinion.

However, I haven't heard of the consequences resulting from this change. Is 1 kg still 1 kg? Or do we need to now convert pre-2019 masses to the new definition?

• They fixed the value of Planck's constant in such a way that $1 kg$ remains $1 kg$. It's just redefined because having a physical object is inconvenient. – harshit54 Feb 26 at 18:43
• It's worth noting that the physical artifact is known to have been changing in any case. Those changes were small, but they were annoyingly unpredictable. – dmckee Feb 26 at 18:59
• @dmckee True. It was like 50 micrograms lighter or something. Plus kg was the last physical unit not to be based on a constant, no? – Lux Claridge Feb 26 at 19:04

For example, before the change, Planck’s constant was a measured value, $$6.626070150(81)\times10^{-34}$$ Joule-seconds. Now it is a defined value, exactly $$6.62607015\times10^{-34}$$ Joule-seconds. It “changed” to the mean measured value.