Timeline of International Kilogram Prototypes

Official copies of the International Prototype of the Kilogram (IPK), the 1 kg platinum–iridium alloy right circular cylinders, disseminated the kilogram from 1889 until the redefinition based on physical constants in 2019. These prototypes underpinned global trust in scientific discovery, industrial manufacturing, and international trade for over a century.

Under the Metre Convention's framework for international collaboration in metrology, the pure platinum "Kilogram of the Archives" standard from 1799 was replaced by the platinum–iridium International Prototype of the Kilogram (IPK) in 1879. Pure platinum was too soft for a durable mass standard, but the addition of just 10% iridium in the alloy greatly increased hardness while still retaining extreme resistance to oxidation, extremely high density, and low magnetic susceptibility. The harder alloy reduced wear and allowed the prototypes to be finished to a high polish, minimising variability.

The IPK and six sister copies are stored under secure environmental controls at the International Bureau of Weights and Measures (BIPM) in the Pavillon de Breteuil. Other copies, manufactured primarily by Johnson Matthey beginning in 1879, were distributed to national metrology institutes of countries that had ratified and conformed to the Treaty of the Metre (and to certain non‑national organisations). Each copy carries a unique identification number and served as a primary mass standard, providing traceability of local measurements to the IPK through periodic comparisons.

The timeline shows the year of assignment and the year of last known calibration. The entries fall into three broad groups:

• Copies 0–40 — Foundational prototypes and early national standards: the IPK itself, its six BIPM sister copies, and the first wave of official allocations to original signatories after the 1st CGPM, with detailed custody and calibration histories.
• Copies 44–63 — Mid‑period issues and expanding membership: mid‑career Johnson Matthey productions allocated to new member states, as well as replacement or supplementary prototypes.
• Copies 75–special designations — Late‑period and special‑purpose prototypes: later allocations, non‑sequential or experimental artefacts, and prototypes intended for particular scientific or commemorative purposes, each with its own custodial context.

List of official copies of the International Prototype of the Kilogram (IPK)

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