150 years of the Metre Convention

On 20 May 1875, seventeen states signed the Metre Convention in Paris, creating the International Bureau of Weights and Measures (BIPM) and the intergovernmental machinery that still governs world measurement. In 2025 the convention marked its 150th anniversary — a rare case of a nineteenth-century treaty not merely surviving into the twenty-first century but becoming more relevant with age.

The anniversary is why 20 May is World Metrology Day, observed annually and — since UNESCO's endorsement in 2023 — recognized as a UNESCO international day. Member states have grown from seventeen to the sixty-plus that fund the BIPM today, with dozens more participating as associates.

The convention's most practical modern offspring is the CIPM Mutual Recognition Arrangement (1999): the framework under which national metrology institutes compare their standards, publish their calibration and measurement capabilities in the BIPM's key comparison database, and accept one another's certificates. It is the reason a calibration traceable to NIST is accepted in Berlin and one traceable to PTB is accepted in Denver — the quiet infrastructure under every 'traceable to SI' statement in this library.