20 May is World Metrology Day, commemorating the anniversary of the signing of the Metre Convention in Paris, in 1875. This treaty provides the basis for a worldwide coherent measurement system that underpins scientific discovery and innovation, industrial manufacturing and international trade, as well as the improvement of the quality of life and the protection of the global environment.

The theme for World Metrology Day 2026 is "Metrology: Building Trust in Policy Making", which highlights the role metrology plays in providing scientific evidence and legal frameworks that policy makers can use to make informed, transparent and enforceable decisions.

Every day, scientific and legal metrology work hand-in-hand to ensure that the global metrology system supports consumer protection and fair trade, enabling the smooth functioning of markets. From ensuring accurate energy and fuel measurements, to facilitating market transactions, transport safety and environmental monitoring, this agreed framework allows measurements to serve as a trusted basis for decisions that affect citizens, markets and cooperation between countries.

Around the world, National Metrology Institutes (NMIs) are central to this process, continually advancing measurement science by developing and validating new measurement techniques at the necessary level of sophistication. NMIs participate in measurement comparisons coordinated by the Bureau International des Poids et Mesures (BIPM) or by the Regional Metrology Organizations, to ensure the reliability of measurement results worldwide.

Alongside NMIs, national legal metrology authorities play a distinct role in implementing and enforcing legally controlled measurements in line with internationally harmonized requirements.

In support of this work, the International Organization of Legal Metrology (OIML) develops International Recommendations, which aim to align and harmonize requirements worldwide in many fields. The OIML also operates the OIML Certification System (OIML-CS) which facilitates international acceptance and global trade of regulated measuring instruments.

In November 2023, a significant milestone was reached as the UNESCO General Conference, during its 42nd Session, officially recognized the annual celebration on 20 May. This endorsement opens up new avenues to promote metrology, aligning with UNESCO's mission to build a better world through science and education.

World Metrology Day is an annual event during which the impact of measurement on our daily lives is celebrated around the world.

This date was chosen in recognition of the signing of the Metre Convention on 20 May 1875, the beginning of formal international collaboration in metrology. Each year World Metrology Day is organised and celebrated jointly by the International Bureau of Weights and Measures (BIPM) and the International Organization of Legal Metrology (OIML) with the participation of the national organisations responsible for metrology.

The international metrology community which works to ensure that accurate measurements can be made across the world endeavours to raise awareness each World Metrology Day through a poster campaign and web site. Previous themes have included topics such as measurements for the global energy challenge, for safety, for innovation, and measurements in sport, the environment, medicine and trade.