NIST’s platinum-iridium kilogram K92 (front) with stainless-steel kilogram masses in the background. Credit: J.L. Lee/NIST

A Turning Point for Humanity: Redefining the World’s Measurement System

The International System of Measurement Units (SI) is set to get a major overhaul on May 20, 2019, when three of the units, the kilogram (mass), the ampere (electric current) and the kelvin (temperature) will be redefined. If all goes according to plan, this will not affect your life in any way, but it will make the measurements more precise and easier for measurement labs around the world to realize them. Learn more with our handy guide, and see the timeline of the past, present and future of the SI below.

Timeline of significant events in the history of the international system of units. Credit: N. Hanacek/NIST



NIST promotes U.S. innovation by advancing measurement science, standards and technology in ways that enhance economic security and improve our quality of life.

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