Revision of the SI

  1.  Chronology – Key steps in the history of the International System of Units (SI)
  2. Key Papers in Metrologia leading up to the 2018 revision of the International System of Units (SI)
  3. Frequently Asked Questions

Watch live broadcast for committee

Connect to the BIPM’s YouTube channel on 16 November 2018 at 11 a.m. Paris time (10:00 UTC), to watch the live open session of the 26th General Conference on Weights and Measures:

Scientists to Vote on Metric Makeover

At its 25th meeting (November 2014) the CGPM adopted a Resolution on the future revision of the International System of Units. This Resolution built on the CGPM’s previous Resolution (2011), which took note of the CIPM’sintention to propose a revision of the SI and set out a detailed roadmap towards the future changes.

The 26th General Conference on Weights and Measures (CGPM) will take place in Versailles in November 2018. At this meeting, we expect the CGPM to approve a resolution to revise the definitions of the International System of Units, the SI, which is based on the second, the metre, the kilogram, the ampere, the kelvin, the mole and the candela (the SI base units).

Over the last 50 years, scientists have measured constants of nature, such as the speed of light and the Planck constant, with increasing accuracy; indeed the metre was already redefined back in 1983 in terms of the speed of light. The inherent stability of these constants make them ideal for underpinning new definitions of the SI units and preparing the measurement system to meet the future demands of science and technology.

If the resolution is approved, the SI will be based on seven physical constants, and thus inherently stable. Most notably, this will mark the end of the last remaining physical artefact in the SI system – a cylinder of metal known as the International Prototype of the Kilogram.

The work needed to reach this point has taken many years, and has been a truly international effort. The universality of access that it will afford has been a long-standing goal for the metric system, dating back more than 200 years.

In the revised SI four of the SI base units – namely the kilogram, the ampere, the kelvin and the mole – will be redefined in terms of constants; the new definitions will be based on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (kB), and the Avogadro constant (NA), respectively. Further, the definitions of all seven base units of the SI will also be uniformly expressed using the explicit-constant formulation, and specific mises en pratique will be drawn up to explain the realization of the definitions of each of the base units in a practical way.



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