Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

International System of Units (SI)


The International System of Units (SI) provides definitions of units of measurement that are widely accepted in science and technology and which set measurement standards agreed to through the Convention of the Meter, a diplomatic treaty between fifty-four nations. The International Bureau of Weights and Measures (BIPM) located in Sèvres near Paris, France has the task of ensuring world-wide uniformity of measurements and their traceability to the SI.


guide to the SI covers

Members of the Fundamental Constants Data Center (FCDC) participate in meetings of the Consultative Committee for Units (CCU) which advises the International Committee for Weights and Measures on matters concerning the SI. The latest meeting of the CCU, held at the BIPM in June of 2013 was attended by Peter Mohr, David Newell, and Barry Taylor, of the FCDC.





latest meeting of the CCU

Guides to the SI

The official document for the SI is the International System of Units (SI). (Bureau International des Poids et Mesures, Sèvres, France, 2006).

The U.S. adaptation of the SI is described in NIST SP 330, The International System of Units (SI).

A practical description of the SI is in NIST SP 811, Guide for the Use of the International System of Units (SI).


There is currently interest in redefining some of the SI units in terms of fundamental constants. For the kilogram this would result in the replacement of the metal prototype standard at the BIPM by a definition based on fixing the value of the Planck constant. In addition, the CCU has recommended that the ampere, kelvin, and mole be redefined by specifying exact values of the elementary charge, the Boltzmann constant, and the Avogadro constant. More details may be found on the BIPM website.

National Public Radio report: This kilogram has a weight-loss problem.


"Reply to Comments on 'Dimensionless units in the SI' " P. J. Mohr and W. D. Phillips, Metrologia 52, 617-618 (2015)

"Dimensionless units in the SI" P. J. Mohr and W. D. Phillips, Metrologia 52 (1), 40-47 (2015)

"A more fundamental International System of Units." D. B. Newell, Physics Today 67(7), 35 (2014)

"Adapting the International System of Units to the twenty-first century" I. M.Mills, P. J. Mohr, T. J. Quinn, B. N. Taylor, and E. R. Williams, Phil. Trans. R. Soc. A 369(1953), 3907-3924 (2011)

"A summary of the Planck constant measurements using a watt balance with a superconducting solenoid at NIST" S. Schlamminger, R. L. Steiner, D. Haddad, D. B. Newell, F. Seifert, L. S. Chao, R. Liu, E. R. Williams, and J. R. Pratt, Metrologia 52(2), L5-L8 (2015)

"Molar mass and related quantities in the New SI," B.N. Taylor, Metrologia 46(3), L16-L17 (2009).

"The Quantum SI: A possible new International System of Units," P.J. Mohr, Adv. Quantum Chem. 53, 27-36 (2008), Chap. 3.

"Defining units in the quantum based SI," (cited in Metrologia highlights of 2008), P.J. Mohr, Metrologia 45(2), 129-133 (2008).

"Redefinition of the kilogram, ampere, kelvin, and mole: A proposed approach to implementing CIPM recommendation 1 (CI-2005)," I.M. Mills, P.J. Mohr, T.J. Quinn, B.N. Taylor and E.R. Williams, Metrologia 43(3), 227-248 (2006).

"Towards an electronic kilogram: an improved measurement of the Planck constant and electron mass," R.L. Steiner, E.R. Williams, D.B. Newell, and R. Liu, Metrologia 42(5), 431-441 (2005).

"Redefinition of the kilogram: A decision whose time has come," I.M. Mills, P.J. Mohr, .J. Quinn, B.N. Taylor, and E.R. Williams, Metrologia 42(2), 71-80 (2005).

"The role of the fundamental constants in the international system of units (SI): Present and future," B.N. Taylor and P.J. Mohr, IEEE Trans. Instrum. Meas. 50(2), 563-567 (2001).

"On the redefinition of the kilogram," B.N. Taylor and P.J. Mohr, Metrologia 36(1), 63-64 (1999).

Created October 19, 2009, Updated August 30, 2023