Skip to main content
U.S. flag

An official website of the United States government

Dot gov

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.

Fundamental Constants Data Center

The Fundamental Constants Data Center is focused on physics relevant to the determination of values of the fundamental constants of physics and chemistry.

Activities include gathering and analyzing information for, and carrying out periodic least-squares evaluations of the constants. When necessary, new theoretical calculations are carried out to provide additional information for the adjustments. The Data Center is also involved in the development of the International System of Units, which is the basis for expressing the values of the constants. Support for research relevant to the fundamental constants and tests of basic laws of physics is provided through the Precision Measurements Grants Program.

News and Updates

Projects and Programs

Quantum Physics Theory

Theoretical work relevant to the determination of values of the fundamental constants, tests of bound-state quantum electrodynamics (QED), and methods of

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


Defining Units in the Quantum SI

Peter J. Mohr
Possible changes to the International System of Units (SI) are being discussed, including a proposal to define the units by specifying the values of a set of


Group Leader