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.


This project provides the basis for a new definition of mass, based on unchanging quantum properties of nature rather than a physical artifact. This is accomplished through a high-accuracy comparison of power as measured in mechanical (force and velocity) and electrical (voltage and resistance) units. Because of the quantum nature of all but the kilogram mass unit (laser length, atomic time, Josephson effect voltage, and Quantum Hall effect resistance), the two measurements provides a high-accuracy determination of the Planck Constant relative to the kilogram artifact. The goal is to establish a consistent measured value that can be adopted as an exact value for the Planck constant, similar to what was done for the second and the speed of light. Following such a redefinition, this experiment would realize the kilogram mass unit in terms of purely quantum units - the meter, second, volt and ohm. This would complete the conversion to a measurement system where all units are derived from unchanging quantum properties of nature, instead of being tied to the properties of a single physical artifact.


NIST-4 watt balance under construction

At NIST we have worked with watt balances since 1980. During the past 35 years we have operated three different watt balances with the goal of measuring the Planck constant. Currently we are building a fourth version that will be used to realize the unit of mass in the US. The photo to the right shows this new watt balance, named NIST-4, during construction.

In 2013, measurements with NIST-3 came to an end with a final determination of the Planck constant. For the final number we obtained

h=6.626 06979(30)× 10-34 Js 

The relative standard uncertainty of this number is 45 x 10-9. The result agrees within two standard deviations with measurements from NRC Canada and the International Avogadro Coordination. This is remarkable, since all three experiments are very different. The NRC watt balance uses a permanent magnet in their watt balance, but NIST-3 has a superconducting magnet system.

Major Accomplishments

  • Precise determination of the Planck constant
  • Designed and built the new NIST-4 watt balance in two years
  • Active in outreach with our demonstration model watt balance made from Lego parts

  • For a detailed but non-technical overview of the science underlying the kilogram redefinition effort, CLICK HERE.
  • For the most recent published value for the Planck constant compared to other measurements, CLICK HERE.
  • For a description of the overall plan and timeline for redefinition of the SI units, CLICK HERE.
Created November 21, 2008, Updated January 19, 2018