The NIST Reference on Constants, Units and Uncertainty

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The information on the SI below was current up until May 20, 2019 (World Metrology Day). Since then the SI has undergone major changes. Hence the pages below are out of date. Updated pages will soon become available.

In the meantime, users may refer to the Bureau International des Poids et Mesures (BIPM) website. Now that the SI Brochure: The International System of Units (SI) has been made available, we are in the process of revising SP 330, SP 811, and the contents of this site.


This is a brief summary of the SI, the modern metric system of measurement. Long the language universally used in science, the SI has become the dominant language of international commerce and trade. These "essentials" are adapted from NIST Special Publication 811 (SP 811), prepared by B. N. Taylor and entitled Guide for the Use of the International System of Units (SI), and NIST Special Publication 330 (SP 330), edited by B. N. Taylor and entitled The International System of Units (SI). Users requiring more detailed information may access SP 811 and SP 330 online from the Bibliography, or order SP 811 for postal delivery. Information regarding the adoption and maintenance of the SI may be found in the section International aspects of the SI.

Some useful definitions

A quantity in the general sense is a property ascribed to phenomena, bodies, or substances that can be quantified for, or assigned to, a particular phenomenon, body, or substance. Examples are mass and electric charge.

A quantity in the particular sense is a quantifiable or assignable property ascribed to a particular phenomenon, body, or substance. Examples are the mass of the moon and the electric charge of the proton.

A physical quantity is a quantity that can be used in the mathematical equations of science and technology.

A unit is a particular physical quantity, defined and adopted by convention, with which other particular quantities of the same kind are compared to express their value.

The value of a physical quantity is the quantitative expression of a particular physical quantity as the product of a number and a unit, the number being its numerical value. Thus, the numerical value of a particular physical quantity depends on the unit in which it is expressed.

For example, the value of the height hW of the Washington Monument is hW = 169 m = 555 ft. Here hW is the physical quantity, its value expressed in the unit "meter," unit symbol m, is 169 m, and its numerical value when expressed in meters is 169. However, the value of hW expressed in the unit "foot," symbol ft, is 555 ft, and its numerical value when expressed in feet is 555.


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