NIST logo

SI Base Units


SI Globe logo

m - Meter - Length

s - Second - Time

mol - Mole - Amount of Substance

A - Ampere - Electric Current

K - Kelvin - Temperature

cd - Candela - Luminous Intensity


kg - Kilogram - Mass



The International System of Units (SI), commonly known as the metric system, is the international standard for measurement. The International Treaty of the Meter was signed in Paris on May 20, 1875 by seventeen countries, including the United States and is now celebrated around the globe as World Metrology Day. NIST provides official U.S. representation in the various international bodies established by the Meter Convention: CGPM - General Conference on Weights and Measures; CIPM - International Committee for Weights and Measures; and BIPM - The International Bureau of Weights and Measures.

si-color-diagram-apr-08The SI is made up of 7 base units that define the 22 derived units with special names and symbols. The international prototype of the kilogram is the only remaining artifact used to define a base unit of the SI (rather than definition by a fundamental constant). The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram. The prototype kilogram is a cylinder with a height and diameter of about 39 mm and is made of an alloy of platinum and iridium. The SI plays an essential role in international commerce and is the commonly used in scientific and technological research and development. Learn more about the SI in NIST SP 330 and SP 811.

Consumers have varying levels of awareness of measurement in daily life. Since the National Bureau of Standards (NBS) Mendenhall Order in the late 1800’s, many everyday measurements have been traceable to metric units. Many are not aware that “below the surface” the SI is the foundation for all measurements or of the extent the SI is used by industry to manufacture and supply the goods and services we all use every day.

Metric transition started early after the 1975 Metric Conversion Law was enacted. One of the first industry sector transitions occurred with Distilled Spirits & Wines, where transition was initiated by U.S. industry and coordinated by the U.S. Department of Treasury. Distilled Spirits and Wines have been successfully sold exclusively and accepted by U.S. consumers in metric units since the early 1980’s.

Since that time, the SI has become more prevalent in U.S. society. Use of SI measurements includes areas such as media reports (e.g., stories concerning global warming refers to metric tons of CO2 emissions, military deployments using kilometers), time (second), electric current (Ampere, Volt, Ohm & Watt), sports (track and field, cycling, Olympics), package labeling (especially beverages), healthcare (blood pressure, cholesterol levels, glucose measurements, prescription and over-the-counter medicines, and dietary supplement dosages), automotive (engines, tires, navigation systems, speedometers), and other consumer products (cameras, film, pens). U.S. consumers are becoming more familiar with grams and milligrams through information provided on mandatory nutritional labeling. Advertising, coupons, and consumer product packaging are increasingly found using metric units exclusively.

*
Bookmark and Share

Highlights

'Sí' on the New SI: NIST Backs Proposal for a Revamped System of Measurement Units

Taking the first steps of what would be a major historical advance in the science of measurement, the National Institute of Standards and Technology (NIST) is participating in a worldwide effort to recommend major revisions to the International System of Units (SI), the modern metric system that is the basis of global measurements in commerce, science and other aspects of everyday life. The new SI, which would be based on seven constants of nature, would enable researchers around the world to express the results of measurements at new levels of consistency and accuracy. Read More.

Federal Register Notice

Together NIST SP 330 and NIST SP 811 provide the legal interpretation of and guidelines for the use of the SI in the United States, as described in the Interpretation of the International System of Units (the Metric System of Measurement) for the United States
FR 28432-28433 PDF
Edition: May 16, 2008