INTERNATIONAL SYSTEM of UNITS (SI)
An Introduction to the International System of Units (SI) and its impact on measurement principles.
The basic principles of measurement related to units of measurement and the International System of Units (SI) form the foundation for scientific, industrial, and everyday measurements. Understanding these principles is essential for ensuring measurements’ accuracy, consistency, and comparability across different contexts and applications.
The International System of Units (SI) is the metric system’s modern form and the most widely used measurement system for science, technology, industry, and commerce. The system is based on a set of base units from which all other units, known as derived units, are defined.
The SI system of measurement is used worldwide and was adopted in 1960 by the General Conference on Weights and Measures (CGPM). The SI has seven base units, each representing a fundamental quantity, and additional derived units that are combinations of the base units.
Units of Measurement
A unit of measurement is a standardized quantity of a physical property used as a factor to express occurring quantities of that property. Units provide a reference point by which weight, length, or capacity objects are described. Standardization of units is crucial for clear communication and interoperability.
The SI rests on a foundation of seven (7) defining constants:
(kg) The Planck constant
(m) The speed of light in vacuum
(s) The cesium hyperfine splitting frequency
(A) The elementary charge (i.e., the charge on a proton)
(K) The Boltzmann constant
(mol) The Avogadro constant
(cd) The luminous efficacy of a specified monochromatic
The SI also defines many derived units, such as the newton (force), joule (energy), watt (power), and volt (electric potential). These derived units are combinations of the base units using mathematical equations.
The SI is constantly evolving to meet the needs of modern science and technology. In recent years, revisions were made to include new definitions for the kilogram, the kelvin, and other units. The ultimate goal of the SI is to provide an accurate and consistent system of measurement that can be used worldwide. 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, ensures worldwide measurements’ uniformity and traceability to the SI.
7 SI Base Units of Measure
The seven defining constants of the SI and the corresponding units they define.
As for any quantity, the value of a fundamental constant is expressed as the product of a number and a unit. The SI Base Unit definitions define the exact numerical value of each constant when its value is expressed in the corresponding SI unit.
As for any quantity, the value of a fundamental constant is expressed as the product of a number and a unit. The SI Base Unit definitions define the exact numerical value of each constant when its value is expressed in the corresponding SI unit.
22 SI Derived Units of Measure
The seven base units and twenty-two units with unique names and symbols may be combined to express the units of other derived quantities.
These are further defined in NIST Special Publication 330 (SP330)
Decimal multiples and sub-multiples of SI units
Decimal multiples and submultiples ranging from 1030 to 10−30 are provided for use with the SI units and frequently utilized in expressing units of measure.
| Name | Symbol | Multiplying factor |
| quetta | Q | 1030 |
| ronna | R | 1027 |
| yotta | Y | 1024 |
| zetta | Z | 1021 |
| exa | E | 1018 |
| peta | P | 1015 |
| tera | T | 1012 |
| giga | G | 109 |
| mega | M | 106 |
| kilo | k | 103 |
| hecto | h | 102 |
| deca | da | 101 |
| deci | d | 10–1 |
| centi | c | 10–2 |
| milli | m | 10–3 |
| micro | µ | 10–6 |
| nano | n | 10–9 |
| pico | p | 10–12 |
| femto | f | 10–15 |
| atto | a | 10–18 |
| zepto | z | 10–21 |
| yocto | y | 10–24 |
| ronto | r | 10–27 |
| quecto | q | 10–30 |
Principles Governing the SI Units
The basic principles of measurement and the International System of Units (SI) provide the foundation for accurate, consistent, and universally accepted measurements, essential for scientific inquiry, industrial processes, and everyday life.
Coherence: The SI units are coherent, meaning derived units are formed by multiplying or dividing the base units without introducing additional factors. This coherence simplifies calculations and conversions within the system.
Constant Definitions: The definitions of SI units are intended to be based on unchanging physical constants, such as the speed of light for the meter and the Planck constant for the kilogram, ensuring that the units are the same everywhere and at any time.
International Acceptance: The SI system is globally accepted and used, facilitating international collaboration, commerce, and scientific research.
Evolution and Adaptation: The SI system is not static; it evolves in response to advancements in measurement science. The system is regularly reviewed and updated to reflect the latest scientific understanding.
Adoption and Implementation
Global Standardization: International treaties and agreements, such as the Metre Convention, underpin the global standardization of the SI, ensuring that the system is uniformly applied worldwide.
Regulatory Compliance: Many countries have incorporated the SI into their national laws and regulations, making it the legal standard for measurements in trade, health, safety, and environmental protection.
Benefits of the SI System
Uniformity and Clarity: The SI system provides a clear, uniform set of widely understood and accepted units, reducing communication confusion.
Precision and Reliability: The reliance on physical constants for definitions enhances the precision and reliability of measurements.
Facilitation of Scientific Advancements: The standardization and precision of the SI system facilitate scientific research, enabling precise replication of experiments and comparability of results across different studies and disciplines.
References
- NIST Redefinition of SI, Retrieved from: https://www.nist.gov/si-redefinition
- NIST Special Publication 330, 2019, The International System of Units (SI), Retrieved from: https://www.nist.gov/pml/special-publication-330
- NIST Special Publication 811, 2008, Guide for the Use of the International System of Units (SI), Retrieved from: https://www.nist.gov/pml/special-publication-811/special-publication-811-extended-contents
- NIST Special Publication 1038, The International System of Units (SI) –Conversion Factors for General Use retrieved from: https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication1038.pdf
- BIPM, Bureau International des Poids st Mesures, Retrieved from: https://www.bipm.org/en/
