SI Units | NIST

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data-drupal-link-system-path="">State Metrology Laboratories</a> </li> <li class="usa-sidenav__item"> <a href="https://www.nist.gov/document/us-wm-and-metrologist-directory" data-drupal-link-system-path="">U.S. W&M and Metrologist Directory</a> </li> <li class="usa-sidenav__item"> <a href="https://www.ncwm.com/" data-drupal-link-system-path="">National Council on Weights and Measures (NCWM)</a> </li> <li class="usa-sidenav__item"> <a href="https://www.ncwm.com/ntep-about" data-drupal-link-system-path="">National Type Evaluation Program (NTEP)</a> </li> <li class="usa-sidenav__item"> <a href="https://www.oiml.org/en" data-drupal-link-system-path="">International Organization of Legal Metrology (OIML)</a> </li> </ul> </li> </ul> </nav> </span></div></div> </div> </div> </div>    </div> </aside> <div class="nist-page__region nist-page__region--content tablet-lg:grid-col-8 desktop-lg:grid-col-9" > <div class="nist-block" > <h1 class="nist-page__title">SI Units</h1> </div> <div class="nist-block" >  <div data-elastic-exclude class="nist-block nist-social__wrapper"> <h2 class="nist-social__title">Share</h2> <div class="social-media-sharing"> <div class="nist-social__wrapper--share"> <div> <a target="_blank" rel="noopener" href="https://www.facebook.com/share.php?u=https://www.nist.gov/pml/owm/metric-si/si-units" title="Facebook" class="nist-social nist-social--facebook-gray"> <span>Facebook</span> </a> </div> <div> <a target="_blank" rel="noopener" href="https://www.linkedin.com/shareArticle?mini=true&url=https://www.nist.gov/pml/owm/metric-si/si-units&source=https://www.nist.gov/pml/owm/metric-si/si-units" title="Linkedin" class="nist-social nist-social--linkedin-gray"> <span>Linkedin</span> </a> </div> <div> <a target="_blank" rel="noopener" href="https://x.com/intent/tweet?url=https://www.nist.gov/pml/owm/metric-si/si-units&status=https://www.nist.gov/pml/owm/metric-si/si-units" title="X.com" class="nist-social nist-social--x-gray"> <span>X.com</span> </a> </div> <div> <a href="mailto:?subject=NIST.gov&body=Check out this site https://www.nist.gov/pml/owm/metric-si/si-units" title="Email" class="nist-social nist-social--envelope-gray"> <span>Email</span> </a> </div> </div> </div> </div>  </div> <div class="nist-block" > <div class="text-with-summary"> <figure class="nist-image"> <img loading="lazy" src="/sites/default/files/styles/960_x_960_limit/public/images/2023/09/20/SI%20Units-banner.png?itok=PJ2toBvl" width="960" height="274" alt="International System of Units banner" typeof="foaf:Image" /> <figcaption class="nist-image__caption"> <div data-caption-editable="true" class="nist-image__caption-content"> </div> <div class="nist-image__credit"> <span class="nist-image__credit-label">Credit:</span> NIST </div> </figcaption> </figure> <div class="nist-callout"><p><a class="ck-anchor" id="top"></a>As of August 16, 2023 the physics.nist.gov historic SI Units site has permanently retired. This page and complete <a href="/pml/owm/metric-si" data-entity-type="node" data-entity-uuid="d897ce5e-ca33-4ca5-b462-08c0e65c928c" data-entity-substitution="canonical" title="Metric (SI) Program">Metric (SI) Program</a> contains current SI information. Contact <span class="spamspan"><span class="u">TheSI</span> [at] <span class="d">nist.gov</span><span class="t"> (TheSI[at]nist[dot]gov)</span></span> with comments, questions or concerns.</p></div> <div class="align-right nist-image"> <img loading="lazy" src="/sites/default/files/styles/220_x_220_limit/public/images/2018/09/13/2018-si.jpg?itok=EoHy4_-p" width="192" height="192" alt="SI Units - 2018" typeof="foaf:Image"> </div> <p>The SI rests on a foundation of seven (7) <a href="https://www.nist.gov/si-redefinition/meet-constants">defining constants</a>: the cesium hyperfine splitting frequency, the speed of light in vacuum, the Planck constant, the elementary charge (i.e. the charge on a proton), the Boltzmann constant, the Avogadro constant, and the luminous efficacy of a specified monochromatic source. NIST provides <em>values</em> and a <em>searchable bibliography</em> for the <a href="/pml/fundamental-physical-constants" data-entity-type="node" data-entity-uuid="35e42963-b84e-4590-adcf-1caa1a6bdd5b" data-entity-substitution="canonical" title="Fundamental Physical Constants">fundamental physical constants</a>. Definitions of all seven (7) SI base units are expressed using an explicit-constant formulation and experimentally realized using a specific <a href="https://www.bipm.org/en/publications/mises-en-pratique" title="BIPM mises en pratique"><em>mises en pratique</em></a> (practical technique).</p> <figure class=" align-right nist-image--lightbox nist-image" role="group"> <div data-lightbox="https://www.nist.gov/sites/default/files/styles/2800_x_2800_limit/public/images/2021/08/23/NIST.SP_.1247.png?itok=CAFgVO0D" data-media-id="620506"> <img alt="SI Base Units Relationships Poster" height="220" loading="lazy" src="/sites/default/files/styles/220_x_220_limit/public/images/2021/08/23/NIST.SP_.1247.png?itok=0JR52Uck" typeof="foaf:Image" width="142"> </div> <figcaption class="nist-image__caption">NIST SP 1247<div class="nist-image__credit"> <span class="nist-image__credit-label">Credit:</span> E. Tiesinga, K. Dill, D. Newell/NIST </div></figcaption> </figure> <p>The seven SI base units, which are comprised of:</p><ul><li><a href="/pml/owm/si-units-length" data-entity-type="node" data-entity-uuid="66688c36-55ed-49db-9da7-648f8c8eba29" data-entity-substitution="canonical" title="SI Units – Length">Length - meter (m)</a></li><li><a href="/pml/owm/si-units-time" data-entity-type="node" data-entity-uuid="82ede736-861a-4fd8-b93b-2d537ff80c20" data-entity-substitution="canonical" title="SI Units – Time">Time - second (s)</a></li><li><a href="/pml/owm/si-units-amount-substance" data-entity-type="node" data-entity-uuid="1af3e5fd-9c49-4c57-9164-2478b9d88ce2" data-entity-substitution="canonical" title="SI Units – Amount of Substance ">Amount of substance - mole (mol)</a></li><li><a href="/pml/owm/si-units-electric-current" data-entity-type="node" data-entity-uuid="117978c4-b2f5-4188-a3fb-5a9dd5320334" data-entity-substitution="canonical" title="SI Units – Electric Current ">Electric current - ampere (A)</a></li><li><a href="/pml/owm/si-units-temperature" data-entity-type="node" data-entity-uuid="d2617def-e7cb-41e8-aca9-3f652e187da1" data-entity-substitution="canonical" title="SI Units – Temperature">Temperature - kelvin (K)</a></li><li><a href="/pml/owm/si-units-luminous-intensity" data-entity-type="node" data-entity-uuid="44e2549f-d3ea-478f-942d-46c057d81198" data-entity-substitution="canonical" title="SI Units – Luminous Intensity ">Luminous intensity - candela (cd)</a></li><li><a href="/pml/owm/si-units-mass" data-entity-type="node" data-entity-uuid="11a1782a-3aed-4169-9100-11d81e8e4936" data-entity-substitution="canonical" title="SI Units - Mass">Mass - kilogram (kg)</a></li></ul><p> </p> <div class="align-left nist-image--lightbox nist-image" data-lightbox="https://www.nist.gov/sites/default/files/styles/2800_x_2800_limit/public/images/2023/08/30/Guides%20to%20the%20SI.png?itok=ybEg6L3q" data-media-id="685406"> <img loading="lazy" src="/sites/default/files/styles/220_x_220_limit/public/images/2023/08/30/Guides%20to%20the%20SI.png?itok=DT8SZwhU" width="220" height="196" alt="guide to the SI covers" typeof="foaf:Image"> </div> <p>The International System of Units (SI), commonly known as the metric system, is the international standard for measurement. <a href="http://www.bipm.org/en/convention/">The International Treaty of the Meter</a> was signed in Paris on May 20, 1875 by seventeen countries, including the United States and is now celebrated around the globe as <a href="http://www.worldmetrologyday.org">World Metrology Day</a>. NIST provides official U.S. representation in the various international bodies established by the Meter Convention: <a href="https://www.bipm.org/en/worldwide-metrology/cgpm/">CGPM</a> - General Conference on Weights and Measures; <a href="https://www.bipm.org/en/committees/cipm/">CIPM</a> - International Committee for Weights and Measures; and <a href="https://www.bipm.org/en/">BIPM</a> - The International Bureau of Weights and Measures.</p><p>The SI is made up of 7 base units that define the 22 derived units with special names and symbols, <span>which are illustrated in </span><a href="/pml/owm/si-base-units-relationships-poster-sp-1247" data-entity-type="node" data-entity-uuid="ed668b91-b4f8-4a82-b469-a39312e3b9c8" data-entity-substitution="canonical" title="SI Base Units Relationships Poster (SP 1247)"><span>NIST SP 1247</span></a><span>, SI Base Units Relationship Poster</span>.  The SI plays an essential role in international commerce and is commonly used in scientific and technological research and development. Learn more about the SI in NIST <a href="https://www.nist.gov/pml/special-publication-330">SP 330</a> and <a href="https://www.nist.gov/pml/special-publication-811">SP 811</a>.</p><p class="text-align-right"><a href="#top">Return to Top of Page</a></p> <figure class=" align-center nist-image--lightbox nist-image" role="group"> <div data-lightbox="https://www.nist.gov/sites/default/files/styles/2800_x_2800_limit/public/images/2021/12/06/si%20base%20units-table%202-nist%20sp%20330-2019.JPG?itok=4Iu9sT0d" data-media-id="630246"> <img alt="Table that lists the seven base units that define the 22 derived units with special names and symbols." height="279" loading="lazy" src="/sites/default/files/styles/480_x_480_limit/public/images/2021/12/06/si%20base%20units-table%202-nist%20sp%20330-2019.JPG?itok=DTRlLCgf" typeof="foaf:Image" width="480"> </div> <figcaption class="nist-image__caption">Table of SI base units.<div class="nist-image__credit"> <span class="nist-image__credit-label">Credit:</span> NIST </div></figcaption> </figure> <div class="nist-callout"><h3><span><strong>Useful Definitions</strong></span></h3><ul><li> <figure class=" align-right nist-image--lightbox nist-image" role="group"> <div data-copyright="Stock Photo" data-lightbox="https://www.nist.gov/sites/default/files/styles/2800_x_2800_limit/public/images/2023/03/05/Washington%20Monument.png?itok=MQ2qwmut" data-media-id="666966"> <img alt="Photo of Washington Monument" height="220" loading="lazy" src="/sites/default/files/styles/220_x_220_limit/public/images/2023/03/05/Washington%20Monument.png?itok=h5_0bbfE" typeof="foaf:Image" width="220"> </div> <figcaption class="nist-image__caption">Washington Monument, Washington, District of Columbia<div class="nist-image__credit"> <span class="nist-image__credit-label">Credit:</span> Adobe Stock </div></figcaption> </figure> <p><span>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.</span></p></li><li><span>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.</span></li><li><span>A physical quantity is a quantity that can be used in the mathematical equations of science and technology.</span></li><li><span>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.</span></li><li><span>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.</span></li><li><span><strong>Example:</strong> the value of the height h<sub>W</sub> of the Washington Monument is h<sub>W</sub> = 169 m. Here h<sub>W</sub> 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.</span></li></ul></div><p class="text-align-right"><a href="#top">Return to Top of Page</a></p><h3><strong>Resources for Students and Teachers</strong></h3><ul><li>Redefining the SI – In November 2018, the <a href="https://www.bipm.org/en/measurement-units/rev-si/">world’s measurement experts</a> voted and unanimously approved a revision of the SI that establishes a measurement system entirely based on <a href="/si-redefinition/meet-constants" data-entity-type="node" data-entity-uuid="1de85004-a27c-454f-b7e3-e753e1f798d4" data-entity-substitution="canonical" title="Meet the Constants">physical constants</a> of nature. The changes became effective on <a href="http://www.worldmetrologyday.org/">World Metrology Day</a>, May 20, 2019. (NIST)</li><li>Learn more about the <a href="https://www.nist.gov/si-redefinition/road-revised-si">Road to the Revised SI</a>. Learn more about the <a href="https://www.nist.gov/si-redefinition">SI Redefinition</a>. (NIST)</li><li><a href="https://www.montanapbs.org/programs/last-artifact/"><em>The Last Artifact documentary</em></a> and companion <a href="https://mpt.pbslearningmedia.org/collection/the-last-artifact/#.X5h9wIhKg2w">Grade 5 to 12 educational resources</a>, which document the work that went on behind the scenes to modernize the International System of Units (SI). (Montana PBS)</li><li><a href="/pml/owm/si-units-card-deck-sp-1297" title="SI Units Card Deck SP 1297" data-entity-type="node" data-entity-uuid="8b98f832-b0fc-434c-8beb-1eb806043b16" data-entity-substitution="canonical">SI Units Card Deck (SP 1297) </a>– this activity offers a fun way to enhance understanding of the International System of Units, including the defining constants, base units, derived units with special names, and prefixes. (NIST)</li><li><a href="/pml/owm/si-base-units-relationships-poster-sp-1247" data-entity-type="node" data-entity-uuid="ed668b91-b4f8-4a82-b469-a39312e3b9c8" data-entity-substitution="canonical" title="SI Base Units Relationships Poster (SP 1247)">SI Base Units Relationships Poster (SP 1247)</a> – a colorful poster illustrating the relationships of the International System of Units (SI) derived units with special names and symbols and the seven traditional base units. (NIST)</li><li>Metric Trivia Quiz – How much do you know about the metric system (SI)? Try the <a href="https://www.nist.gov/quiz/nist-metric-trivia-quiz">NIST Metric Trivia Quiz</a> online or use the <a href="https://www.amazon.com/pcorey-NIST-Metric-Trivia-Quiz/dp/B075QNJ7T7/ref=sr_1_1?s=digital-skills&ie=UTF8&qid=1507913957&sr=1-1&keywords=NIST">Alexa skill</a> to test your knowledge and be on your way to thinking metric! (NIST)</li><li><a href="https://www.nist.gov/pml/weights-and-measures/metric-si/si-education-training">SI Education and Training</a> – Explore NIST metric system education resources that build SI measurement system familiarity and fluency. (NIST)</li><li><a href="https://www.nist.gov/kids">Kids Education Resources</a>. (NIST)</li><li><a href="/blogs/taking-measure/top-10-tips-teaching-metric-system" title="Top 10 Tips for Teaching the Metric System" data-entity-type="node" data-entity-uuid="1c0f9db1-5e82-444c-8dc0-b1d7230556a2" data-entity-substitution="canonical">Top 10 Tips for Teaching the Metric System</a>. (NIST Taking Measure Blog)</li><li><a href="https://www.nist.gov/blogs/taking-measure/busting-myths-about-metric-system">Busting Myths about the Metric System</a>. (NIST Taking Measure Blog)</li></ul><p class="text-align-right"><a href="#top">Return to Top of Page</a></p><div class="nist-callout"><h3 class="text-align-center">EXPLORE THE METRIC PROGRAM</h3><p class="text-align-center"><a href="/pml/owm/becoming-familiar-si" data-entity-type="node" data-entity-uuid="d5664148-a600-40f7-9177-4bc8122a63e9" data-entity-substitution="canonical" title="Becoming Familiar with SI">Becoming Familiar with SI</a> | <a href="/pml/owm/metric-si/everyday-estimation" data-entity-type="node" data-entity-uuid="d6cbadf5-1189-42e9-9f84-59729483414f" data-entity-substitution="canonical" title="Everyday Estimation">Everyday Estimation</a> | <a href="/pml/owm/metric-si/us-metrication" data-entity-type="node" data-entity-uuid="5cc2c990-4058-4fde-8419-be492029a0e2" data-entity-substitution="canonical" title="U.S. Metrication">U.S. Metrication</a> | <a href="/pml/owm/metric-si/us-metrication-frequently-asked-questions-faqs" title="U.S. Metrication Frequently Asked Questions FAQs" data-entity-type="node" data-entity-uuid="f678040b-4a6b-4996-92c0-28caaa22aeb0" data-entity-substitution="canonical">U.S. Metrication FAQs</a> | <a href="https://www.nist.gov/pml/owm/metric-si-prefixes">Prefixes</a> | <a href="/pml/owm/metric-si/metric-kitchen" title="Metric Kitchen" data-entity-type="node" data-entity-uuid="a95984f4-2f3e-4fc6-9d26-b82c82ce32e1" data-entity-substitution="canonical">Metric Kitchen</a> | <a href="/pml/weights-and-measures/metric-si/si-education-training" data-entity-type="node" data-entity-uuid="e13f21a5-6a4e-4e81-ad91-f55af089a731" data-entity-substitution="canonical" title="SI Education & Training">SI Education and Training</a> | <a href="https://www.nist.gov/pml/owm/metric-publications">SI Publications</a> | <a href="/pml/owm/metric-si/understanding-metric" data-entity-type="node" data-entity-uuid="776bd31b-aac9-48ea-b117-a71f08304fd2" data-entity-substitution="canonical" title="Understanding Metric">Understanding Metric</a> | <a 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