SI Redefinition

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background-image:url('https://web.archive.org/web/20231216120721im_/https://www.nist.gov/sites/default/files/styles/720_wide/public/images/2018/09/14/si_redef_full_photo-updated-her1.jpg?itok=egVDWA6H'); } @media (min-width: 720px) { #row-229716 { background-image:url('https://web.archive.org/web/20231216120721im_/https://www.nist.gov/sites/default/files/styles/2400_wide/public/images/2018/09/14/si_redef_full_photo-updated-her1.jpg.webp?itok=Sd2ldtyt'); } } </style> <div class="nist-image-bg nist-image-bg--right nist-image-bg--dark" id="row-229716"> <div class="nist-image-bg__overlay"> <div class="nist-image-bg__overlay-inner"> <div class="text-long"> <div class="text-overlay text-overlay--right" style="margin-top:5vw;margin-bottom:5vw;"> <h1><svg height="75px" viewbox="0 0 198.43 198.43" width="75px" xmlns="http://www.w3.org/2000/svg"> <defs> <style type="text/css">.si-logo {fill: #fff;} .si-logo-text {fill: #3c3c3b;} </style> </defs> <title></title> <circle class="si-logo" cx="99.21" cy="99.21" r="99.21"></circle> <path class="si-logo-text" d="M54.14,148.05l7.48-8.68c7,7.33,16.9,12.27,27.07,12.27,12.87,0,20.5-6.44,20.5-16,0-10-7-13.16-16.31-17.35l-14.06-6.13c-9.27-3.89-19.89-10.92-19.89-25.28,0-15,13-26,30.81-26a40.72,40.72,0,0,1,29,12.12L112,81c-6-5.68-13.16-9.27-22.29-9.27-10.92,0-18.25,5.53-18.25,14.36,0,9.42,8.53,13,16.31,16.3l13.91,6c11.36,4.93,20.19,11.66,20.19,26.17,0,15.56-12.71,28-33.36,28A47.61,47.61,0,0,1,54.14,148.05Z" transform="translate(-9.75 -9.78)"></path> <path class="si-logo-text" d="M141.2,62.64h12.41v98.12H141.2Z" transform="translate(-9.75 -9.78)"></path> </svg>  <span style="vertical-align:super;">Redefinition</span></h1> <p>SI stands for the International System of Units, also known as the metric system.</p> </div> </div> </div> </div> </div> <div class="nist-content-row nist-content-row--width-normal paragraph paragraph--type--text-bg-color paragraph--view-mode--default"> <div class="nist-content-row__inner"> <div class="text-long"> <p>On November 16, 2018, in Versailles, France, a group of 60 countries made history. With a unanimous vote, they dramatically transformed the international system that underpins global science and trade. This single action finally realized scientists’ 150-year dream of a measurement system based entirely on unchanging fundamental properties of nature.</p> <p>On that day, the International System of Units, informally known as the metric system — the way in which the world measures everything from coffee to the cosmos — changed in a way that is more profound than anything since its establishment following the French Revolution. </p> <p>It was a turning point for humanity.</p> <p style="text-align: center;"><a class="usa-button" href="/web/20231216120721/https://www.nist.gov/si-redefinition/introduction-redefining-worlds-measurement-system">READ MORE</a></p> </div> </div> </div> <style> #row-229721 { background-image:url('https://web.archive.org/web/20231216120721im_/https://www.nist.gov/sites/default/files/styles/720_wide/public/images/2018/05/15/si_draft_rev2.png?itok=E11KMnat'); } @media (min-width: 720px) { #row-229721 { background-image:url('https://web.archive.org/web/20231216120721im_/https://www.nist.gov/sites/default/files/styles/2400_wide/public/images/2018/05/15/si_draft_rev2.png.webp?itok=tdMwOV17'); } } </style> <div class="nist-image-bg" id="row-229721"> <div class="nist-image-bg__overlay"> <div class="nist-image-bg__overlay-inner"> <div class="text-long"> <div class="nist-slogan"> <h2 class="nist-slogan__title">THE SI</h2> </div> <p style="text-align: center; color: white;">The SI — the modern metric system — has seven base units from which all other measurement units can be derived. On May 20, 2019, four of them — the kilogram, kelvin, ampere and mole — were redefined in terms of constants of nature. The remaining three — the second, meter, and candela — are already based on universal constants.</p> <p style="text-align: center; color: white;">Click on the SI symbols below for more information.<br>  </p> <div style="text-align:center"><a href="/web/20231216120721/https://www.nist.gov/si-redefinition/kilogram"> <svg height="170px" id="kilogram_symbol" viewbox="0 0 341 341" width="170px" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"> <defs> <style type="text/css">.kilo-circle { fill: #ce2031; stroke: #fff; stroke-miterlimit: 10; stroke-width: 9.79px; } .cls-2 { fill: #fff; pointer-events: none; } .kilo-circle:hover { stroke: #12659c; stroke-width: 10px; -webkit-transition: all 0.7s ease; -moz-transition: all 0.7s ease; -o-transition: all 0.7s ease; transition: all 0.7s ease; } .kilo-circle { cursor: pointer; } </style> </defs> <title></title> <circle class="kilo-circle" cx="170.5" cy="170.5" id="circle" r="165.6"></circle> <g> <path class="cls-2" d="M137.69,189.45h11.5v32.11h.29l12.59-15.75h12.76l-13.9,16.36,15,23.33H163.15L154.22,230l-5,5.71v9.77h-11.5Z" transform="translate(-5.08 -5.08)"></path> <path class="cls-2" d="M178.26,252.31c0-3.29,2-5.95,5.63-8V244a7.68,7.68,0,0,1-3.63-6.74c0-2.86,1.9-5.65,4.43-7.43v-.32a12.69,12.69,0,0,1-5.33-10.3c0-9.52,7.87-14.36,16.41-14.36a19.35,19.35,0,0,1,6.15,1h14.52v8.57h-6.38a10.42,10.42,0,0,1,1.34,5.23c0,9.13-7,13.33-15.63,13.33a17,17,0,0,1-4.84-.77,3.39,3.39,0,0,0-1.38,3c0,2,1.6,2.9,5.93,2.9h6.44c9.73,0,15.16,3,15.16,10.09,0,8.25-8.6,14.26-22.22,14.26C185.63,262.38,178.26,259.37,178.26,252.31Zm27.55-2.18c0-2.5-2.14-3.08-6.05-3.08h-4.12a21.57,21.57,0,0,1-5.56-.5,5,5,0,0,0-2.14,3.89c0,3,3.48,4.55,8.79,4.55S205.81,252.88,205.81,250.13Zm-4.62-30.92c0-4.23-2.29-6.45-5.42-6.45s-5.41,2.21-5.41,6.45,2.32,6.62,5.41,6.62S201.19,223.56,201.19,219.21Z" transform="translate(-5.08 -5.08)"></path> <path class="cls-2" d="M59.39,105.73h8.84v16h.22l11.65-16h9.72L78,121.34l14,23.49H82.3l-9.51-16.52-4.56,6v10.48H59.39Z" transform="translate(-5.08 -5.08)"></path> <path class="cls-2" d="M94.83,106.1c0-2.7,2.14-4.61,5.1-4.61s5.1,1.91,5.1,4.61-2.14,4.61-5.1,4.61S94.83,108.79,94.83,106.1Zm.68,9h8.82v29.76H95.51Z" transform="translate(-5.08 -5.08)"></path> <path class="cls-2" d="M112.09,135.58V102.8h8.82v33.14c0,1.9.88,2.5,1.6,2.5a3.84,3.84,0,0,0,1.08-.12l1,6.5a11.37,11.37,0,0,1-4.43.73C114.16,145.55,112.09,141.61,112.09,135.58Z" transform="translate(-5.08 -5.08)"></path> <path class="cls-2" d="M127.52,130c0-9.9,7-15.61,14.5-15.61s14.5,5.71,14.5,15.61-7,15.59-14.5,15.59S127.52,139.84,127.52,130Zm20,0c0-5.15-1.81-8.49-5.46-8.49s-5.46,3.34-5.46,8.49,1.81,8.48,5.46,8.48S147.48,135.11,147.48,130Z" transform="translate(-5.08 -5.08)"></path> <path class="cls-2" d="M160.73,149.94c0-2.47,1.48-4.46,4.22-6v-.24a5.78,5.78,0,0,1-2.72-5.05,7.27,7.27,0,0,1,3.32-5.58v-.24a9.5,9.5,0,0,1-4-7.72c0-7.14,5.9-10.77,12.31-10.77a14.47,14.47,0,0,1,4.61.72h10.88v6.42h-4.78a7.91,7.91,0,0,1,1,3.93c0,6.84-5.2,10-11.71,10a13,13,0,0,1-3.64-.58,2.55,2.55,0,0,0-1,2.21c0,1.47,1.21,2.18,4.46,2.18h4.83c7.3,0,11.36,2.24,11.36,7.57,0,6.19-6.44,10.69-16.66,10.69C166.26,157.49,160.73,155.24,160.73,149.94Zm20.67-1.63c0-1.88-1.61-2.31-4.54-2.31h-3.09a15.86,15.86,0,0,1-4.17-.38,3.76,3.76,0,0,0-1.61,2.92c0,2.26,2.61,3.41,6.6,3.41S181.4,150.37,181.4,148.31Zm-3.47-23.19c0-3.17-1.72-4.84-4.06-4.84s-4.07,1.66-4.07,4.84,1.75,5,4.07,5S177.93,128.38,177.93,125.12Z" transform="translate(-5.08 -5.08)"></path> <path class="cls-2" d="M194.62,115.07h7.2l.63,5.2h.22c2.17-4,5.42-5.92,8.46-5.92a8.28,8.28,0,0,1,3.55.6l-1.47,7.61a12.22,12.22,0,0,0-3.21-.48c-2.25,0-5,1.45-6.56,5.5v17.25h-8.82Z" transform="translate(-5.08 -5.08)"></path> <path class="cls-2" d="M215.47,136.58c0-6.27,5-9.67,16.76-10.91-.18-2.6-1.48-4.3-4.71-4.3-2.49,0-5,1-7.87,2.69l-3.14-5.79a24.31,24.31,0,0,1,12.71-3.92c7.61,0,11.83,4.35,11.83,13.47v17h-7.2l-.63-3H233c-2.51,2.2-5.31,3.76-8.66,3.76C218.91,145.55,215.47,141.59,215.47,136.58Zm16.76-.39V131c-6.27.84-8.32,2.64-8.32,4.89,0,1.91,1.32,2.85,3.42,2.85S230.66,137.75,232.23,136.19Z" transform="translate(-5.08 -5.08)"></path> <path class="cls-2" d="M248.47,115.07h7.19l.64,3.82h.22c2.44-2.46,5.11-4.54,9-4.54,4.18,0,6.71,1.8,8.21,5,2.62-2.73,5.42-5,9.35-5,6.43,0,9.31,4.57,9.31,12v18.48h-8.82V127.48c0-4.28-1.17-5.62-3.68-5.62-1.49,0-3.18.93-5.06,2.89v20.08H266V127.48c0-4.28-1.17-5.62-3.68-5.62-1.47,0-3.19.93-5.06,2.89v20.08h-8.82Z" transform="translate(-5.08 -5.08)"></path> </g> </svg></a>  <a href="/web/20231216120721/https://www.nist.gov/si-redefinition/candela"> <svg height="170px" id="candela_symbol" viewbox="0 0 341 341" width="170px" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"> <defs> <style> .candela-circle { fill: #3d2f82; 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A vote to adopt the change happened on November 16, 2018, at Versailles, France, and the change went into effect on May 20, 2019.</p> <figure class="align-center nist-video" data-media-id="469471"><div class="video-embed-field"><div class="video-embed-field-lazy" data-video-embed-field-lazy="<div class="video-embed-field-provider-nist-kaltura video-embed-field-responsive-video"><iframe width="854" height="480" frameborder="0" allow="autoplay *; fullscreen *; encrypted-media *" allowfullscreen="allowfullscreen" src="https://www.kaltura.com/p/684682/sp/68468200/embedIframeJs/uiconf_id/31013851/partner_id/684682?iframeembed=true&amp;entry_id=1_5k6domhl?iframeembed=true&amp;entry_id=1_5k6domhl&amp;flashvars%5BautoPlay%5D=1"></iframe> </div> "><img alt="Scientists Voted on Metric Makeover - English" loading="lazy" src="https://web.archive.org/web/20231216120721im_/https://cdnsecakmi.kaltura.com/p/684682/sp/68468200/thumbnail/entry_id/1_5k6domhl/width/1280" typeof="foaf:Image"/><button class="video-embed-field-lazy-play"></button> <div class="video-embed-field-lazy-title">Scientists Voted on Metric Makeover - English</div> </div> </div> <figcaption class="nist-video__caption" data-caption-editable="true"></figcaption></figure> </div> </div> </div> <div class="nist-content-row nist-content-row--width-normal paragraph paragraph--type--text-bg-color paragraph--view-mode--default"> <div class="nist-content-row__inner"> <div class="text-long"> <div class="nist-slogan"> <h2 class="nist-slogan__title">THE CONSTANTS</h2> </div> <figure class="align-right nist-image"><img alt="Person holding wallet card showing the values of fundamental constants being used for revising the International System of Units, the modern metric system." height="412" loading="lazy" src="/web/20231216120721im_/https://www.nist.gov/sites/default/files/styles/480_x_480_limit/public/images/2018/10/30/17pml015_si-constants-card.jpg?itok=yrzK-gxq" typeof="foaf:Image" width="480"/><figcaption class="nist-image__caption"><div class="nist-image__caption-content" data-caption-editable="true"> </div> <div class="nist-image__credit"> <span class="nist-image__credit-label">Credit:</span> Stoughton/NIST </div> </figcaption></figure><p>The units in the revised SI are based completely on seven unchanging quantities or “universal constants,” including the speed of light, the amount of electric charge in an electron, and the Planck constant. Learn more about each of these “invariants of nature” and how they come into play in the revised SI.</p> <p><a class="usa-button" href="https://web.archive.org/web/20231216120721/https://www.nist.gov/si-redefinition/meet-constants"><strong>Meet The Constants</strong></a></p> </div> </div> </div> </div> </div> <div data-elastic-exclude>  <footer class="nist-footer padding-bottom-4"> <div class="grid-container nist-footer__info"> <div class="grid-row"> <div class="tablet:grid-col-6"> <div class="nist-footer__logo"> <a href="/web/20231216120721/https://www.nist.gov/" title="National Institute of Standards and Technology" rel="home"> <img class="nist-footer__logo-img" src="/web/20231216120721im_/https://www.nist.gov/libraries/nist-component-library/dist/img/logo/NIST-Logo-Brand-White.svg" alt="National Institute of Standards and Technology logo" width="300px" height="42px"/> </a> </div> <div class="nist-footer__contact"> <h3 class="nist-footer__contact-heading">HEADQUARTERS</h3> <address> 100 Bureau Drive<br> Gaithersburg, MD 20899<br> <a href="https://web.archive.org/web/20231216120721/tel:301-975-2000">301-975-2000</a> </address> <p> <a href="https://web.archive.org/web/20231216120721/mailto:do-webmaster@nist.gov">Webmaster</a> | <a href="https://web.archive.org/web/20231216120721/https://www.nist.gov/about-nist/contact-us">Contact Us</a> | <a href="https://web.archive.org/web/20231216120721/https://www.nist.gov/visit">Our Other Offices</a> </p> </div> </div> <div class="tablet:grid-col-6"> <div class="nist-footer__social-links"> <a class="nist-social nist-social--twitter-white" href="https://web.archive.org/web/20231216120721/https://twitter.com/NIST"> <span>Twitter</span> </a> <a class="nist-social nist-social--facebook-white" href="https://web.archive.org/web/20231216120721/https://www.facebook.com/NIST"> <span>Facebook</span> </a> <a class="nist-social nist-social--linkedin-white" href="https://web.archive.org/web/20231216120721/https://www.linkedin.com/company/nist"> <span>LinkedIn</span> </a> <a class="nist-social nist-social--instagram-white" href="https://web.archive.org/web/20231216120721/https://www.instagram.com/nist/"> <span>Instagram</span> </a> <a class="nist-social nist-social--youtube-white" href="https://web.archive.org/web/20231216120721/https://www.youtube.com/NIST"> <span>YouTube</span> </a> <a class="nist-social nist-social--giphy-white" href="https://web.archive.org/web/20231216120721/https://giphy.com/nist"> <span>Giphy</span> </a> <a class="nist-social nist-social--rss-white" href="https://web.archive.org/web/20231216120721/https://www.nist.gov/news-events/nist-rss-feeds"> <span>RSS Feed</span> </a> <a class="nist-social nist-social--envelope-white" href="https://web.archive.org/web/20231216120721/https://public.govdelivery.com/accounts/USNIST/subscriber/new"> <span>Mailing List</span> </a> </div> <div class="nist-footer__feedback"> How are we doing? 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