JSFiddle - React, Tailwind, and code Playground

HTML

<div id="element"></div>
<div id="log"></div>

CSS

* {
    margin:0;
    padding:0;
}
body {
    font-family: sans-serif;
    font-size: 16px;
}
#element {
    width: 15.526em; /* 248.417px */
    height: 10px;
    background-color: #f80000;
    display: block;
    margin-bottom: 10px;
}

JavaScript

jQuery(document).ready(function ($) {
    var elWidth = parseFloat($("#element").width());
    $("#log").append("<p>the element width = " + elWidth + "</p>");
}); // ready