JSFiddle - React, Tailwind, and code Playground

HTML

<div id="div-1" style="width: 1000px;">
    <div id="div-2" style="width: 10%;background:red;"></div>
</div>

JavaScript

var percent = parseInt(document.getElementById("div-2").style.width);
 var parentWidth = parseInt(document.getElementById("div-1").style.width);
 var pixels = parentWidth*(percent/100);
    alert(pixels);