<style> body { overflow-y: scroll } div { width: 10px; height: 10px; background-color: #d64203; } </style> <body> <div id="a"></div> <div style="height: 50px"></div> <!-- Unaffected by scrollbars. --> <div id="c"></div> <div style="height: 50px"></div> <!-- Unaffected by scrollbars. --> <!-- Deliberately make scrollbars appear. --> <div style="width: 500px; height: 500px; background-color: black"> </body> <script> // client{Width, Height} consist of the area _inside_ the scrollbars, so these // manually calculated units reflect the fact that with 'overflow: scroll' set, // viewport units are sized taking the scrollbars into account. Since we're // only dealing with 'overflow-y' here, only the width units are affected. var vw = 0.01 * document.body.clientWidth; document.getElementById('a').style.width = (50 * vw) + "px"; document.getElementById('c').style.width = (35 * vw) + "px"; </script>