Detect scrollbar width with JavaScript. Based on a technique by David Walsh http://davidwalsh.name/detect-scrollbar-width

scroll_width.js

var scrollWidth = (function() {
    var scrollDiv = document.createElement("div");
    scrollDiv.className = "scrollbar-measure";
    document.body.appendChild(scrollDiv);
    
    var scrollbarWidth = scrollDiv.offsetWidth - scrollDiv.clientWidth;
    
    document.body.removeChild(scrollDiv);

    return scrollbarWidth;
})();

scroll_width_styles.css

.scrollbar-measure {
    width: 100px;
    height: 100px;
    overflow: scroll;
    position: absolute;
    top: -9999px;
}