How can I get the browser's scrollbar sizes?

From Alexandre Gomes Blog I have not tried it. Let me know if it works for you.

function getScrollBarWidth () {
  var inner = document.createElement('p');
  inner.style.width = "100%";
  inner.style.height = "200px";

  var outer = document.createElement('div');
  outer.style.position = "absolute";
  outer.style.top = "0px";
  outer.style.left = "0px";
  outer.style.visibility = "hidden";
  outer.style.width = "200px";
  outer.style.height = "150px";
  outer.style.overflow = "hidden";
  outer.appendChild (inner);

  document.body.appendChild (outer);
  var w1 = inner.offsetWidth;
  outer.style.overflow = 'scroll';
  var w2 = inner.offsetWidth;
  if (w1 == w2) w2 = outer.clientWidth;

  document.body.removeChild (outer);

  return (w1 - w2);
};

Using jQuery, you can shorten Matthew Vines answer to:

function getScrollBarWidth () {
    var $outer = $('<div>').css({visibility: 'hidden', width: 100, overflow: 'scroll'}).appendTo('body'),
        widthWithScroll = $('<div>').css({width: '100%'}).appendTo($outer).outerWidth();
    $outer.remove();
    return 100 - widthWithScroll;
};

This is only script I've found, which is working in webkit browsers ... :)

$.scrollbarWidth = function() {
  var parent, child, width;

  if(width===undefined) {
    parent = $('<div style="width:50px;height:50px;overflow:auto"><div/></div>').appendTo('body');
    child=parent.children();
    width=child.innerWidth()-child.height(99).innerWidth();
    parent.remove();
  }

 return width;
};

Minimized version:

$.scrollbarWidth=function(){var a,b,c;if(c===undefined){a=$('<div style="width:50px;height:50px;overflow:auto"><div/></div>').appendTo('body');b=a.children();c=b.innerWidth()-b.height(99).innerWidth();a.remove()}return c};

And you have to call it when document is ready ... so

$(function(){ console.log($.scrollbarWidth()); });

Tested 2012-03-28 on Windows 7 in latest FF, Chrome, IE & Safari and 100% working.

source: http://benalman.com/projects/jquery-misc-plugins/#scrollbarwidth