How to calculate the width of the scroll bar?

There is a jQuery plugin that can help with this: https://github.com/brandonaaron/jquery-getscrollbarwidth/blob/master/jquery.getscrollbarwidth.js

Also, from http://www.alexandre-gomes.com/?p=115
Here is some code that may help.

This creates a hidden <p> element at 100% width inside a <div> with a scrollbar, then calculates the <div> width – the <p> width = scroll bar width.

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); 
};