If you need the width of something that's hidden and you can't un-hide it for whatever reason, you can clone it, change the CSS so it displays off the page, make it invisible, and then measure it. The user will be none the wiser if it's hidden and deleted afterwards.
Some of the other answers here just make the visibility hidden which works, but it will take up a blank spot on your page for a fraction of a second.
Example:
$itemClone = $('.hidden-item').clone().css({
'visibility': 'hidden',
'position': 'absolute',
'z-index': '-99999',
'left': '99999999px',
'top': '0px'
}).appendTo('body');
var width = $itemClone.width();
$itemClone.remove();