cssDimensionInPx.js

// Sometimes you want to know the exact pixel dimension of a CSS property, but it’s
// in non-pixel units. This hack might help you out.
//
// Note: assumes a jQuery object `$`.
//
// License: http://github.com/stilist/ratafiacurrant/blob/master/License

var cssDimensionInPx = (function () {
	return function ($element, property) {
		var dimension = $element.css(property);
		var dimensionInt = parseInt(dimension, 10);
		var fontSize = parseInt($element.css("font-size"), 10);
		if (dimension && !isNaN(dimensionInt)) {
			// no unit
			if ("" + dimensionInt === dimension) {
				return dimensionInt;
			// px
			} else if (dimension.length - 2 === dimension.indexOf("px")) {
				return dimensionInt;
			// em
			} else if (dimension.length - 2 === dimension.indexOf("em")) {
				return dimensionInt * fontSize;
			// %
			} else if (dimension.length - 1 === dimension.indexOf("%")) {
				return (dimensionInt * 0.01) * fontSize;
			}
		} else {
			return 0;
		}
	};
})();
alert(cssDimensionInPx($("body"), "width"));