Please check this. is a solution using canvas

function get_tex_width(txt, font) {

        this.element = document.createElement('canvas');

        this.context = this.element.getContext(2d);

        this.context.font = font;

        return this.context.measureText(txt).width;

    }

    alert('Calculated width ' + get_tex_width(Hello World, 30px Arial));

    alert(Span text width +$(span).width());

Demo using

EDIT

The solution using canvas is not the best, each browser deal different canvas size.

Here is a nice solution to get size of text using a temporary element.
Demo

EDIT

The canvas spec doesn't give us a method for measuring the height of a string, so for this we can use parseInt(context.font).
TO get width and height. This trick work only with px size.

function get_tex_size(txt, font) {

    this.element = document.createElement('canvas');

    this.context = this.element.getContext(2d);

    this.context.font = font;

    var tsize = {'width':this.context.measureText(txt).width, 'height':parseInt(this.context.font)};

    return tsize;

}

var tsize = get_tex_size(Hello World, 30px Arial);



alert('Calculated width ' + tsize['width'] + '; Calculated height ' + tsize['height']);