Can jquery return height of an element as a percentage?

You can use .height(), which returns just the number (i.e, without the unit symbol). Perfect for percentage, which of course needs no units. From the docs:

The .height() method is recommended when an element's height needs to be used in a mathematical calculation.

So you can try something like this:

var height_pct = Math.round( 
    $('#my_div').height() / 
    $('#my_div').parent().height() * 100
    );

you can convert get height in px and convert them to percenatge

1em = 12pt = 16px = 100%