$('selector').css('line-height');
If this returns a number with px, you can use the number.
If it returns a % or a number with no units, you can multiply it by the font size and use that.
If it returns normal, I managed to work this up:
var height = $element.height();
var font_size = parseInt($element.css('font-size'));
var num = Math.floor(height / font_size);
for(; height % (height / num) != 0; --num)
;
return height / num;
This only works with "normal" because you can assume that the line height will always be greater than the font size.
It is based off the idea height % line_height == 0.
Technically num == height / line_height which makes it unknowable. Fortunately the floor of height / font_size is a good heuristic because it and height / line_height are usually the same or close, and if you're off you always overshot - again because "normal" guarantees the line height is greater than the font size - so you can test and try again.
