Referring go this example
http://jsfiddle.net/uzgJX/
The result is the height of the box containing the text (the one you can see if you select the text with
Get the computed font-size for your text element instead:
parseInt(window.getComputedStyle(text[0]).fontSize, 10);
font-size represents the size of an em square for a font. It should be noted that, while most glyphs will stay inside the bounds of an em square, some may exceed those bounds. This doesn't usually occur on the vertical dimentions, though.
Give it a try: http://jsfiddle.net/uzgJX/1/. Tip: screenshot and copy into your favourite image editor, then select the pixels exactly to the height of the text and compare with the value given in the fiddle.