Edit: No, there isn’t.
To get the rendered font size of a given element, without affecting the DOM:
parseFloat(getComputedStyle(parentElement).fontSize);
This is based off the answer to this question.
Edit:
In IE, you would have to use parentElement.currentStyle["fontSize"], but this is not guaranteed to convert the size to px. So that’s out.
Furthermore, this snippet won’t get you the default font size of the element, but rather its actual font size, which is important if it has actually got a class and a style associated with it. In other words, if the element’s font size is 2em, you’ll get the number of pixels in 2 ems. Unless the font size is specified inline, you won’t be able to get the conversion ratio right.