Length of a literal string containing embedded tags

Hi,
 
Could anyone tell me how I can calculate the length of a string containing
inline elements?
 
E.g.
 
With a context node like this:
 
<text>this is some text<SUP>3</SUP>some more</text>
 
"string-length(.)" should return 43, but it returns less than that.
 
My assumption is that the embedded tags are being treated as proper nodes
rather than literal text.
 
Any idea how I can make sure that each character of the embedded tags counts
as a character when calculating the length of the overall string?
 
E.g.   <SUP> would count as 5 characters
 
Thanks

D.