estimation

I want to estimate the amount of space needed to bcp a table to a file. When I use sp_spaceused {tablename} it gives me a pretty aproxiate amount of space. I have done this with a couple of tables which are not so big. The amount given by sp_spaceused is real close to the amount taken up by doing the bcp out. But when I use sp_spaceused to a huge table it will give the output as 0k. There is an overflow so it will not give me that info..

Is there an equation that I may use to get real close to the bcp file.

Thank you....