Tar overhead

[Kaushik Roy]

Hello,
 Is there any way to estimate the size of a tar file - i.e is there
 any way to know approximately what overhead the tar adds to a file
 or directory it archives?
Im trying to find the size of a tarred directory for archiving without
having to do the tar itself to save time.so if i know the size of the
directory itself..how accurately can i predict the size of the tar file?

Thanks,
Kaushik