[magick-users] Uncompressed file size

[Beginner]

Hi,

I have been using the ping method to return the size of a TIFF file. 
This size though, seems to be the size of the file on disk. The TIFFs 
I have been looking at had used LZW compression so the actually size 
of the raw image is larger than the disk size.

Is there a way to retrieve the un-compressed file size from a file, 
hopefully as quickly as the ping method? Or is there a formula for 
working out the un-compressed size of a file; width x height x no. 
channel x bits per channel perhaps?

Any ideas appreciated.
Thanx.
Dp.