I recently started using this program. It's great. I imaged a few drives at work. These were 70GB drives that only had 15GB of data. When I imaged them they only ended up being 8GB.

Is there a way I can tell before I image a partition with this program, how big will the image be after compression? Lets say I have 150GB of data is that depended on the kind of data?

Thanks,
JOe K.

Definitely depending on content and packer routine used
Same as with archivers.

As an example, big differences can even show up on the very same file type:
I am recording from DVB-S frequently.
Normally the resulting .mpg files can't get compressed noticeably, as they are already heavily compressed inside.
BUT if the source is a truly digital test screen (like those I am using for TV repair), a good engine like ace or 7zip might compress to far less than 10%.
A special flicker test sequence I've created with mute audio compressed from >100 MB to around 100 kB.
Forget zip on this...
A matter of packer intelligence.

So no way for a realistic prediction.