I have a North Face Big Shot backpack that I purchased around 2003. It was a great general purpose backpack that I used for day hikes, rock climbing, and traveling. Unfortunately time and the abuse I've inflicted on the pack have taken their toll and the pack needed to be replaced. I've replaced it with a REI Trail 40 pack since I didn't like the current evolution of the North Face Big Shot pack.
I'm curious if I've gained or lost any space with my replacement pack. While I know I can try to cram as much stuff into both packs to see if one pack fits more, I'd prefer to have something more quantifiable than an "X shirt increase." So what is the best way to get a proper volume measurement on an old pack? I tried to google the older pack but I kept finding newer versions of my pack. The different newer versions all hovered around 2,000 cubic inches, which is quite a bit smaller than the 3,000 cubic inch volume I seem to recall. My memory isn't perfect though so maybe my old pack really is smaller than I recall.
Here is a pic of my old pack for reference. While I currently want an answer for this specific pack I'd prefer a more general solution. Also while I'm sure it would be the most accurate I don't know if I'd be able to handle the math needed to calculate the volume based on the complex curves and angles that make up most packs.