Are you trying to measure the volume of your existing pack, or the volume you need to hold the gear you have?

If it's the volume of the existing pack, the packing-peanut method should work well.

If it's the volume of your stuff, I'd suggest that you get a large box, and put your gear in it (compressing anything that would be compressed in the pack, like your sleeping bag.) Then mark the inside of the box at the height of the load. Empty it out, then measure length, width, and depth, and do the math. (Divide cubic inches by 60 to get an approximation of the liters.) I'd allow some extra volume when shopping for packs, simply because you'll end up with some unused space (such as in the lid) because gear won't fit in the pack like it does in the box.