So, I've been looking through some of my old materials because it occurred to me that I don't really know how much "stuff" can fit in my character's backpack. Sure, the standard backpack holds "1 cubic foot of material" or 1,728 cubic inches. Okay. How does that help me unless you include the amount of space that each other item takes up?
For example, how much space do trail rations take? Well, that depends on a lot of factors. Are the rations scattered about my pack, or are they in their own container. What do the rations consist of? Are they nuts, jerky, and hardtack? Panforte? Apples and spinach? Who knows? Some things, such as standard vials, are given this treatment. A standard vial is 1 inch across (or, in diameter) and 3 inches deep. That's 3 cubic inches (pretty big, actually. Draw it out on 1/4" grid paper and it's 4 squares x 12 squares in two dimensions). That means I could have 576 vials in my backpack. If they're empty, that's 57.6 lbs. If they're full (1oz of liquid each) then that's 93.6 lbs.
Take the "Standard Adventurer's Kit" as an example. The kit includes a backpack, a bedroll, flint & steel, a belt pouch, 50ft of hemp rope, 2 sunrods, 10 days worth of trail rations and a waterskin. Okay, the backpack (obviously) doesn't take up any space in the pack. The rope, belt pouch, bedroll, and waterskin are all assumed (by me at least) to be "outside" of the pack (either attached to the pack, or to my character's belt). So, inside the backpack is the flint & steel, 2 sunrods, and 10 days worth of trail rations. So, what else can I put in there?
The sunrods appear to be about the same size and shape as a torch. That's roughly 1 inch in diameter and 12 inches long (or 12 cubic inches). The "flint & steel" is a little strange, though, because it's not just a piece of flint and a piece of steel. It (historically) would be a small "tinder box" made up of a piece of steel in the shape of something like the letter D, a bit of flint or other stone, and some kind of tinder (either a bit of shredded cotton/linen/burlap etc, or a "char-cloth" or anything else that catches fire easily). This would fit nicely in a box that is about 3"x4"x1" (or 12 cubic inches). What about the trail rations?
This one's tough because, despite all of the seemingly useless information floating around on the internet out there, I can't find a single example of what a D&D trail ration might look like. It's 1lb, so, maybe another 12 cubic inches per day? That seems to be the trend for 1lb items, so that's where I'll go for now.
2 sunrods = 24 cubic inches
1 flint & steel = 12 cubic inches
10 trail rations = 120 cubic inches
So, my adventuring kit items take up approximately 156 cubic inches of space in my backpack. That leaves 1,572 cubic inches of space in my pack for whatever else I can think of. Plus whatever fits in my belt pouch. Maybe I should invest in a coin purse as well? All of those coins have to go somewhere.
My point is simply this: if you're going to pique my interest by telling me how much a backpack can hold, then follow through and tell me how much space the other stuff takes up. Finding examples and making estimations gives me anxiety, yet not knowing the answers to my questions ALSO gives me anxiety. Maybe my head is just all messed up and I should leave well enough alone?