The system of choice for this campaign will be Pathfinder, because of its many options and fiddly bits for players. I'm drawing the line at using the standard D&D/Pathfinder per-gram inventory system. There's much sleeker stuff to be found in the OSR-blogs.
Sources: Raging Owlbear, Goblin Punch, Games with Others, the Black Hack (usage die).
What you can carry
- You have an amount of inventory slots equal to your strength. Most items (shields, one-handed weapons, a crowbar or lantern) take up 1 slot. This is more a function of unwieldiness than basic weight.
- Two-handed weapons and medium armor take up two slots, heavy armors take up four.
- Sets of up to 5 smaller items (pitons, potions, arrows, coinage) can be bundled in packs that take up 1 inventory slot. See below under Usage die.
- Of your [strength] inventory slots, dexterity/2 (round down) are quick-access slots. Use these for items you need to find at a moment's notice, such as weapons or potions. These are easy to access (free as part of a move or standard action) but are also vulnerable.
- All other items are assumed to be in containers, bundles or your backpack. They are stored more safely but take d6 x 10 movement to dig out (half if you just spill everything on the ground).
Bundles of small items
- Jewelry and other ultra-light materials don't need to be tracked via inventory slots.
- Coins and gems run to 300 per inventory slot. 250 gold pieces + 300 copper pieces = 2 inventory slots. And if you find gems big enough to break this rule, rejoice. Also, run from the inevitable horde of cultists whose temple you just robbed.
- Some items are small and light enough that it takes 5 of them to fill up an inventory slot: for example, daggers, vials and flasks, scroll cases, pens and signal whistles.
- Not encumbered: carrying up to [strength] inventory slots worth of items. Use only the armor check penalty from your armor or shield.
- Encumbered: carrying up to 5 more items. Movement speed -10. Dex and strength based skills take a -5 penalty. Maximum dex bonus to AC is 3. (Use this or your armor check penalty, whichever is worse).
- Heavily encumbered: carrying up to 10 more items than your strength. Movement speed becomes 5. Maximum dex bonus to AC is 0. Penalty to dex and strength skills becomes -10. (Use this or your armor check penalty, whichever is worse).
Consumable itemsMany small items are perishables or consumeables. Examples: ammunition, rations, ink, oil, paper, parchment, pitons, rope (50ft), wax, soap (for gods sake stock up on soap), torches.
Rather than track each arrow and drop of ink, or foot of rope, we'll borrow the usage die from the Black Hack. This goes from d20-d12-d10-d8-d6-d4-gone. After intensive use (for instance writing for ink; exploring with torches; rope climbing; shooting arrows; eating), roll the usage die and on a result of 1-2, decrease the die to the next lower step. A roll of 1-2 on 1d4 means your resource is gone. Usage dice d4, and d8 take up 1 slot. Usage dice d10 and d12; d20 takes up 3 slots.
Cost per die size: d4 - regular price; every step thereafter: price x2.
Quiver of arrows / case of crossbow bolts: regular price buys you usage die d12.
Gruesomely expensive items such as magic potions or magic arrows are tracked per item. What, keeping track of how many +5 godbothering arrows you have is too much work?
Extra inventory slots
- A traveling backpack adds 2 inventory slots without triggering encumbrance (below). A fancy bandolier or belt with pouches and straps adds 1 quick-access slot. Does not add to the total inventory slots. You can use either a backpack or a bandolier/belt, not both.
- A Bag of Holding takes up 1-4 inventory slots and has internal space for 5/10/20/30 slots. And it will cost you a fortune to get one. Put a Bag of Holding in a quick access slot to have all its contents available as quick access items.
Damaging stuff(You can thank Goblin Punch.)
If you fail a save and get crushed by rocks, torched by a dragon, sprayed with acid or other unconventional forms of greeting in Xen'drik, the DM rolls 2d8:
- doubles: count this number upwards from the bottom of
the backpack (and continue into quick access slots). This item gets
damaged (a multiple-slot items such as a medium armor counts as one).
Roll 2,2: damage the second-lowest item in the backpack.
- different numbers: take the lowest of the two, count down this many
slots from your topmost quick access slot (continuing into the backpack)
and this item gets damaged.
Roll 5,7: damage the 5th item down on your list.