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They hardly take up any space and will make up for the weight in usefulness
Internal pack organization, packing up whatever you feel like belongs in a bag
2.5 liter: snacks, wallet & keys4 liter: a shell jacket or lightly insulated jacket6.5 liter: essentials: gloves, a warm hat, a headlamp, a dry base layer9 liter: a warmer puffy jacket15 liter: up to five days worth of food (for storage in your backpack)20 liter: a tent canopy (for storage in your backpack)30 liter: food, toiletries etc when used as a critter bag