>>2001522stolen from leddit:
Assemble all of your gear you intend to carry.
Get a cardboard box that all of your gear will fit into.
Put everything into the box. Make sure it all fits below the top of the box.
Measure height, width, and depth of your gear within the box. Solve for the volume of your gear (H x W x D = CID). Make sure to allow for and add the volume of your food to the above result, then add about 10 to 20 percent more to the total cubic inch displacement. (divide cubic inches value by 61 to calculate volume in liters)
Weigh your gear. Anything over 25 pounds including your carried water is the point at which you might consider a pack with some sort of frame sheet, internal frame or external frame. Hip belts come into the equation at or about 18 to 20 pounds IMO.
Start looking at packs that will hold your calculated volume and weight.
If at all possible try the pack(s) on with the requisite weight inside of the pack.
Make your pack purchase.
also, having a bag that's too big is fine as long as its not too much bigger - most packs can compress, and it'll let you get away with compressing your insulation less, packing bulkier foods, etc. etc.
