Find an appropriately sized cardboard box. Put in all your gear plus equivalent in weight and bulk of a week's food. Level off the top. Measure volume (length x width x height). If you want liters, for rough estimates, 1 liter equals approximately 60 cubic inches. The weight is also important because pack manufacturers specify a maximum weight (subtract 5 lbs. from their number).

Be sure to have the gear available to load into the pack when you get it! That's why it's strongly recommended that you buy the pack last. You should load up the pack with all our gear and take a couple hours' "hike" around the house while you can still return the pack.