Roll the tape. Find a friend and a soft tape measure (available at fabric stores) in order to follow the curve of your spine.
Tuck in your chin so the C7 vertebra at the base of your neck protrudes. This knob marks the top of your torso measurement.
Grab your love handles. Then use your thumbs to feel for your iliac crests–the tops of your hipbones. The line between your thumbs marks the bottom of your measurement. Use this number (most adults fall between 15 and 22 inches) to find the correct size.
Load it. When shopping, fill a pack with at least 20 pounds (try ropes).
Loosen up. Release all the straps when you throw the pack on.
Shrug your shoulders to lift the pack, then buckle the hipbelt. Tighten shoulder straps and load lifters. Weight should feel evenly distributed, and there should be no visible gaps at the top of the shoulders.