How to Properly Fit a Backpack | Stop the Shoulder Ache

The secret to a good backpack fit isn’t the brand or price—it’s measuring your torso length before you even touch a strap. Most people skip this step and end up with a pack that’s two sizes too big or too small, then wonder why their shoulders hurt after two miles. Here’s how to get it right in about ten minutes with just a measuring tape and fifteen pounds of gear.

Measure Your Torso and Hip First

Your torso length—not your height—determines backpack size. Tilt your head forward and find the prominent bump at the base of your neck (C7 vertebra). Place one hand there. Slide your other hand down your ribcage until you feel the top edge of your hip bones (iliac crest). Stand up straight and measure the distance between those two points along the curve of your spine. Adult torsos typically range from 15 to 22 inches.

Backpack sizes by torso length: XS 15–17 in, S 16–19 in, M 18–21 in, L 20–23 in. If you’re between sizes, choose the smaller one—a slightly short pack is more stable than one that droops below your hips. Also measure your hip circumference right at the top of your iliac crest; the hip belt needs to wrap snugly there, not around your soft waist.

Strap Adjustment Order Matters

Never adjust straps on an empty pack. Load it with 15–20 pounds—sleeping bag, water bottles, whatever’s handy—so the pack behaves like it will on trail. Then tighten everything in this exact sequence:

1. Hip belt first. Position it directly on top of your iliac crest, with the padded center cupping your hip bones. The top of the belt should sit about an inch or two above the crest, padding split roughly half above and half below. Tighten until snug, buckle centered on your stomach with at least one inch of clearance on each side. This is where 80% of the pack weight should rest, not on your shoulders.
2. Shoulder straps next. Tighten just enough to hug your shoulders without pinching your armpits. The strap attachment points should align with your shoulder blades, about an inch or two below the top of your shoulders. The yoke (where straps connect to the pack) should sit about an inch below C7. Shoulder straps carry only 5–15% of the total weight when the hip belt is set correctly.
3. Load lifters. These small straps at the top of the shoulder straps pull the pack top closer to your body. Pull them forward until they angle upward at roughly 45 degrees. Too flat and they don’t help; too steep and they pull the pack away from your back.
4. Sternum strap last. Position it about one inch below your collarbone—not directly on the bone, which restricts breathing. Tighten only for stability; if it’s constricting your breathing or arm movement, loosen it.

If you’re still adjusting and shopping for packs, our top picks for commuter gym backpacks include models that fit this adjustment system well for daily use.

Common Mistakes That Ruin the Fit

The most frequent error is placing the hip belt on the natural waist (above the hip bones) instead of the iliac crest. That puts weight on your lower back, not your hips. The second most common is tightening shoulder straps before the hip belt, which overloads the shoulders before the hips get a chance to carry anything. An empty pack guarantees a bad fit because the weight distribution will shift completely once loaded. And a load lifter angled too flat or too steep will never pull the pack tight against your body.

These guidelines apply to standard backpacking and hiking packs with padded hip belts and technical frames. Simple packs with only webbing waist belts rest differently—those sit on the hip sockets, not the iliac crest, and carry less weight off the shoulders.

REI’s expert advice on adjusting backpack fit confirms this five-step sequence and the torso measurement method. The same principles apply whether you’re hiking the Appalachian Trail or carrying a day’s worth of gear.

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