Proper sizing and fitting: The ultimate guide to backpack comfort

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Last Updated on 2026-01-31 by NatureCrank

Whether you’re section hiking a local trail or tackling a grueling thru-hike, a properly fitted pack is the difference between striding in comfort and wincing in pain. If you’ve ever wondered why your shoulders ache after only three miles, the culprit is likely a sizing mismatch.

This post will dive into the technical side of sizing and fitting your backpack to ensure your gear supports your journey rather than hindering it.

Table of Contents

Why backpack sizing matters more than brand

Most hikers buy gear based on brand reputation or price. However, the best backpack is the one that matches your torso length, not your height.

Backpacks typically come in small, medium, and large. Unlike a t-shirt, these sizes correspond to the distance between your neck and your hips. If the frame is too long, the pack will pull away from your shoulders; if it’s too short, the weight will crush your traps.

How to measure your torso length at home

Before shopping for Osprey, Gregory, or Deuter packs, you need your “torso number.” Follow these steps:

Locate the C7 vertebra: Tilt your head forward and feel for the bony bump at the base of your neck.
Locate your iliac crest: Place your hands on your hip bones, pointing your thumbs toward your spine. This shelf is your iliac crest.
Measure the distance: Have a friend use a flexible tailor’s tape to measure from the C7 down the curve of your spine to the point between your thumbs.

An anatomical diagram showing the human pelvis and the iliac crest, highlighting the top of the hip bone where a backpack hip belt should rest. — The iliac crest is the “shelf” of your hip bone. Aligning your hip belt here ensures 80% of the pack weight is supported by your legs.

Torso length	Common pack size
15″ – 17″	Small
18″ – 19″	Medium
20″ – 22″+	Large

Pro tip: Always check the specific manufacturer’s size chart. A “Medium” in a Gregory pack may differ from a “Medium” in an Osprey.

Measuring your hip size

While torso length determines the frame size, your hip measurement determines the hip belt size. Most of your pack’s weight (approx. 70-80%) should rest on your hips.

How to measure: Wrap your tape measure around the top of your hips (the iliac crest). This is usually higher than your pants’ waistline.
The gap rule: When tightened, the padded sections of the hip belt should have a 3 to 6-inch gap between them over your buckle. If the pads touch, the belt is too large and won’t transfer weight effectively.

How to adjust your backpack in 4 easy steps

Before adjusting, add 15–20 pounds of weight to the bag. An empty pack sits differently than a loaded one, and adjusting a “ghost” pack will result in a poor fit on the trail.

1. The hip belt (your primary support)

Tighten the hip belt so it cups your iliac crest. It should feel snug but not pinching. If the belt sits too high or low, readjust your shoulder straps to reposition the pack on your back.

A hiking backpack leaning against a tree with a red circle highlighting the padded hip belt to show proper placement for weight distribution. — The hip belt is the foundation of pack fit; it should be centered over your hip bones to take the strain off your shoulders.

2. Shoulder straps

Tighten these until they wrap closely around your shoulders. They should follow the contour of your back without a gap, but they shouldn’t be carrying the weight. The strap anchors should sit 1 to 2 inches below the top of your shoulders.

Fitting backpacks shoulder straps — Fitting backpacks — the shoulder straps

3. Load lifters (the 45-degree rule)

These straps connect the top of the shoulder harness to the pack frame.

The Goal: Tighten them to pull the weight of the pack toward your center of gravity.
The Angle: Ideally, these should sit at a 45-degree angle. If they are too flat or too vertical, your torso length might be incorrect. A common mistake is over-tightening load lifters until they create a gap between your shoulder and the strap. If you see a ‘tunnel’ over your shoulder, loosen them slightly.

A side view of a hiking backpack with the load lifter straps circled, demonstrating how they connect the shoulder harness to the main pack body. — Load lifters should ideally sit at a 45-degree angle. Tightening them pulls the pack’s weight closer to your back to prevent swaying.

4. Sternum strap

Slide the sternum strap until it sits about one inch below your collarbone. Tension it just enough to pull the shoulder straps inward, allowing your arms to move freely without the straps slipping off. If it’s too tight, it will restrict your breathing.

A close-up of a hiker wearing a backpack with a blue circle highlighting the sternum strap buckled across the chest at the correct height below the collarbone. — Position the sternum strap about an inch below your collarbone. It should be snug enough to keep the shoulder straps in place without restricting your breathing.

Frequently asked questions about backpack fitting (FAQ)

An ill-fitting pack causes “sore spots” and fatigue. A pack that is too long will rub against your lumbar or tailbone (sacrum), while a pack that is too short puts 100% of the weight on your shoulders, leading to neck strain and numbness in the arms.

Often, yes. Women-specific packs usually feature narrower shoulder dimensions and “S-shaped” straps to clear the chest, as well as hip belts contoured for a different pelvic angle.

While online sizing tools are excellent, visiting a retailer like REI allows you to use a weighted “sandbag” to test the fit under a real load. A pack that feels great empty may feel completely different when carrying 30 pounds.

If you fall between sizes, try both if possible. Usually, it is safer to go with the smaller size. This ensures the hip belt can sit high enough on your iliac crest without the shoulder straps “bottoming out” or leaving a gap above your shoulders.

It depends on the design. Suspended mesh backs (like the Osprey Atmos) only touch your hips and shoulders for airflow. Traditional foam-back packs should contour closely to your spine to keep the weight centered.

This is often caused by improper packing. Ensure your heaviest items (stoves, water, bear canisters) are centered close to your spine, not at the very bottom or top of the bag.

Final thoughts for the trail

A backpack is your most intimate piece of gear. By measuring your torso and fine-tuning your load lifters, you transform a heavy burden into a seamless extension of your body. Remember to readjust your straps every few miles—as your load shifts or your clothes compress, a quick “tweak” can prevent a hotspot from becoming a blister.

Want to see these steps in action? Check out this expert fitting guide from Osprey Packs:

This video tutorial by Osprey Packs provides a visual walkthrough of the torso measurement and strap adjustment steps covered in this guide.

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Proper sizing and fitting: The ultimate guide to backpack comfort