Backpack fit: torso length, hip belts and where the load goes

A backpack is sized by the length of your back, not by how tall you are. Two people of the same height can differ by 8 cm in torso length, and the pack that fits one will bruise the other. Once the frame length is right the job is simple: put 70 to 80 percent of the weight on the iliac crest, and leave the shoulders to steady the load rather than carry it.

Why torso length and not height#

Height is the sum of leg length, torso length, and head and neck, and only one of the three carries a pack. Leg length varies more between individuals than torso length does, which makes height a poor proxy: a 1.80 m person with long legs may need the same frame as a 1.65 m person with a long back.

The frame has to span two load points. Its top must be high enough that the shoulder straps wrap over the shoulder without a gap, and the hip belt has to land on bone. Both constraints are set by your skeleton, and the distance between them is your torso length.

On a boundary, the tiebreaker is belt position: take the size that centres the belt on the crest with the shoulder straps still in contact. An adjustable harness covers that case, at a cost of roughly 100 to 250 g.

How to measure your torso and your hips#

You need a soft tape measure, a friend and two minutes.

1. Find C7. Tip your head forward. The most prominent bump at the base of the neck is the seventh cervical vertebra.
2. Find the iliac crest. Put your hands on your hips, thumbs back, and press down until you feel the top of the pelvic shelf. Take an imaginary line between your thumbs, across the spine.
3. Measure the curve. Stand normally, not stretched tall, and run the tape from C7 down the contour of the spine to that line.
4. Measure the hips for the belt. Wrap the tape around the body at the level of the crest, not at the trouser waistband. This circumference is usually 3 to 10 cm larger than your trouser waist size.

Repeat twice. If the readings differ by more than 1 cm, you are losing the crest line, so press harder.

Where the load goes, and the arithmetic behind it#

Load transfer works because the pelvis sits directly above the femurs, so weight on the hips runs down the skeleton into the legs. Weight on the shoulders compresses the whole spine and passes through soft tissue on the way in.

The second half of the problem is the moment. A pack's mass sits behind your spine, so gravity applies a rotating force that would tip you backward, and your body cancels it by leaning forward and tensioning the shoulder straps. That torque is the load multiplied by how far it sits behind the spine.

Take a 12 kg loaded pack whose centre of mass sits 15 cm behind the spine. Gravity pulls with 12 kg times 9.81 m/s squared, which is 117.7 newtons, and the moment about the spine is 117.7 N times 0.15 m, which is 17.7 newton metres. Let the pack sag away from your back by another 5 cm and the moment becomes 117.7 N times 0.20 m, which is 23.5 newton metres.

That is a 33 percent increase in the rotating force your back and shoulders must hold, from a gap you could close with one belt adjustment. Nothing about the weight changed. It is also the argument for putting dense items against the spine, covered in packing a backpack.

The adjustment order, and why it is not negotiable#

Every strap changes the effect of every strap set before it, so order determines the result. Load the pack with the weight you will actually carry: an empty pack tells you nothing, because the frame has not deflected and the belt has not settled.

1. Loosen everything. Hip belt, shoulder straps, load lifters, sternum strap, stabilisers. Start from slack, always.
2. Set the hip belt. Swing the pack on, bend the knees slightly so the crest stands proud, centre the padding over the crest, and pull the belt tails outward and slightly forward. Firm, not crushing: you should be able to breathe deeply without the belt moving.
3. Snug the shoulder straps. Pull down and back until the straps follow the shoulder with no gap at the front of the armpit. In contact, not tight. The anchor where the strap meets the pack body should sit 3 to 5 cm below the top of your shoulder.
4. Set the load lifters. Take up slack until the top of the pack draws in toward the shoulder blades, then stop. Cranking them lifts the shoulder straps off the shoulders and puts load back onto the collarbone.
5. Sternum strap. Position it about 2 to 3 cm below the collarbone and tension it only enough to stop the shoulder straps sliding outward. Overtightening restricts breathing on climbs.
6. Hip belt stabilisers. A short tug to pull the lower pack body toward the belt wings.

Repeat step 2 after ten minutes of walking. Loads settle, and the belt almost always wants another 1 to 2 cm.

The load lifter angle tells you whether the frame length is right at all. Roughly 30 to 45 degrees is correct: the strap can pull the pack top inward without lifting the shoulder strap off your shoulder. Close to horizontal means the frame is too short, so the strap can only pull backward and the pack leans away on climbs. Steeper than about 60 degrees means the frame is too tall, so tension goes straight up and unloads the shoulder straps. Under about 7 kg total, load lifters do little and many light packs omit them, so settle your base weight before choosing a pack.

Fit it yourself: a home protocol with a bathroom scale#

This is a repeatable check you can run in a hallway with no specialist equipment.

Step 1: know the number you are fitting. Stand on a bathroom scale and note your body weight. Step off, pick up the loaded pack, and stand on the scale again holding it. The difference is the pack weight, and 75 percent of it is what you are aiming to put on your hips. For a 12 kg pack: 12 times 0.75 is 9 kg on the pelvis, leaving 3 kg across the shoulders and back.

Step 2: the strap-drop test. With the pack fitted and a friend standing behind you, loosen both shoulder straps fully and stand still for ten seconds. A correctly fitted pack stays put on the hips and simply leans away from your upper back. If it slides down or drags you backward at once, the belt is above the crest or too loose.

Step 3: the two-finger check. Retighten, then slide two fingers under a shoulder strap at the collarbone and lift. The strap should lift a little with mild resistance. If it will not budge, the shoulders are carrying too much.

Step 4: photograph the profile. Shoot from the side, camera level with the pack top, then compare the load lifter with the shoulder strap it joins. The angle should be nearer 30 to 45 degrees than flat or vertical.

Step 5: walk stairs for fifteen minutes. Flat walking hides almost every fit fault. Stairs change your trunk angle and expose belt migration and frame length errors within a few flights.

Step 6: note where it hurt, then use the table below.

Hot spots: what the pain is telling you#

Change one thing at a time. Adjusting three straps at once teaches you nothing.

Men's, women's and youth frames are real specifications#

The differences are dimensional, not decorative, and the manufacturer publishes them if you look.

• Shoulder yoke width. Women's harnesses are cut narrower at the top and set closer together, with more curve to clear the chest. A wide yoke on a narrow frame lets the straps slide toward the arms.
• Hip belt flare. Women's belts carry more flare, because the female pelvis is on average wider at the crest relative to the waist. That flare is what lets the belt grip.
• Torso range. Women's and youth frames cover shorter bands, often 38 to 48 cm rather than 43 to 56 cm.
• Youth frames. Almost always adjustable across 8 to 12 cm of torso, because the wearer is growing. Expect to re-fit every season.

Ignore the label and match the numbers: plenty of people are best fitted by the harness sold under the other category name. If you are assembling a first kit, gear for a first camping trip covers what else has to be right before pack fit matters.

Common mistakes#

Fitting an empty pack in a shop. Frames flex, belts settle and straps stretch under load. Fit with 8 to 12 kg in the pack and walk with it.

Treating the hip belt as a waist belt. A belt sized from your trouser size is likely 5 to 10 cm too small in circumference and sitting too high.

Cranking the load lifters. They are trim tabs, not lifting straps: tension them until the gap closes, then stop.

Wearing the pack over a thick belt or bulky layers. A stiff waistband under a hip belt creates a hard ridge on the crest, and a compressible insulated layer flattens under the belt and lets the load creep down. Fit over the layering system you will actually walk in.

Solving a fit problem with a bigger pack. Overcapacity leads to overpacking, which raises weight and makes every fit fault worse. Size volume from your kit, as set out in how many litres you need.

Frequently asked questions#

How do I measure my torso length for a backpack?#

Tip your head forward and find the bump at the base of your neck, the C7 vertebra. Place your hands on your hips with thumbs pointing backward and press until you feel the top of the pelvic bone, the iliac crest. Have someone measure along the spine between the two points, following the contour rather than a straight line. Most adults measure 41 to 53 cm.

Should a backpack hip belt sit on the hip bones or above them?#

On them, straddling the top edge. Aim for roughly half the belt padding above the crest and half below, so the padding clamps around the bony shelf rather than resting on it. A belt entirely above the crest sits on soft abdomen and slides down. A belt entirely below it presses on the greater trochanter and blocks your stride.

How much of the weight should a backpack put on my hips?#

Roughly 70 to 80 percent for a loaded pack on rolling terrain. The shoulder straps take the rest and mainly stop the pack rotating away from your back. On technical ground where you need shoulder freedom, some people deliberately move a little more onto the shoulders, trading fatigue for mobility.

Why do my hands go numb when I carry a backpack?#

Almost always compression of the brachial plexus, the nerve bundle that passes under the collarbone, by the shoulder strap or an overtightened sternum strap. Loosen the sternum strap and drop it a couple of centimetres, ease the shoulder straps, and move load onto the hip belt. If numbness persists after a trip, it warrants medical advice rather than another strap adjustment.

What is the correct load lifter strap angle?#

Roughly 30 to 45 degrees between the strap and the top of the shoulder strap where it anchors. At that angle the strap can pull the top of the pack inward toward your shoulder blades. Flat straps mean the frame is too short, steep straps mean it is too tall, and neither can be fixed by tightening.

Do adjustable torso harnesses work as well as fixed sizes?#

They work, and they are the right answer if you are between sizes, sharing a pack, or fitting a growing teenager. The trade-off is 100 to 250 g of hardware and a mechanism that can slip under heavy loads. For a settled adult above about 15 kg, a fixed frame in the correct size is stiffer and simpler.

How much weight should I carry relative to my body weight?#

Up to about 20 percent of body weight is carried by most people without a noticeable change in gait. Between 20 and 25 percent your pace and daily distance drop. Above 25 percent injury risk climbs sharply, particularly to knees, feet and the lower back. Working out that number for your own kit is what the base weight calculator is for.