The number on a backpack - 30 litres, 50 litres, 65 litres - is the specification everybody compares and the one that predicts the least about whether you will enjoy carrying it. Volume tells you what will fit. It tells you nothing about where the weight ends up, and that is the only thing your body notices all day.
Volume as a starting point
Litres are still a useful first filter. As a rough convention rather than a rule:
- Around 15 to 30 litres covers a day out with water, food, a shell and a spare layer.
- Around 30 to 50 litres covers long days, winter day gear, or a light overnight trip.
- Around 50 to 70 litres and beyond covers multi-day trips with a shelter, sleeping system, food and fuel.
Two people carrying the same kit can need very different volumes, because volume depends on how compactly a sleeping bag packs, how bulky insulation is and how much food is being carried. A pack that is only two-thirds full carries badly, since the load moves; a pack stuffed to the rim strains zips and puts pressure on the shape of the frame.
Buy the pack that fits the trip you do most often, and accept that a second, smaller pack is usually cheaper than one compromise.
The frame is what transfers the load
A pack carries well when the weight sits on your hips rather than hanging from your shoulders. Everything else follows from that.
- Frameless packs rely on a careful, compact load against your back. They are light and cheap and can be comfortable up to a point, but past a modest weight they stop transferring anything.
- Internal frames and framesheets are the norm for anything above day-pack size. A frame of aluminium stays, a plastic sheet or a shaped hoop spreads the load across the back panel and pushes it down into the hip belt.
- External frames are rare outside specialist and heavy-load use, where their airflow and huge capacity matter more than their bulk.
The harness then decides how well the frame’s work reaches you. On a good pack, tightening the hip belt first and the shoulder straps second should put most of the weight on your hip bones, with the shoulder straps doing little more than holding the pack against your back.
Torso length, not height
Pack sizing is by torso length, measured along the spine, roughly from the base of the neck to the top of the hip bones. Two people the same height can need different sizes, which is why pack sizes are not simply small, medium and large in the way clothing is.
Get this wrong and the consequences are predictable. Too long and the hip belt sits below your hips, so the weight hangs on your shoulders. Too short and the belt rides up onto your waist, crushing your stomach and still failing to carry the load. Most packs have a small range of adjustment, but adjustment cannot fix a size that is wrong by several centimetres.
The hip belt itself should wrap around the front of your hip bones, not squeeze them from above. Padding should be firm enough to distribute pressure without collapsing, and the belt should have a fine adjustment - a strap you can pull while walking - so you can shift the load on a long descent.
Load-lifter straps, which run from the top of the shoulder straps to the pack, pull the top of the pack in towards your back. A sternum strap holds the shoulder straps in place on your chest; it is a stability feature and it should never be tightened enough to restrict breathing.
Weight, fabric and how long the pack will last
Packs get lighter when the fabric gets thinner. Denier measures the weight of the yarn in the weave, and a lower denier generally means a lighter and less abrasion-resistant fabric. A very light pack can be a good choice if you treat it well. It is a poor choice if it will be dragged over rock, thrown into a van and used as a seat.
Where the fabric has to survive, look at the base and the sides near the hip belt and the shoulder straps. Reinforcement in those areas matters more than an unbroken spec sheet of ultralight fabric. Hardware matters too: a worn buckle or a failed zip is what usually ends a pack’s life, and both are replaceable if the maker sells spares.
Comfort, on a long carry, is also a question of harness sturdiness. Very light packs use thin foam and narrow webbing; they save weight, and they concentrate the load into smaller areas of contact.
Weather protection
No fabric makes a pack waterproof on its own. Water gets in through the lid, the seams, the zip and the fabric itself under sustained rain, and a pack cover mainly keeps the outside from soaking up water - useful, but easily defeated by wind-driven rain and by water running down your back.
The reliable approach is a liner: one large dry bag or a roll-top liner inside the pack, with the important contents inside that, plus small dry bags for a sleeping bag and spare clothing. Inside the pack, a liner protects the load whatever happens to the outside.
If you regularly carry electronics or a camera, a small dry bag is worth more than any feature on the pack.
Packing, briefly
A load carries best when the heavy items sit close to your spine and are held firmly, with nothing shifting as you walk. Beyond that, preference matters: a low, compact centre of gravity helps on rough ground and while scrambling, while some walkers prefer weight slightly higher for long flat approaches. What degrades carrying most is a loose load at the bottom of the pack, swinging away from your back.
Keep the things you need often - water, food, gloves, a hat - where you can reach them without taking the pack off, and pack the rain shell so it can be added without unpacking everything.
Fitting and buying
- Load the pack with something heavy - a few litres of water is ideal - before you judge it in a shop.
- Tighten the hip belt first, then the shoulder straps, then the load lifters, then the sternum strap.
- Walk around, bend forward, and lean side to side. Look for any point where you can feel a single area of pressure rather than a distributed load.
- Check the return policy. Torso length is easy to get wrong, and a pack that is uncomfortable after an hour is not one you will grow to like.
Volume is still the number you will use to narrow the shelf down to three packs. It should be the last thing that makes the decision.
