Backpacks are sold in litres, which sounds precise and is not. The figure tells you roughly how much volume the bag encloses, and brands measure it differently enough that two 30-litre packs can differ by several litres of usable space.
What a litre figure includes
Some manufacturers measure only the main compartment. Others include every pocket, the lid, the hip belt pouches and the stretch panel on the front. There is a voluntary standard for measuring pack volume, and not every brand follows it.
So compare litres within a brand confidently, and across brands loosely.
Rough sizes by use
10–20 litres. A day in a town, a commute, a laptop and a lunch. Too small for winter layers.
20–30 litres. The most useful single size for most people. A full day on the hill in summer with waterproofs, food, water and a spare layer. Also the largest size that reliably passes as airline personal item.
30–45 litres. An overnight in a hostel or a hut, or a long winter day with heavy layers. This is also the usual cabin baggage maximum — check the airline's dimensions, not the litres.
45–65 litres. Multi-day with your own shelter and food, or a week's travel without checking a bag.
65 litres and up. Expedition, winter camping, or carrying for more than one person. Bigger packs get filled, which is worth knowing before buying one.
Fit matters more than volume
A well-fitted 40-litre pack carries better than a badly fitted 30. The measurement that matters is not your height but your torso length — the distance from the bony bump at the base of your neck to the top of your hip bones.
Measure it, then check it against the manufacturer's size chart. Many packs come in two or three back lengths, and some adjust.
With a loaded pack on, roughly 70–80 per cent of the weight should sit on your hips, not your shoulders. If your shoulders are taking it, either the hip belt is not on your hip bones or the back length is wrong.
Features worth having, and worth skipping
Worth having
- A hip belt that actually transfers load, with padding over the bone. On packs above 30 litres this is the difference between a comfortable day and a miserable one.
- Compression straps. A half-empty pack that can be cinched down carries far better than one that lets the contents move.
- Access that matches how you pack. Top-loading is lighter and stronger; a front panel zip is much easier to live out of when travelling.
- A rain cover, or a liner. Almost no pack is genuinely waterproof at the seams and zips.
Usually skipped
- A built-in hydration bladder you did not choose.
- A great many small pockets, which add weight and hide things.
- Heavily structured frames on packs under 30 litres, where they add weight for little gain.
Weight, empty
Pack weight is the load you carry every day before you put anything in it. A 45-litre pack can weigh anywhere from 900g to 2.5kg depending on frame, fabric and features.
On a light load, a heavy pack is most of the discomfort. On a heavy load, a frame that transfers weight is worth every gram it costs. Match the pack's substance to what you will actually carry, not to the worst case you can imagine.
Fabric and the number after the D
Fabric is quoted in denier — 210D, 420D, 600D and so on — which measures the thickness of the yarn. Higher is more abrasion resistant and heavier.
Denier does not measure waterproofing. That comes from a coating, usually polyurethane, and from the seams. And it does not measure tear strength, which depends on the weave: ripstop fabrics use a reinforcing grid that stops a small tear becoming a long one, and a 210D ripstop can outlast a plain 600D in practice.
The short version
Pick the size from what you actually carry on a normal day, not the biggest day you can imagine. Measure your torso and use the size chart. Then check the hip belt, because on anything over 30 litres it does most of the work.
More kit notes are filed under Outdoors & Travel.