How many litres should a trekking backpack be? What the number on the label doesn't tell you
When you buy a trekking backpack, the first number you look at is the litres: 30, 50, 65. It’s part of the pack’s name, the most visible bit of information on the shelf and, for most people, the only thing they compare. But what those litres actually measure, who measured them and how much of the space you can really use is less clear than it looks.
Below, you’ll first find which litre range will do the job for a trip of a given number of nights. Then we’ll look at why the number on the label can mislead you. For that we have two independent lab measurements, and both come to the same conclusion.
How many litres should a trekking backpack be?
What decides how many litres you need isn’t distance. It’s how many nights you’ll spend out and how much of your shelter and food you’re carrying on your back. If you’re home by evening, a twenty-kilometre route fits comfortably into 25 litres; do the same route with a tent, a sleeping bag and two days’ food and you’ll be pushing 50 litres. The season has a direct effect too, because a winter sleeping bag takes up almost twice the space of its summer equivalent.
| Litre range | Typical use | What fits |
|---|---|---|
| 20–30 litres | Day hikes | Water, lunch, waterproof jacket, thin fleece, first aid kit |
| 30–50 litres | Light trips of one or two nights | All of the above + sleeping bag, mat, room for a light tent |
| 50–70 litres | Three nights or more, carrying your own food | Tent, stove, several days’ food, spare clothes |
| Over 70 litres | Winter trips, technical gear | Winter sleeping bag, thick mat, crampons and related gear |
These ranges are a starting point, not a hard rule. The most common mistake beginners make is buying a bigger pack than they need: empty space invites you to fill it, and every litre you fill ends up on your back. For trips you do at a run, the numbers change completely; for those ranges, see How to choose a running backpack.
The litres on a trekking backpack label aren’t the main compartment’s litres
Outside Lab, run by Backpacker magazine at CU Denver, measured 18 traditional and ultralight packs this year, compartment by compartment, filling every pocket with standard-size plastic balls. Here’s what they found: a pack sold as 60 litres had a 45-litre main compartment. The other 15 litres were spread across the lid pocket, the front mesh, the side pockets and the hip belt pockets.
So the advertised total isn’t wrong. But it isn’t the volume you can put into the pack in one go, either. A winter sleeping bag that won’t pack down, or a bulky tent, might not fit in a 60-litre pack, because the space it actually has to go into is the 45-litre main compartment. Lots of pockets make you more organised, but they cost you main-compartment volume. The details of the test were published in Backpacker’s lab notes.
There’s no mandatory standard for measuring litres
The reason brands give numbers that don’t line up is simple: there’s no shared, mandatory method. ASTM F2153 sets out a defined test method for backpack volume: the pack is filled with 20-millimetre spheres, then compressed to squeeze out the dead space. But no manufacturer has to use it, no retailer asks for a uniform measurement, and the test itself costs money. For small workshop brands, that cost is an obstacle in its own right.
When the Treeline Review team noticed they could fit the same gear into both a 35-litre and a 55-litre pack, they filled the main compartments of six ultralight packs with beans and measured them one by one. The results ranged from 20 per cent below to 22 per cent above the main-compartment volume the brands advertised. Forty litres from two different brands aren’t the same thing, and the gap comes less from bad faith than from the lack of a common way to measure.
| Pack | Advertised main compartment | Measured main compartment |
|---|---|---|
| Hyperlite Mountain Gear 2400 Junction | 40 L | 32 L |
| Zpacks Arc Blast 55 | 42 L | 34 L |
| Six Moon Designs Swift X | 36 L | 44 L |
| ULA Equipment Circuit | 40 L | 38 L |
| Gossamer Gear Kumo 36 | 28 L | 29 L |
| Gossamer Gear Mariposa 60 | 36 L | 42 L |
You can’t read the empty weight off the label either
The same lab team also weighed the empty packs and reported that the advertised weights often didn’t match reality. The quickest way to find out the first load your back will carry is to put the pack on the scales before you buy it.
Back length matters more than litres
Picking the right litre range is half the job. Two trekking backpacks of the same volume can carry completely differently, depending on how well they match your back length. It’s measured from the bony vertebra that sticks out at the base of your neck down to the top of your hip bones, and brands base their S/M/L sizes on this measurement, not on a height and weight chart.
Where the hip belt sits matters just as much, because the part of you that really carries the load is your hips, not your shoulders. Using force sensors fitted to the hip belt and shoulder straps, Outside Lab defined a pack’s load limit as the moment more than half the weight shifts onto the shoulders. Most of the strain you feel on long hikes comes not from the litres you chose but from that balance going wrong.
Three checks before you buy
First, take your bulkiest item, your sleeping bag or your tent, and see whether it fits in the main compartment; a total litre figure that fails this test is no use to you. Next, try the pack on loaded in the shop, because an empty pack tells you nothing, and most shops have test weights you can use. Finally, tighten the hip belt and check that the shoulder straps go slack; if they do, the load is sitting in the right place. What to put in the pack and how to arrange it is a separate topic, and you’ll find it in How to pack your first trekking backpack.