Rucksack size: how many litres by trip length and season

The night before you leave, 9pm. Tent, sleeping bag, sleeping mat, stove and one fleece too many lie spread across the bed, and the 30-litre rucksack bought for day walks refuses to close. You push, you squash, you end up sitting on it. To no avail: a rucksack is chosen for what it has to swallow, and the number on the label tells you less than you would think.

This guide puts the suggested volumes side by side, explains why one brand’s litres aren’t another’s, then moves on to back length and weight. It answers ‘The night before’ in our guide to your first wild camp. For what goes inside, see the wild camping kit list or, for a day out, the day walk kit list.

How many litres for a day, a weekend or a week

No official rule sets these volumes: they are pointers. Here are those from Gregory [1], Deuter [2] and Decathlon [3], three brands that sell these rucksacks, the last of them a French sports retailer, along with the Swiss Council for Accident Prevention (BFU, or BPA in French) [4] and the FFRandonnée, France’s national walking federation [5]. Read them as advice, each with its own source’s needs in mind.

For a day, everyone meets around 20 to 30 litres. The gaps open up once you sleep out. For a wild camp, Gregory stops at 50 L and Decathlon climbs to 60 L, whereas Alpkit, the British brand, puts spring-to-autumn nights out in its 30 to 45 L rucksacks [6]. Huts bring the same wobble: Deuter talks of 45 to 60 L with meals included, the BFU of a volume barely above a day’s, and our guide to the Tour du Mont-Blanc aims for 35 to 45 L. The FFRandonnée, which doesn’t sell rucksacks, settles on 55 L for more than 4 days, possibly with fold-away pouches [5].

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Winter and summer: what the season changes

For winter, Gregory puts a number on the extra: 10 to 15 L more [1]. Alpkit explains what they are for, namely extra clothing, crampons and a storm shelter, and places winter day rucksacks in its 30 to 45 L range [6]. In hot weather, the brand adds, you sometimes need more water: that means kilos more than litres, to be worked out with our guide to water and food.

Why two 35-litre rucksacks don’t offer the same room

Alpkit says so plainly: nothing guarantees that two 35 L rucksacks have the same capacity, because each brand measures a little differently [6]. There is a standard, though, an American one: ASTM F2153, dated 2007 and reapproved in 2018. It costs money to read, and we cite only its official summary, which distinguishes an unextended and an extended capacity, and excludes anything not completely enclosed by fabric, such as mesh pockets, water bottle holders and compression pockets [7]. We found no ISO or European equivalent.

The principle is easier to grasp with a method. CabinZero, a travel bag maker that cites this standard, loosens the straps, then fills the main compartment and all the closed pockets with 10 to 20 mm plastic balls, or dried beans, using a 1 L measuring jug, and counts the jugs [8]. Alpkit pours polystyrene balls into a large measuring cylinder, but keeps only the main compartment [6]. On a rucksack with pockets, the second method’s figure will therefore be smaller than the first’s.

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Not everyone follows the standard. Minaal, a travel bag brand that says it applies it, claims that many manufacturers do without, to the point where one company’s 35 L could equal another’s 50 L [9]. That’s a competitor talking, with no measurements to back it up; the thing to remember is that the figure doesn’t compare from one brand to the next.

Then there’s the plus sign. On Deuter’s Aircontact Lite 40+10, the spec sheet gives 40 L [10], and the adjustable lidThe flap that closes the top of the rucksack, often with a pocket. Adjustable in height, it frees up room above the main compartment. offers up to 10 L more [2]. Compare base volumes with base volumes, and save the extension for the days after a resupply.

Working out litres: the tape measure misleads, adding up helps

The DIY reflex is to multiply height, width and depth. On a removal box, that works. On a rucksack, here is what the sum gives using the dimensions manufacturers publish.

The calculation overshoots the claimed figure by 21 to 49%, and the gap changes from one rucksack to the next: a rucksack isn’t a box, and nothing lets you work back from its dimensions to its litres. Your own needs, on the other hand, add up better. The big items of a wild camp have a published packed size, and each is roughly a cylinder.

Here’s an example with three spec sheets. Deuter’s synthetic Orbit +5 sleeping bag measures 20 cm in diameter by 39 cm, or 12 L, a figure the spec sheet also gives [16]. Hilleberg’s one-person Akto tent, 43 cm by 13 cm, takes up 5.7 L [17], and Sea to Summit’s Ether Light XT mat, 24 cm by 11 cm, 2.3 L [18]. Total: 20 L. Add them to a 20 to 30 L day rucksack and you reach 40 to 50 L, squarely in Gregory’s range, before water and the evening meal.

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It’s an example, not a recommendation: a two-person tent or a different sleeping bag will change the total. But the method takes one evening, with the spec sheets and a calculator.

Back length: measure it before you look at litres

Volume tells you what goes in; back length tells you whether the rucksack can be carried. Deuter defines it as the distance between the seventh cervical vertebra and the upper edge of the iliac crestThe top edge of the hip bone, which you can feel under your fingers when you put your hands on your hips. [19]. Gregory sets out the routine on its spec sheets [14]: head tilted forward to find the bump at the base of your neck, hands on your hips sliding up to the top of the bone, then thumbs towards the small of your back, where you read the tape. Decathlon talks more simply of a measurement from the waist to the cervical vertebrae [3], a lower and less precise reference point.

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Deuter puts its SL rucksacks at around 42 cm, Regular around 46 cm and EL around 50 cm, give or take 2 cm, and points out that two people of the same height can have different backs [19]. The measurement is only a starting point: for the brand, only trying a rucksack on is certain [19], and Decathlon advises trying it loaded [3]. Deuter also sets a threshold on its trekking rucksacks: a back length adjustment from 35 L, decisive above all with more than 10 kg and on long walks, and none below that, to keep the empty rucksack light [20].

Size sometimes affects the litres. Deuter adapts the volume of its EL models [19], and the SL version of the Aircontact Lite claims 35 L rather than 40 L [13, 10]. Gregory, for its part, lists 65 L for its Baltoro in both size S and size L, even though the rucksack grows from 79 to 84 cm tall [14, 15]. One more reason to look at the rucksack filled rather than at the label.

How much weight to put in those litres

More litres often means more kilos: the bigger the rucksack, notes the FFRandonnée, the more we tend to load it [5]. The published limits look alike without being equal.

The Spanish study published in January 2026 measured the pressure under the feet of 39 Camino de Santiago walkers, standing and barefoot: compared with no rucksack, it rises by about 5% at 10% of body weight and by nearly 10% at 20% [21]. The authors speak of an exploratory marker, not a universal safety limit, and their protocol says nothing about the risk of injury. Michael O’Shea, a physicist at Kansas State University, disputes the fixed percentage altogether: in his model, strength follows muscle cross-section rather than body weight, so a smaller person need not necessarily carry less [22]. He himself judges the model too simple to set one person’s load [22], and finds the idea of a single percentage somewhat misleading [23].

The rucksack has its say too. Deuter advises 6 to 10 kg on the Futura 30 SL and 12 to 15 kg on the Aircontact Lite 40+10 [12, 10], Gregory 27.2 kg at most on the Baltoro [14]. For Deuter, those values are comfort guidelines, not the rucksack’s strength [20]. Do the sum: at 80 kg, the FFRandonnée’s 16 kg ceiling already exceeds the 12 to 15 kg Deuter advises on this 40-litre model. The empty rucksack counts too: 640 g for the Vaude Agile 20, 1.59 kg for the Aircontact Lite 40+10, 2.13 kg for the Baltoro in size S [11, 10, 14]. Gregory aims for 70% of the weight on the hips and 30% on the shoulders [1], hence the value of a hip belt at the right height. For a child, see our guide to walking with children.

From day rucksack to wild camping rucksack: which to buy

• You walk by the day: 20 to 30 L falls within almost every source’s figures; go bigger for the mountains or winter.
• You sleep in huts: the figures run from 35 to 60 L depending on the source; start at the low end, and do a trial pack before buying bigger.
• One or two nights in a tent: 35 to 50 L, more if your sleeping bag takes up a lot of room.
• A week self-sufficient: 50 to 70 L, with the FFRandonnée’s 55 L as a marker.

If you can only buy one rucksack, I would look around 40 L: that’s near the middle of Gregory’s range for a tent, the top of Alpkit’s, and just above the 35 L from which Deuter adjusts the back length. Gram counters, Gregory admits, will get by with fewer litres [1]; I suspect their toothbrushes lost their handles some time ago.

A good habit to get into, the night before you leave: lay everything on the bed, do one trial pack, then stand on the bathroom scales with the rucksack on and subtract yourself. If it closes without anyone sitting on it and the weight stays under your limit, the volume is right. If not, the rucksack isn’t necessarily what needs replacing: start with that extra fleece.

Worth watching

REI’s Katie and Miranda measure a torso to find the right pack size, then show how to fit the pack: the back-length routine from this guide, seen in motion.

The video is hosted by YouTube (Google): nothing loads until you play it, and Google may then set cookies.

Guillaume Hoarau

Camping, wild camping, bushcraft and hiking

Guillaume Hoarau writes for Outfarer about camping, wild camping, bushcraft and hiking. A survival-skills enthusiast and a collector of old field manuals, he likes clear explanations, lists that earn their place and other people’s stories, which he tells well.

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