How many litres of pack you actually need

Pack capacity is measured by filling the pack with small beads and weighing or metering them, and no international standard governs how that is done. One brand counts every pocket, another counts only the main compartment. So a stated 50 litres is a reliable comparison within one manufacturer's range and a rough guide across ranges. Size the pack from the volume of your own kit instead.

Why a stated litre figure is not a measurement#

The bead-fill method itself is sound: fill the compartment, level it, read the volume. The problem is what gets filled. There is no ISO or EN test method for rucksack capacity, so each manufacturer defines its own protocol and its own boundaries.

The variables that move a number without changing the pack:

• Pockets. Hip belt, side, front stretch and lid pockets can add 6 to 12 litres. Include them or not, and a pack is 50 or 60 litres.
• The extension collar. A roll top or drawcord collar can add 8 to 15 litres above the nominal body. Some brands quote packed-down capacity, some quote fully extended.
• Frame size. A single model in three torso lengths has three different volumes, often 3 to 5 litres apart. The quoted figure is usually the medium.
• Compression state. Volume measured with compression straps slack is not the volume you carry when they are pulled in.

The practical consequence is that a 45 litre pack from one range can hold more than a 55 litre pack from another. Comparing capacities across brands is only useful at the level of "roughly weekend sized" against "roughly week sized".

Working ranges by trip type#

These bands assume kit that is dry, compressible and packed rather than stuffed at random. A synthetic sleeping bag, a closed cell foam pad, or a heavy shelter can push you a whole band upward without any change in trip length. Shelter choice moves this more than anything else, which is the volume side of tent, tarp or hammock.

What your kit actually occupies#

The only way to size a pack properly is to add up what goes in it. These are typical packed volumes, and your own items will vary by roughly 30 percent either way.

Cook kit is the category most often underestimated, because nested sets rarely nest as tightly as the packaging suggests; camp cookware covers what is worth carrying. Add one more factor to any total: packing voids. Items do not tessellate. A stuff sack cylinder inside a rectangular pack leaves air in the corners, and in practice 20 to 35 percent of the pack's volume ends up as space between objects. Any budget that ignores this comes out one size too small.

Three volume budgets, worked#

Summer overnight, light kit. Quilt 6, inflatable pad 2.5, tarp or light tent 5, clothing 4, cook kit 2.5, one day of food at 1 kg giving 1.4, other items 3. Subtotal 24.4 litres. Add 30 percent for voids: 24.4 times 1.3 is 31.7 litres. Buy a 35 litre pack.

Four day three season trip. Sleeping bag 8, pad 3, tent 7, clothing 6, cook kit 3, food at 3 kg giving 4.1, one litre of water inside 1, other items 3.5. Subtotal 35.6 litres. Add 30 percent: 35.6 times 1.3 is 46.3 litres. Buy a 50 litre pack.

Winter, or a trip requiring a bear canister. Winter bag 12, insulated pad 4, four season shelter 9, insulation and spare layers 12, cook kit and extra fuel 4, canister 11, other items 4. Subtotal 56 litres. Add 25 percent, because rigid items pack more predictably: 56 times 1.25 is 70 litres. Buy a 70 litre pack.

Three budgets, three answers, and none of them came from the length of the trip. They came from what the kit occupies, which is set by the decisions in base weight.

Compression: what it buys and what it costs#

Compression reduces volume. It does not reduce weight, and this gets forgotten regularly.

• What it buys. A sleeping bag at 15 litres loose goes to 6 to 8 litres in a compression sack. That is 7 to 9 litres of pack space, often the difference between two pack sizes.
• What it costs in grams. A compression sack is 30 to 80 g of fabric and webbing, which partly offsets the smaller pack you can now carry.
• What it costs in packing. A hard, over-compressed cylinder is the worst possible shape to pack around. It creates voids at the corners, so some of the volume you saved comes straight back. A bag stuffed loose into the bottom of the pack, with no sack at all, fills the corners and often occupies less total space than the same bag compressed into a rigid drum.
• What it costs over time. Long-term compression damages loft in both down and synthetic fill. Compress for the walk, store loose at home.

The useful conclusion is that compression sacks earn their place for items you want as a defined block (spare clothing, a tent inner) and lose it for the sleeping bag, which is better used as the material that fills the awkward space. Whether your sleep system compresses at all is one of the differences covered in quilts against sleeping bags.

External carry, and when it is a bad idea#

Strapping items outside is how a 45 litre pack carries a 55 litre kit. It works for a narrow set of things.

Reasonable outside:

• A closed cell foam pad, which is bulky, weatherproof and effectively weightless in aerodynamic terms.
• Trekking poles, stowed vertically against the side compression straps.
• A wet flysheet in a side pocket, keeping the water out of everything else.
• Footwear you are not wearing, if it is dry.

A bad idea outside:

• Anything you cannot afford to lose. External items come off on scrub, rock and stiles, and you will not hear it happen.
• Anything heavy. External carry pushes the mass away from your spine, and as the fit article shows, moving 12 kg from 15 cm to 20 cm behind the spine increases the rotating force on your back by 33 percent.
• Anything that must stay dry, unless it is inside a sealed bag.
• Food, in any area with bears or habituated wildlife.
• Anything that sits above shoulder height, on scrambling terrain or under a low canopy.

If more than two items are strapped on, the pack is too small for the kit. That is a sizing signal, not a packing skill.

Measure it yourself before you buy#

You can measure the volume your kit needs with a tape measure and a bin liner, before you spend anything.

1. Pack everything as you would for the trip, compression sacks and all, into a large bin liner. Include the sleeping bag, pad, shelter, clothing, cook kit and a realistic amount of food. Leave out the water.
2. Twist the liner closed and squeeze it into a rough cylinder, the way it would sit in a pack.
3. Measure two numbers: the circumference around the fattest part, and the height.
4. Compute the volume. Radius equals circumference divided by 2 times pi. Volume equals pi times radius squared times height.

Worked example: a 90 cm circumference gives a radius of 90 divided by 6.283, which is 14.3 cm. With a height of 55 cm, the volume is 3.1416 times 14.3 squared times 55, which is 35,336 cubic centimetres, or 35.3 litres.

1. Add 10 to 15 percent for the packing voids a rigid pack body creates but a soft liner does not.
2. Compare with the stated capacity of any pack you are considering, and remember that the stated figure may or may not include pockets. Ask which.

The second half of the check is easier: fill a candidate pack with 1 litre cartons or bottles and count them. It is tedious, but it tells you exactly what a stated 50 litres means on that particular pack.

Common mistakes#

Buying big "for flexibility". An oversized pack is heavier, carries a load less stably when half empty, and gets filled. Empty space in a pack is not neutral, it is an invitation.

Sizing by trip length. A four day trip with light gear can fit 45 litres. A single winter night can need 65. Food volume changes with days, but the sleep system and insulation do not.

Forgetting the canister. A bear canister is 8 to 12 litres of rigid, non-compressible volume that must go inside the pack. Plan a whole size band around it if the area requires one.

Ignoring pack shape. Two 50 litre packs, one tall and narrow and one short and wide, hold the same volume but pack completely differently. Tall and narrow keeps the mass near your spine, which matters for load placement.

Compressing everything. Over-compression makes rigid drums that will not tessellate. Leave the sleeping bag loose in the bottom and let it fill the corners.

Counting the extension collar as usable capacity. It is a surge space for a full food carry, not a routine part of the volume, and loading it raises the centre of mass considerably.

Frequently asked questions#

Is a 50 litre backpack big enough for a weekend?#

Comfortably, and often more than you need. A weekend with light three season kit fits inside 30 to 45 litres once the sleeping bag is compressed. Fifty litres becomes the right answer when the kit is bulkier: a synthetic bag, a foam pad carried inside, a heavier tent, or a colder forecast that means more insulation.

How many litres do I need for a week of backpacking?#

Usually 50 to 65 litres, because the change between a weekend and a week is mostly food. Trail food occupies roughly 1.2 to 1.5 litres per kilogram, so five extra days at 750 g a day is about 3.75 kg, or 4.5 to 5.6 litres. The sleep system, shelter and clothing do not change at all.

Why do backpack litres differ so much between brands?#

Because no international standard defines the measurement. Manufacturers fill the pack with beads, but each one decides whether pockets, lids and extension collars are included, and whether the figure refers to the medium frame size. A quoted capacity is reliable for comparing packs within one range and unreliable across ranges.

How much space does a sleeping bag take in a pack?#

Between 6 and 10 litres compressed for a typical three season bag or quilt, and 12 to 15 litres for a winter bag. Down compresses smaller than synthetic for equivalent warmth. A useful alternative is to stuff the bag loose into the bottom of the pack with no sack, which fills the corners and often uses less total space.

Should I strap my sleeping pad to the outside of my pack?#

A closed cell foam pad, yes: it is bulky, weatherproof, and would otherwise consume 10 to 15 litres inside. An inflatable pad, no: it packs to 2 to 4 litres and a puncture from scrub or rock ends your night. Keep anything you cannot replace in the field inside the pack.

Does a bigger pack make me carry more weight?#

In practice, yes. Empty volume gets filled, because the marginal item always looks free when there is space for it. The pack itself is also heavier: a 65 litre framed pack typically runs 400 to 900 g more than a 45 litre one. Buying the size your kit needs is a weight decision as much as a volume one.

How do I measure how much space my gear needs?#

Pack everything into a bin liner, twist it closed into a rough cylinder, and measure the circumference and height. Radius is circumference divided by 6.283, and volume is pi times radius squared times height. A 90 cm circumference and 55 cm height comes out at about 35 litres. Add 10 to 15 percent for the voids a rigid pack creates.

Does pack volume depend on base weight?#

Closely, within the normal three season range. A 5 kg base weight fits about 40 litres and a 10 kg base weight about 60, which is roughly 20 litres plus 4 litres per kilogram. The relationship breaks for winter kit, where insulation is bulky for its mass, and for bear canisters, which add fixed volume regardless of weight.