Pack Volume by Trip Length: Litres You Need

Pack size is decided by bulk, not by weight. Two hikers with identical 14 lb base weights can need a 45 litre pack and a 65 litre pack, and the difference is entirely down to how small their shelter, insulation and food pack down. This chart gives the litres each trip length actually needs at three kit levels, then breaks the number down by category so you can see where your own volume is going and which single item is costing you a pack size.

The Granite Gear Crown3 60L is on this page as a reference point rather than a recommendation for everyone: at 60 litres and 2 lb 3 oz with a removable framesheet and lid, it covers a loaded five day carry and strips down for a weekend, which is exactly the range most people need one pack to span.

How many litres does each trip length actually need?

The table below assumes one person, three season conditions unless the row says winter, and no bear canister. Add roughly 10 litres to any row where a canister is required. Ranges rather than single figures are used because the same trip length can differ by 10 litres depending on whether you carry a puffy, a book and a camp chair or none of the above.

Look at the ultralight column across the whole table. It moves from 30 litres to 60 litres, and almost all of that movement is food and winter insulation rather than the base kit. The base kit itself barely changes: the same shelter, quilt and pad go on a one night trip and an eight night trip. That is why a hiker with a compressible kit can genuinely run a 50 litre pack year round in three season conditions and simply accept a fuller pack on the long carries.

Now look at the traditional column. It starts at 50 litres for a single night, which is more than an ultralight hiker needs for eight nights. The gear is not eight times heavier. It is simply bulkier, item for item, and bulk is what you buy pack litres to hold.

Why does bulk decide pack size rather than weight?

Because a pack is a container before it is a load carrier, and containers are limited by the space inside them. Two kits that both weigh 14 lb can occupy 26 litres and 44 litres, and the second one needs a bigger pack even though your legs feel the same load. Weight and bulk are correlated, which is why people conflate them, but they come apart badly at exactly the point where it matters most: the sleep system.

Down is the clearest example. Fill power is a measure of how many cubic inches one ounce of down lofts to, so 800 fill down lofts roughly 45 percent more per ounce than 550 fill down. To reach the same warmth with lower fill power you need more ounces of down, and those extra ounces bring their own volume with them. The result is a bag that is both heavier and considerably bulkier for identical warmth. That single specification, quoted on every down product page and ignored by most buyers, moves pack volume more than any other number in a gear list.

Shelter behaves the same way. A trekking pole shelter is not just lighter than a freestanding tent, it has no poles to pack, so a rigid 15 inch bundle that could not be squashed around anything simply stops existing. Anything rigid is disproportionately expensive in volume terms, because soft gear packs into the gaps around soft gear but nothing packs into the gaps around a pole set, a canister or a fuel bottle.

Compression helps, but less than the marketing suggests and at a real cost. A compression sack with straps can take a down quilt from 8 litres to 5, and it weighs 2 to 3 ounces to do it. It also stresses the baffles and the down itself, and down that lives compressed for months loses loft permanently, which is why every maker tells you to store it in a large breathable sack at home. The more common approach among experienced light packers is to skip the stuff sack entirely and push the quilt loose into the bottom of a pack liner, letting it fill the corners that a cylindrical sack would waste. That takes no extra weight, packs faster and is kinder to the insulation, but it does need a pack liner you trust to keep water out.

The practical hierarchy is simple. Compressible items are cheap in volume terms and you can carry plenty of them. Rigid items are expensive and each one sets a floor. Non-compressible soft items like a foam pad are somewhere in between and usually end up strapped outside. Sort your gear list into those three groups and you can see your pack size without adding anything up.

Where does the volume actually go?

This is the table to work from if your pack is always full and you cannot see why. Volumes are the space each item occupies once it is packed the way it is normally packed, which for down means compressed into a stuff sack and for foam means not compressed at all.

The three rows worth staring at are the bag rows. A 20 degree down quilt at 4 to 6 litres, a premium 20 degree down bag at 6 to 8 litres and a 550 fill 20 degree bag at 11 to 13 litres are all the same warmth. The difference between the first and the last is 7 litres, which on its own is the gap between a 45 litre pack and a 52 litre one. That is the whole mechanism behind the advice to buy the sleep system before the pack.

The Kelty Cosmic 20 Down is the honest illustration of this: it is a genuinely good budget bag and it will keep you warm, but 550 fill down means the loft you need takes far more space than the same warmth in 800 fill. An Enlightened Equipment Revelation 20 quilt does the same job in roughly half the volume and a pound less. The quilt versus sleeping bag comparison covers whether the sleeping arrangement suits you before you spend the money.

The foam pad row is the other one people miss. A folded Therm-a-Rest Z Lite Sol occupies 8 to 10 litres and cannot be compressed at all, which is why nearly everyone straps it to the outside of the pack. That is a legitimate solution, and it means a foam pad does not count against your internal volume. It also means the pad flaps in wind, catches on brush and gets wet, so decide deliberately rather than by default.

How much volume does a bear canister take?

More than any other single item, and unlike everything else in the pack it cannot be squeezed, folded or packed around. A canister is a rigid cylinder that sets a hard floor under your pack volume. Where a canister is required by the managing agency, size the pack around the canister first and fit everything else in afterwards.

An 11.5 litre BV500 inside a 50 litre pack is 23 percent of the total volume, occupied by a cylinder you cannot deform. That is why hikers on trails with canister requirements so often run a 55 or 60 litre pack while carrying an otherwise ultralight kit. Check the managing agency's current requirement before you plan around a hang, because the list of places where canisters are mandatory keeps growing, and read the bear canister roundup for which sizes are accepted where. Pack the canister upright against the back panel with soft items around it, as covered in how to pack a backpack, or it will roll and pull the load away from your spine.

What do the real packs hold, and what do they weigh?

The weight per litre column is the useful comparison, because it separates the pack's efficiency from its size. A 60 litre pack at 0.35 oz per litre and a 60 litre pack at 0.88 oz per litre hold the same gear and differ by two pounds. What you give up moving down that column is frame, fabric thickness, pockets and load transfer, in roughly that order. The frameless versus framed comparison covers where the trade stops being worth it, and the general rule is unchanged: a frameless pack carries well to about 25 lb total and badly above 30 lb.

Note also that a smaller pack is lighter twice over. It is lighter because there is less of it, and lighter again because a smaller volume needs less frame to hold its shape. Cutting bulk out of the kit therefore pays a compounding dividend, which is exactly the argument for choosing the shelter and sleep system before the pack.

Do external pockets and strapped-on gear count?

Manufacturers are inconsistent about this, which makes headline litre figures less comparable than they look. Some brands quote the main body only. Others quote main body plus lid plus all external pockets, which on a pack with a large rear mesh pocket and two deep side pockets can be 8 to 12 litres of the total. Two packs both labelled 50 litres can therefore differ by nearly a fifth in usable capacity, and the only reliable way to compare is to read the specification breakdown rather than the name on the label.

External storage is genuinely useful volume and not a cheat. A rear mesh pocket is where a wet fly, a rain jacket you are about to need again and a bag of rubbish belong, and keeping those items out of the dry main compartment is a real benefit rather than a compromise. Side pockets hold water bottles, which is the single best place for them: bottles are dense, and dense items carried low and close to your spine ride far better than dense items floating in the middle of a packed body.

Strapping gear to the outside has limits worth respecting. A foam pad under the compression straps is normal and effectively free. A tent lashed under the lid is fine. Beyond that, anything swinging on the outside changes your centre of gravity, catches on brush, snags in scrambling terrain and gets soaked in rain that the pack liner would have kept off it. The traditional test is whether you could fall over without damaging the item or yourself. If a pack only works with four things dangling from it, it is too small for the kit rather than cleverly loaded.

Where things sit inside the pack matters as much as whether they fit. Dense items belong high and against your back, soft items fill the bottom and the gaps, and the things you need during the day belong in pockets rather than at the bottom of the body. Getting that wrong makes a correctly sized pack feel unstable and makes a marginal one feel unbearable, which is why how to pack a backpack is worth reading before you conclude you need more litres.

How do you decide your own number?

Add up your base kit from the category table, add your longest realistic food carry at 1.2 to 1.5 litres per day, add a canister if one is required, then add about 15 percent as working margin so you are not fighting the pack every morning. That total is your pack volume. The pack volume calculator does the arithmetic and lets you test what happens when a single item changes.

Two corrections apply to the result. First, external pockets are real volume that many manufacturers exclude from the headline figure, so a 50 litre pack with large side and rear mesh pockets can swallow considerably more than a 50 litre pack without them. Second, buy for the kit you carry rather than the kit you might one day carry. Packs get filled to their capacity, and the spare 10 litres you bought as insurance become 10 litres of things you did not need. Compare specific models on the ultralight backpack roundup once you have a number.