Backpack Volume Calculator (What Size Pack Do I Need?)
Most three season backpacking trips of 3 to 5 nights fit a 45 to 55 litre pack, and 1 litre equals 61.02 cubic inches, so a 50 litre pack is about 3,051 cubic inches. A bear canister or winter gear adds 10 to 15 litres.
Pack volume is the one gear number people get wrong in both directions. Buy too small and the quilt will not fit alongside four days of food. Buy too big and you will fill it, because empty space is the most reliably consumed resource in backpacking. This calculator combines the four things that actually move the answer: trip length, season, whether a bear canister is required, and your base weight.
Nights between resupplies, not total trip length.
2,746 to 3,356 cubic inches
Volume follows bulk, not weight, so this is a starting range rather than a fitting. Load your actual gear into a candidate pack before you commit, and check the base weight calculator first if you are not sure which tier you are in.
How many litres do you actually need?
Pack volume is the usable carrying capacity of a pack measured in litres, where 1 litre equals 61.02 cubic inches. The anchors below are the ranges nearly every retailer, guide and gear list converges on, and they assume a lightweight three season kit rather than a traditional one.
| Trip length | Litres | Cubic inches | What decides it |
|---|---|---|---|
| 1 to 2 nights | 30 to 40 L | 1,831 to 2,441 | Shelter bulk. Food is barely a factor at two days |
| 3 to 5 nights | 45 to 55 L | 2,746 to 3,356 | Food volume starts to dominate the main body |
| 5 to 7 nights | 55 to 65 L | 3,356 to 3,966 | A full week of food is roughly 15 to 20 L on day one |
| Winter or bear canister | Add 10 to 15 L | Add 610 to 915 | Rigid canister volume, or loft and layers that will not compress |
Two of those rows deserve a caveat. The 1 to 2 night range assumes a compressible quilt and a trekking pole shelter, and a 4 lb freestanding tent with poles eats a third of a 35 litre pack on its own. The 5 to 7 night range is really a food range: a week of food at 110 calories per ounce is roughly 15 to 20 litres of bulk on day one, and by day four the same pack looks half empty. Size the pack for day one, then use the compression straps for the rest of the trip. The full table by trip length lives on the pack volume reference chart.
Why does base weight change the pack size?
Because light gear is almost always small gear. Down compresses, synthetic does not. A Dyneema shelter packs to the size of a water bottle, a double wall tent with poles does not. A 13 ounce inflatable pad disappears and a closed cell foam pad rides outside the pack forever. So a 10 lb base weight is usually also a 30 litre base volume, and a 22 lb base weight is often 45 litres before any food goes in.
That is why the calculator drops the range for lighter kits and raises it for heavier ones. It is also why the honest answer to "what size pack should I buy" is often "finish the rest of the kit first". A pack is the one item that should be bought last, because everything else decides what it needs to hold. The buying order that avoids this trap is set out in the first ultralight kit buying order.
Why is a bear canister worth more than its volume?
A BearVault BV500 holds about 11.5 litres and weighs 2 lb 9 oz, but it costs more pack space than 11.5 litres because it is a rigid cylinder in a soft bag. Nothing packs around it, nothing squashes into the gaps, and the space it wastes is real. Allowing 10 to 15 litres of extra pack rather than a straight swap is the standard correction.
Shape matters as much as volume here. Wide, straight-sided packs take a canister horizontally near the top of the main body, which is where you want the weight anyway. Tapered ultralight packs frequently will not take one at all without it riding on the outside, which is both unpleasant and, in some jurisdictions, against the regulation you bought the canister to satisfy. Check the canister requirement for your permit area before you size the pack, and see the bear canister roundup for the volume and weight of each common model.
What does a manufacturer's litre figure include?
Not the same thing at every brand, which is why two 50 litre packs can look nothing alike. Some quote the main body only. Others include the lid, the front stretch pocket, the side pockets and the hipbelt pockets, which on a modern ultralight pack can be 8 to 10 litres of the total. A 55 litre pack measured generously and a 55 litre pack measured conservatively are effectively different sizes.
Two practical checks. Look for a main body figure quoted separately, since that is the number your quilt and food have to fit inside. And treat external pockets as a different kind of volume: they are excellent for a wet tent, a filter and snacks, and useless for anything that has to stay dry or compressed. How to load all of it so the pack carries well is covered in how to pack a backpack.
When is a smaller pack the wrong choice?
When you are not carrying the base weight it was designed for. Frameless and lightly framed packs carry beautifully up to about 25 to 30 lb of total load and become genuinely uncomfortable above it, because there is no frame to transfer weight to your hips. Putting a heavy traditional kit into a 40 litre frameless pack does not make it lighter, it makes it hurt. The trade-offs are compared directly in frameless versus framed packs.
The other case is regulatory or seasonal rather than personal. If your route requires a canister, if you carry an ice axe and traction, or if the forecast means bringing insulation you may never wear, the volume has to be there. Do not solve a volume shortfall by leaving rain gear, insulation, navigation or water treatment behind. Going ultralight must not mean going unprepared, and a pack that is 5 litres too small is a much smaller problem than a night out without a shell.
Frequently asked questions
What size backpack do I need for a 3 day trip?
For most people a 45 to 55 litre pack covers a 3 to 5 night trip in three season conditions. Under 45 litres works if your base weight is already near 10 lb and your quilt and shelter compress small. Over 55 litres for three nights usually means the gear inside is bulky rather than the trip being long, and a bigger pack will not fix that.
How many litres is a 60 litre pack in cubic inches?
One litre equals 61.02 cubic inches, so a 60 litre pack is about 3,661 cubic inches. American brands historically listed cubic inches and most now print both. Watch what is being measured too, because some brands quote the main body only and others include lid, hipbelt and side pockets, which can differ by 8 to 10 litres on the same pack.
Does a bear canister change what pack size I need?
Yes, and more than the litre figure suggests. A BearVault BV500 is roughly 11.5 litres of rigid cylinder that cannot be squashed into a corner, so allow 10 to 15 litres of extra pack volume rather than 11.5. Packs with a wide, tall main body swallow a canister horizontally near the top. Narrow tapered packs often will not take one at all without it riding outside.
Is it bad to buy a pack that is too big?
Usually yes. Empty volume gets filled, that is the oldest rule in the hobby, and a half full pack also carries badly because the load shifts and the frame is not loaded the way it was designed to be. A slightly small pack forces good decisions and compresses down to nothing. Size for your normal trip, not for the one big trip you might do next year.
How much pack volume does winter gear need?
Add 10 to 15 litres over your three season figure. The extra is not weight so much as bulk: a heavier bag with more loft, a second sleeping pad, a puffy that lives in the pack during the day, more layers, more fuel and more food. A 50 litre kit in summer commonly becomes a 65 litre kit once there is snow on the ground.
Can I use a 40 litre pack for a thru-hike?
Plenty of people do, and it depends almost entirely on base weight and resupply length. Under a 10 lb base weight with four days of food, a 40 litre pack is comfortable. Add a bear canister, a five day carry or winter layers and it stops working. Most long-trail hikers land in the 50 to 60 litre range because that is where a full food carry still fits.
How we choose: we compare published manufacturer specifications, listed weights, and verified owner reviews across retailers. We do not test gear in person, and every weight quoted here is the manufacturer's figure unless we say otherwise, so weigh your own kit before you trust a total. Going lighter also means carrying less margin, so treat insulation, navigation, water treatment and an emergency plan as fixed costs rather than places to save grams.