What Size Backpack Do I Need?

Backpack sizing is two questions wearing one label, and the shops mostly answer the wrong one. "Size" on a spec sheet means capacity in liters, which is about how much you can fit. "Size" on the fit chart means torso length, which is about whether the pack transfers weight to your hips or hangs off your shoulders for three days.

Get the liters wrong and you carry something slightly too big or slightly too small. Get the torso length wrong and no amount of adjustment will fix it, because you are wearing a frame built for a different body.

Quick Answer

For capacity, match liters to nights out: 10 to 20 L for daily carry, 20 to 35 L for a day hike, 35 to 50 L for one or two nights, 50 to 70 L for three to five nights, and 70 L and up for a week or for winter gear. For fit, the number that matters is your torso length, measured from the bony bump at the base of your neck (the C7 vertebra) down to the line between the tops of your hip bones. That measurement, not your height, determines whether you take a small, medium, or large frame. A correctly fitted pack puts roughly 75 to 80 percent of the load on your hips, not your shoulders. And whatever size you buy, keep the loaded weight near or below 20 percent of your body weight, because that ceiling, not the volume, is what actually limits what you can carry.

Key Takeaways

  • Torso length is the size. Height is not. Two people the same height routinely need different frame sizes, because the difference lives in leg length rather than back length. Measure before you buy.
  • The hip belt does the work. Around three quarters of a properly carried load rides on the iliac crest, the shelf of bone at the top of your pelvis. Shoulder straps stabilize. If your shoulders hurt, the belt is either the wrong size, sitting in the wrong place, or the torso length is off.
  • Liters measure volume, not carrying ability. A 65 L pack does not mean you can carry 65 L of gear comfortably. Weight is the binding constraint and it arrives long before you run out of space.
  • Buying bigger "just in case" backfires. Empty pack volume gets filled. A larger frame is also heavier before you put anything in it, and a partly loaded big pack carries worse than a full smaller one.
  • Twenty percent of body weight is the practical ceiling for a loaded pack, and 10 percent is a comfortable target for a daypack. For children the guidance is stricter still, with pediatric and safety bodies putting the limit closer to 10 to 15 percent.
  • For an emergency bag, water is the whole sizing problem. Three days of stored water weighs more than most people should be carrying in total. That single fact is why a serious go-bag carries a filter and not a reservoir.
  • Hip circumference is measured at the iliac crest, not the waist. It is also the more forgiving of the two measurements, since most belts have real adjustment range.

What the Liter Number Actually Measures

Pack capacity is the total internal volume including lid, pockets, and any external stash areas the maker chooses to count. That last part is why two 50 L packs from different brands can hold visibly different amounts of gear. Measurement conventions vary, so treat liters as a comparison tool within a brand and a rough guide across brands.

What the number does reliably tell you is the class of trip a pack is designed for, because frame stiffness, belt padding, and suspension all scale with intended load.

Capacity Trip length What it holds
10 to 20 L Daily carry, commute, short walk Water bottle, layer, small kit, laptop
20 to 35 L Full day out Water, food, shell, first aid, navigation, headlamp
35 to 50 L One to two nights The above plus sleep system, shelter, stove, more food
50 to 70 L Three to five nights Multi-day food, fuel, extra layers, bulkier sleep gear
70 to 90 L A week or more, winter, carrying for others Cold-weather bag, insulated pad, bulky clothing, group gear

Two things shift you up a category regardless of nights out. Cold weather is the big one: a winter sleeping bag and pad take roughly twice the space of their summer equivalents, and the clothing bulk on top of that is real. Carrying for someone else, a child or a partner, is the other.

Two things shift you down. Modern lightweight gear compresses far smaller than gear from a decade ago, and experience does the rest. Most people's first multi-day pack is a size larger than their fifth.

Torso Length: How to Measure It

This is the measurement that decides frame size, and it takes two minutes with a soft tape measure and a second person.

  1. Find the top point. Tilt your head forward. The bony bump that stands out most at the base of your neck is the seventh cervical vertebra, C7. That is the top of the measurement.
  2. Find the bottom point. Put your hands on your hips with thumbs back and fingers forward, so your index fingers rest along the top ridge of your pelvis. That ridge is the iliac crest. Now draw an imaginary line between your thumbs, straight across your spine. Where that line crosses your spine is the bottom point.
  3. Measure between them, following the curve of the spine rather than a straight line through the air. Stand naturally with your head level while someone else reads the tape.

Then match it to the maker's chart. Ranges vary between brands and this is a genuine difference rather than marketing, so check the specific model rather than assuming your size carries over.

Torso length Typical frame size
Under about 16 inches Extra small
About 16 to 17.5 inches Small
About 18 to 19.5 inches Medium
About 20 inches and over Large

Many packs have an adjustable harness that slides several inches, which covers people near a boundary and lets one pack fit two members of a household. Adjustability costs a little weight and a little rigidity, which is why the lightest packs tend to come in fixed sizes.

Hip circumference is the second measurement and the easier one. Wrap the tape around the top of the iliac crest, not around your natural waist, which sits noticeably higher. Most hip belts cover a wide range and many are swappable, so this rarely drives the purchase. Torso length almost always does.

Why the Hip Belt Is the Whole Point

A loaded pack is a lever trying to pull you backward. The reason a framed pack feels dramatically better than a school bag with the same weight in it is that the frame transfers that load down to a belt sitting on bone.

Done right, the hip belt carries somewhere around 75 to 80 percent of the weight. Your shoulders take the remainder and mostly just keep the pack against your back. That is the difference between arriving tired and arriving sore.

The belt has to sit on the iliac crest, wrapping it so the padding straddles that ridge with roughly equal coverage above and below. Too high and it squeezes soft abdomen. Too low and it slides down over the hip and fouls your stride. If it will not stay put once tightened, the usual cause is not the belt but the torso length: a pack that is too long puts the belt below where it should be no matter how you cinch it.

The load lifter straps running from the top of the shoulder harness to the top of the frame should form roughly a 45 degree angle when the pack is fitted. That angle is a quick, reliable diagnostic. Much flatter and the frame is too short for you. Much steeper and it is too long.

The Weight Ceiling, Which Matters More Than Volume

Volume is what you buy. Weight is what stops you. A common working figure for a loaded backpacking pack is about 20 percent of body weight, and around 10 percent for a daypack.

Body weight Daypack target (10%) Loaded pack ceiling (20%)
120 lb 12 lb 24 lb
150 lb 15 lb 30 lb
180 lb 18 lb 36 lb
200 lb 20 lb 40 lb
230 lb 23 lb 46 lb

These are guidelines rather than physical limits. Fit, conditioning, terrain, and distance all move them, and people carry more than this routinely. What the numbers are good for is a sanity check before you buy volume you will then feel obliged to fill.

⚠️ SAFETY: For children, the guidance is stricter and better established. The National Safety Council and pediatric sources recommend a child's loaded backpack stay within roughly 10 to 15 percent of their body weight, with both shoulder straps worn and the heaviest items closest to the back. Verify against current guidance from the National Safety Council or your pediatrician for a specific child.

Sizing a Bug-Out Bag: The Water Problem

Emergency bag sizing looks like the same question and is not, because the contents are weighted very differently from a hiking load.

Start from the standard. Ready.gov's basic emergency kit specifies one gallon of water per person per day for three days, covering drinking and sanitation, plus a three-day food supply. That is the benchmark almost every go-bag list is built on, and it was written for a kit that sits in your house.

Now weigh it. A US gallon of water weighs about 8.34 pounds. Three gallons is 25 pounds, before a single other item goes in the bag. For a 150 pound adult, that is 83 percent of the entire 20 percent weight ceiling spent on water alone.

This is the single most useful number in emergency pack planning and it settles the sizing debate immediately: you cannot carry three days of stored water, so a carried kit must produce water rather than transport it. One to two liters on your back, a filter, and a plan for where the next source is. That is not a compromise, it is the only arrangement that fits inside the weight budget, and it is why the size of a go-bag is decided by everything except water.

Which puts most people's bug-out bag in the 30 to 45 L range: large enough for a 72-hour kit with shelter and a sleep system, small enough to stay under the weight ceiling, and small enough to move in. The failure mode we describe in The Bug-Out Bag Nobody Needs (And What to Pack Instead) is almost always a 65 L pack filled to capacity by someone who has never walked ten miles with it.

Household water storage is the other half of this and it works on completely different rules, since weight stops mattering the moment the container is not on your back. That is covered in The Water Storage Mistake Almost Everyone Makes.

Choosing a Size for What You Actually Do

Use Capacity What decides it
Everyday carry 15 to 25 L Whether a laptop and a day's water both have to fit
Day hike, temperate 20 to 30 L Water volume and how many layers the forecast demands
Day hike, exposed or cold 30 to 35 L Insulation bulk and emergency shelter
Get-home bag, vehicle or office 20 to 30 L Footwear, one night of contingency, staying unremarkable
72-hour bug-out bag 30 to 45 L The weight ceiling, not the volume
Weekend backpacking 40 to 55 L Sleep system bulk and food for two days
Multi-day backpacking 55 to 70 L Food volume, which grows fastest with trip length
Winter or extended 70 L and up Cold-weather sleep gear, roughly double the summer bulk

One overlooked category deserves a mention. A get-home bag lives in a car or at a desk and only has to cover the distance between where you are and where you live, so it is usually smaller than people expect and heavily weighted toward footwear, water, and light. It is the bag most likely to actually get used, and the one most often skipped in favor of the more dramatic three-day kit.

Checking the Fit Before You Commit

A pack should be fitted with 15 to 20 pounds in it, because an empty pack tells you nothing about how a frame behaves under load. Most outdoor shops keep weighted bags for exactly this.

  1. Loosen everything, then put the pack on and settle the hip belt across the top of your iliac crest, centered on the bone. Tighten it firmly.
  2. Snug the shoulder straps until they follow the contour of your shoulders with no gap. They should not be carrying the load, just holding the pack in.
  3. Check the anchor point. The shoulder straps should meet the frame about one to two inches below the top of your shoulders. Higher or lower than that is a torso-length problem.
  4. Set the load lifters to roughly 45 degrees, snug but not straining. If you cannot reach that angle, the frame length is wrong.
  5. Fasten the sternum strap an inch or so below your collarbones. It stops the shoulder straps splaying outward. It is not a load-bearing part.
  6. Walk with it. Ten minutes, and stairs if there are any. Most fit problems only appear once you are moving.

If the belt keeps sliding down, the torso is too long. If the pack pulls away from your back at the top, it is too short. Neither is fixable with straps, and both are common when a pack is bought by height rather than by measurement.

FAQ

What size backpack do I need for a 3 day trip?

For three nights out in reasonable weather, 50 to 65 L is the usual range, with lighter and more experienced packers managing at 40 to 50. The variable that moves it most is your sleep system: a bulky synthetic bag and a foam pad can take twice the space of a compressed down bag and an inflatable pad. Winter conditions push a three-day trip toward 70 L. Note that a three-day emergency bag is a different problem from a three-day hiking trip, and usually needs less volume, because it is limited by weight rather than by gear bulk.

Is a 40 L backpack big enough for backpacking?

For one or two nights, yes, comfortably, provided your gear is reasonably modern and compact. For three nights it is possible but tight, and it stops being possible once food volume and cold-weather insulation enter the picture. Forty liters is also close to the upper limit of what most airlines accept as carry-on, which is why so many travel packs cluster right at that number. As a single do-everything pack for a temperate climate, 40 to 50 L is the most useful range there is.

How do I know what size backpack to buy?

Two measurements and one decision. Measure your torso length from the C7 vertebra to the line across the tops of your hip bones, and match that to the maker's frame-size chart. Measure your hips at the iliac crest for the belt. Then choose capacity by the longest trip you realistically take rather than the longest trip you imagine, because empty volume always gets filled. If you are between frame sizes, an adjustable harness solves it, and if you are between capacities, take the smaller one.

Does backpack size depend on my height?

No, and this is the single most common sizing mistake. Frame size is set by torso length, and torso length correlates only loosely with height because people vary far more in leg length than in back length. Two people who are both five foot ten can easily need a small and a large. Height is a reasonable first guess when you have no tape measure and nothing more than that. Always measure before buying, and if you are ordering online, measure twice.

How much should my loaded backpack weigh?

Around 20 percent of your body weight is the commonly used ceiling for a loaded backpacking pack, and about 10 percent is a comfortable daypack target. For a 160 pound adult that is roughly 32 pounds and 16 pounds. These are guidelines, not limits, and fitness and terrain move them in both directions, but they are a useful check against a pack that has quietly filled up. For children, guidance from safety and pediatric bodies is stricter, at roughly 10 to 15 percent of body weight.

What size backpack do I need for a bug-out bag?

Thirty to 45 L for a 72-hour bag, for one specific reason: three days of stored water at the standard one gallon per person per day weighs about 25 pounds, which alone would eat most of a sensible weight budget. Since that is not carryable, a real go-bag carries one to two liters plus a filter, and the pack only has to hold shelter, sleep, food, and clothing. That fits in 30 to 45 L. Bags built at 65 L and above tend to get filled to capacity and then never actually get carried.

What does the liter number on a backpack mean?

Total internal volume, including the main compartment, lid, and pockets, though exactly which spaces get counted varies between manufacturers. That is why two packs both labeled 50 L can hold visibly different amounts. Use liters to compare within a brand and as a rough class indicator across brands. What the number does communicate reliably is the load a pack's frame and suspension were designed for, since those scale with capacity.

Can a backpack be too big?

Yes, in two ways. A frame longer than your torso will not let the hip belt sit on your hip bones, so the load ends up on your shoulders no matter how you adjust it, and no strap fixes that. Separately, excess capacity is self-filling: an oversized pack tends to arrive at the trailhead full, and it weighs more empty than a smaller pack does. A partly loaded large pack also carries worse than a full smaller one, because the contents shift around inside instead of forming a stable block against your back.

Where should a hip belt sit?

Straddling the iliac crest, the ridge of bone at the top of your pelvis, with roughly equal padding above and below it. Find it by putting your hands on your hips with fingers forward. If the belt sits above that ridge it compresses your abdomen and carries nothing, and if it sits below it slides down and interferes with walking. A belt that will not stay in place after being tightened almost always means the pack's torso length is wrong rather than the belt being the wrong size.

Do I need a different size backpack for winter?

Usually one category up, yes. Cold-weather sleeping bags and insulated pads take roughly twice the packed volume of their summer equivalents, and the extra insulating layers, gloves, and hats add real bulk on top. A trip that fits in 50 L in July often needs 65 to 70 L in January. Frame size does not change, since your torso does not, but pay attention to whether the hip belt still fits over heavier clothing, which is a genuine adjustment issue on packs with a snug belt range.

The Bottom Line

Pick capacity from the longest trip you actually take, not the one you picture, and then take the smaller of the two options you are torn between. Pick frame size from a tape measure across your back, never from your height. Fit it loaded, get the belt on the bone, and check the load lifters sit at about 45 degrees.

And if the pack in question is an emergency bag rather than a hiking one, do the water arithmetic before anything else. Twenty-five pounds for three days is the number that decides the size of the bag, the contents of the bag, and whether you will ever pick it up. Be prepared. Stay savage.

References