Backpack Sizes: Choosing Liters and Fitting a Pack to Your Body
A backpack has two sizes: how much it holds, in liters, and the torso length it is built for. This guide covers both, plus the right backpack size for children at each age.
Choose backpack capacity by use: 15–30 liters for school, commuting and day hikes, 30–50 liters for 1 to 3 nights, and 50–80 liters for longer backpacking trips. For packs with a hip belt, the fit size comes from your torso length, measured from the C7 vertebra at the base of the neck to the top of the hip bones: roughly under 16 in is XS, 16–17.5 in Small, 18–19.5 in Medium and 20 in or more Large.
- Day pack
- 15–30 L
- 1–3 nights
- 30–50 L
- Multi-day
- 50–80 L
- Medium torso
- 18–19.5 in
On this page
Backpack size in liters by activity
Capacity is the internal volume of the main body and pockets, in liters. It tells you how much fits, not how big the pack looks. Pick the smallest size that holds what you need, because a larger pack weighs more empty and invites overpacking.
| Use | Capacity | Cubic inches | What it holds |
|---|---|---|---|
| Running, cycling, hydration | 5–15 L | 300–900 | Water reservoir, jacket, snacks, phone |
| Everyday, school, commuting | 15–30 L | 900–1,800 | Laptop, books or binder, lunch, water bottle |
| Day hike | 15–30 L | 900–1,800 | Water, food, layers, first aid; 30–40 L in winter |
| Travel personal item | 18–28 L | 1,100–1,700 | Electronics, a change of clothes, documents |
| One-bag travel (carry-on) | 35–45 L | 2,100–2,750 | A week of clothes with laundry |
| Overnight to weekend, 1–3 nights | 30–50 L | 1,800–3,050 | Tent or shelter, sleeping bag, stove, food |
| Multi-day, 3–5 nights | 50–80 L | 3,050–4,900 | More food and fuel, bulkier sleep system |
| Extended trips, winter, carrying for children | 70 L and up | 4,300 and up | Four-season gear, extra food and shared kit |
Your gear decides the size more than the number of nights. A synthetic sleeping bag typically packs to roughly twice the volume of a down bag of similar warmth, and that difference alone can move you up a pack size. Compare packed sizes in our sleeping bag size guide before you pick a pack.
Brands measure capacity differently: some count only the main compartment, others include the lid, side pockets and stretch front pocket. Two "40-liter" packs can differ by 5 liters in usable space. For the outside dimensions of school, laptop and travel packs, and the airline sizes they have to fit, see backpack dimensions by type.
Liters to cubic inches
US brands sometimes list capacity in cubic inches. One liter is 61.02 cubic inches.
| Liters | Cubic inches | Typical pack |
|---|---|---|
| 10 L | 610 cu in | Hydration pack |
| 20 L | 1,220 cu in | Small daypack |
| 30 L | 1,831 cu in | Large daypack, school pack |
| 40 L | 2,441 cu in | Carry-on travel pack |
| 50 L | 3,051 cu in | Weekend backpacking |
| 65 L | 3,967 cu in | Multi-day backpacking |
| 80 L | 4,882 cu in | Expedition |
How to measure your torso length
Packs with a load-bearing hip belt come in sizes, or with an adjustable back, set by torso length. If the torso size is wrong, the hip belt rides on your waist or your buttocks instead of your hip bones, and the weight ends up on your shoulders. You need a helper and a soft tape measure.
- Find the C7 vertebra. Tilt your head forward. The most prominent bony bump at the base of your neck is C7. This is the top of the measurement.
- Find the iliac crest. Put your hands on your hips with thumbs pointing backward and index fingers forward, resting on top of the hip bones. Imagine a line between your thumbs across your spine. This is the bottom.
- Measure along the spine from C7 to that line, following the curve of your back while standing straight.
- Measure twice and use the average. Results within half an inch are normal; if you fall between sizes, an adjustable harness or the smaller size with a longer setting usually fits better.
| Pack size | Torso length (in) | Torso length (cm) |
|---|---|---|
| Extra small | Up to 15.5 in | Up to 39 cm |
| Small | 16–17.5 in | 41–44 cm |
| Medium / Regular | 18–19.5 in | 46–50 cm |
| Large / Tall | 20 in and up | 51 cm and up |
Many brands use overlapping sizes such as S/M and M/L, each covering 3 to 4 inches. Women's-specific packs typically cover shorter torso ranges and have narrower, S-curved shoulder straps and more angled hip belts; anyone can use either fit.
Hip belt size and fit
The hip belt carries most of the load on a pack above about 30 liters, so its size matters as much as the torso length. It is sized by hip circumference, measured around the top of the hip bones (the iliac crest), not around your trouser waist.
- Position: the padded wings of the belt should sit centered over the iliac crest, with about half the padding above and half below the top of the hip bone.
- Length: when tightened, the padded sections should not meet in front. If the buckle is at the end of its adjustment, the belt is too large; many brands sell interchangeable belts in other sizes.
- Load lifters: the short straps from the top of the shoulder straps to the pack should run back at roughly 45 degrees. If they are flat or pointing down, the torso size is too short.
- Shoulder straps: should wrap over the shoulder with no gap between strap and body, and anchor 1 to 2 in below the top of the shoulder at the back.
An empty pack feels comfortable in almost any size. Try a pack with 15 to 25 lb (7 to 11 kg) inside, tighten the hip belt first, then the shoulder straps, then the load lifters, and walk for several minutes before deciding.
Kids' backpack size by age
A child's school backpack should be sized to the child's back, not to what the school list requires. Two guidelines apply at every age: the pack should be no taller than the distance from the shoulders to the waist, with its bottom no more than 4 in (10 cm) below the waist, and the loaded pack should weigh no more than 10 to 20 percent of the child's body weight.
| Age / grade | Pack height | Capacity | Max loaded weight (10–20% rule) |
|---|---|---|---|
| Preschool (3–5) | 10–12 in (25–30 cm) | 5–10 L | 3–8 lb for 30–40 lb children |
| Kindergarten to grade 2 (5–8) | 12–14 in (30–36 cm) | 10–15 L | 4–11 lb for 40–55 lb children |
| Grades 3–5 (8–11) | 14–16 in (36–41 cm) | 15–20 L | 5.5–16 lb for 55–80 lb children |
| Middle school (11–14) | 16–18 in (41–46 cm) | 20–28 L | 8–24 lb for 80–120 lb children |
| High school (14+) | 17–19 in (43–48 cm) | 25–35 L | 11–30 lb for 110–150 lb teens |
Children of the same age vary widely in size, so measure: from the top of the shoulder to the waist is the maximum pack height. A 7-year-old with a 13 in shoulder-to-waist length should have a pack about 12 to 13 in tall, whatever the store labels say.
- Use both straps. Wide, padded straps worn over both shoulders spread the load; slinging the pack over one shoulder concentrates it.
- Heaviest items closest to the back. Books and laptops against the back panel shift the center of mass toward the spine.
- Waist or chest strap on packs above about 20 liters keeps the load from swinging.
- Weigh it. Put the packed bag on a bathroom scale once a week; a 70 lb child's pack should stay under 7 to 14 lb.
Adult daypack and commuter fit
Packs under about 30 liters usually come in one size, with no hip belt or only a webbing strap. Fit comes down to length: the bottom of the pack should sit at or just above the top of the hips, and the top should not rise above the shoulders. If the pack hangs below your hip bones with the straps snug, it is too long for you; many brands make a shorter or "small" version of their commuter packs.
For carrying a laptop, match the sleeve to the laptop body using the laptop bag size chart. For one-bag travel, the 35 to 45 liter range is set by the airline limit rather than by your body; the carry-on bag size guide explains that limit, and a soft duffel bag is the alternative when you will not carry the bag far. For sports kit, see gym bag sizes.
Frequently asked questions
What size backpack do I need in liters?
For school or work, 20 to 30 liters. For a day hike, 15 to 30 liters, more in winter. For an overnight or weekend trip with a tent, 30 to 50 liters; for 3 to 5 nights, 50 to 80 liters. For one-bag travel within airline carry-on limits, 35 to 45 liters.
How many cubic inches is a 40 liter backpack?
About 2,440 cubic inches. One liter is 61.02 cubic inches, so 20 L is 1,220 cu in, 30 L is 1,830 cu in, 50 L is 3,050 cu in and 65 L is 3,970 cu in.
How do I know my backpack torso size?
Have someone measure along your spine from the bony bump at the base of your neck (the C7 vertebra) down to the line between the tops of your hip bones (the iliac crest). Compare that length with the manufacturer's size chart, since a Medium at one brand can be a Small at another.
Is torso length the same as height?
No. Two people of the same height can have torso lengths 2 to 3 in apart, because leg length varies. Height is only a starting point; measure the torso before buying a pack with a hip belt.
What size backpack should a kindergartener have?
A pack about 12 to 14 in (30 to 36 cm) tall and 10 to 15 liters. It should be no taller than the distance from the child's shoulders to the waist, and the loaded pack should weigh no more than 10 to 20 percent of the child's body weight.
How heavy should a backpack be?
For children, no more than 10 to 20 percent of body weight, the range the American Academy of Pediatrics recommends. A 60 lb (27 kg) child should carry 6 to 12 lb (2.7 to 5.4 kg) at most. For adult backpackers, a frequently used rule of thumb is about 20 percent of body weight for a multi-day load.