Backpack Size Guide: Litres, Carry-On Limits & Trip Length

A backpack’s volume in litres is its height × width × depth in centimetres divided by 1,000, so a 55 × 40 × 20 cm bag holds about 44 litres. As a backpack size guide, 15–25 L suits everyday use, 30–45 L a weekend away, and 35–45 L is the usual range for a week with carry-on only.

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Your conversion appears here. The full chart is below.

Backpack size by trip

Use	Volume
Everyday / commute	15–25 L
School or laptop bag	20–30 L
Overnight	25–35 L
Weekend (2–3 nights)	30–45 L
One week, carry-on only	35–45 L
Multi-week travel (checked)	50–70 L

How to measure a backpack

Airlines measure the outside of your bag, so that is what the calculator needs. Measure the bag packed, because a full rucksack (backpack) is deeper than an empty one.

1. Pack the bag roughly the way you will carry it on the trip.
2. Stand it upright and measure the height from the base to the highest point, including the top handle.
3. Measure the width across the widest part, including any side pockets that stick out.
4. Measure the depth from the back panel to the front of the furthest pocket.
5. Enter all three in centimetres or inches. The calculator orders them from longest to shortest, so it does not matter which one you call height.
6. Read the volume and the two carry-on checks. If the maker lists a volume, compare it with the result, since makers may measure the inside of the bag rather than the outside.

Carry-on size limits

Airline cabin limits change, and they vary between airlines and even between fare types on the same airline. Always check your airline’s current rules before you fly. With that said, two limits come up often.

Many airlines use a cabin bag limit of 55 × 40 × 20 cm, which works out at about 44 litres. US airlines commonly allow 22 × 14 × 9 inches, about 45 litres. The two are not interchangeable: a 55 × 40 × 20 cm bag is too wide for a 14-inch limit, and a 22 × 14 × 9 inch bag is too deep for a 20 cm limit. The calculator checks both.

IATA, the airline trade body, publishes a guideline size of 56 × 45 × 25 cm. That is larger than many airlines actually allow, so treat it as a reference point rather than a promise. Many airlines also set weight limits and a smaller allowance for a personal item that fits under the seat.

What size backpack for your trip

The table under the calculator gives typical ranges. For commuting and everyday use, 15–25 L is the usual range, enough for a day’s essentials and often a laptop and a jumper (sweater). A school or laptop bag usually falls between 20 and 30 L. An overnight trip suits 25–35 L, and a weekend of two or three nights suits 30–45 L. For a week with carry-on only, 35–45 L is the usual range, which sits right at the edge of the two common cabin limits above. For multi-week trips with a checked bag, 50–70 L is typical.

In the UK, “rucksack” and “backpack” mean the same thing, and both words are used for everyday bags as well as travel ones.

Quick answers

• 55 × 40 × 20 cm: 44 L
• 22 × 14 × 9 in: about 45.4 L
• 56 × 45 × 25 cm (IATA guideline): 63 L
• 50 × 30 × 20 cm: 30 L, fits both common carry-on limits
• 45 × 30 × 15 cm: about 20 L, an everyday daypack
• Weekend of 2–3 nights: 30–45 L

FAQs

What size backpack do I need for a week with carry-on only?

Around 35–45 litres, as long as the outside dimensions fit your airline’s cabin limit. Packing cubes and a light jumper you wear on the plane help a week’s clothes fit.

Is a 40-litre backpack allowed as carry-on?

Often, but litres alone don’t decide it. Airlines check height, width and depth, so a 40 L bag that is tall and narrow can still fail a limit. Enter its dimensions above and check your airline’s rules.

How many litres is 22 × 14 × 9 inches?

About 45 litres. That is the outside box size, so the usable space inside a padded bag will be a little less.

What size backpack is best for school or a laptop?

Usually 20–30 litres. Check that the laptop sleeve fits your laptop’s actual dimensions, not only its screen size.

Is a rucksack the same as a backpack?

Yes. Rucksack is the usual British word and backpack the usual American one, and the sizing is the same.

For packing advice by trip, see our travel guides in the Journal. To check a new bag, enter the dimensions from its product page. Our backpacks and rucksacks are in the shop, and every other converter is in the size guide.

Volume is outer height × width × depth ÷ 1,000, so it is an upper estimate of the space inside. The volumes by trip are typical ranges, not a standard. The cabin sizes quoted are common airline limits and IATA’s guideline; each airline sets and changes its own rules.