What size backpack do I need?

For a day, 15 to 25 liters. For a weekend, 30 to 40. For a week with laundry, 40 to 50. For multi week travel, 55 to 70. Those are starting points: the actual constraint is usually the longest rigid item you are carrying rather than the total volume.

How many liters is a carry on bag?

A bag at the common 56 x 45 x 25 cm allowance is about 40 to 45 liters. At the stricter 40 x 30 x 20 cm personal item size it is about 20 liters. Because allowances differ so widely between carriers, check the specific airline rather than buying to a generic carry on label.

How do I calculate what bag capacity I need?

Add up your packing list by volume. A rolled t shirt is roughly 1 liter, a pair of jeans 2, a fleece 3, a pair of shoes 3 to 4, and a wash bag 3. Add the total, then add 20 percent, because a bag packed to its stated capacity will not close comfortably.

Should I measure the bag inside or outside?

Both, for different reasons. Internal dimensions decide whether your items fit. External dimensions decide whether the airline accepts it, because gate sizers measure the outside including wheels, handles and any expansion. External is typically 3 to 5 cm larger per dimension.

Do compression straps make a real difference?

On soft luggage, yes. They recover 10 to 20 percent of packed volume by squeezing air out of clothing, which is often the difference between a bag that closes and one that does not. On anything with a rigid frame or a hard shell they achieve nothing.

ItemFits

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Home / Container & Packing Fit

Bag Size Calculator

Get the capacity you need from your packing list, then check it against the cabin limit.

Trusted across thousands of fit checks · updated daily

400 questions answered

Room Preview·Visual Results·Simulations

Fit summaryFIG. VOLUME

Whether it fits comes down to the measurements most people skip.

A 56 x 45 x 25 cm cabin bag is about 40 to 45 liters, measured outside.

The spaceDay pack 15 to 25 liters. Weekend 30 to 40. Week 40 to 50. Multi week 55 to 70. Common cabin allowance 56 x 45 x 25 cm, about 40 to 45 liters

What decides itTotal packed volume plus 20 percent, and the longest rigid item vs. bag internal length

What to measureRolled t shirt about 1 liter. Jeans 2. Fleece 3. Shoes 3 to 4. Wash bag 3. Enter your list for an exact figure

Measure the item and the narrowest usable space yourself. Small differences in trim, packaging, hardware, and model configuration can change the result. Check your own measurements before you buy or move.

ReferenceFurniture Dimensions

About 10 seconds · no signup

Verdicts compare the supported item orientations against the measurements shown above. See our methodology

10,000+ fit checks run·Last check: a few minutes ago

“Checked my carry-on against 3 airlines before a trip. Saved me a gate-check fee.” — Frequent flyer

Measure smart

What to measure.

Four numbers decide nearly every fit check. Get these right and the rest follows.

01The largest single item going in, at its widest points. Capacity in liters is irrelevant if one rigid item is longer than the bag
02Your total packing list by rough volume, counting a rolled t shirt at about 1 liter and a pair of shoes at 3 to 4 liters
03The cabin allowance of the airline you are flying, in all three dimensions, since these vary more than the marketing suggests
04Bag external dimensions rather than internal, because airline sizers and gate checks measure the outside including wheels, handles and straps
05Whether the bag has compression straps, which reclaim 10 to 20 percent of packed volume on soft luggage

Don't make these

Common mistakes.

Most “it didn't fit” stories trace back to one of these oversights.

⚠Buying by liters alone. A 40 liter bag shaped long and narrow will not take a 15 inch laptop that a 30 liter bag swallows easily
⚠Measuring the internal capacity and presenting it at the gate. Airline sizers measure the external dimensions including wheels and handles, which add 3 to 5 cm
⚠Assuming carry on limits are standard. They are not: common allowances run from 40 x 30 x 20 cm to 56 x 45 x 25 cm, and the small end will reject a bag the large end waves through
⚠Packing to the stated capacity. A bag filled to its nominal liters will not close comfortably, and 80 to 85 percent is the practical working figure
⚠Forgetting that a rigid item sets a floor on the bag length. Skis, a tripod, or a guitar decide the bag before capacity is even considered

Frequently asked

Questions we keep getting.

What size backpack do I need?
For a day, 15 to 25 liters. For a weekend, 30 to 40. For a week with laundry, 40 to 50. For multi week travel, 55 to 70. Those are starting points: the actual constraint is usually the longest rigid item you are carrying rather than the total volume.
01
How many liters is a carry on bag?
A bag at the common 56 x 45 x 25 cm allowance is about 40 to 45 liters. At the stricter 40 x 30 x 20 cm personal item size it is about 20 liters. Because allowances differ so widely between carriers, check the specific airline rather than buying to a generic carry on label.
02
How do I calculate what bag capacity I need?
Add up your packing list by volume. A rolled t shirt is roughly 1 liter, a pair of jeans 2, a fleece 3, a pair of shoes 3 to 4, and a wash bag 3. Add the total, then add 20 percent, because a bag packed to its stated capacity will not close comfortably.
03
Should I measure the bag inside or outside?
Both, for different reasons. Internal dimensions decide whether your items fit. External dimensions decide whether the airline accepts it, because gate sizers measure the outside including wheels, handles and any expansion. External is typically 3 to 5 cm larger per dimension.
04
Do compression straps make a real difference?
On soft luggage, yes. They recover 10 to 20 percent of packed volume by squeezing air out of clothing, which is often the difference between a bag that closes and one that does not. On anything with a rigid frame or a hard shell they achieve nothing.
05

Related fit checks.

Will It Fit in a Bag?Check if your items will fit in a backpack, duffel bag, or tote. Compare bag dimensions and capacity for packing.Will It Fit in a Suitcase?Check if your items will fit in a suitcase. Compare carry-on, checked, and large suitcase dimensions for packing.Shipping Box Size CalculatorFree shipping box size calculator. Enter your item dimensions to get the smallest standard box that fits, with the 2 in per side padding already allowed for.Will a Sleeping Bag Fit in a Backpack?Check if a sleeping bag will fit in a backpack. Compare item dimensions with clearances, openings, and practical movement space before you buy or move it.Will a Stroller Fit in a Travel Bag?Check if a stroller will fit in a travel bag. Compare item dimensions with clearances, openings, and practical movement space before you buy or move it.

All container fit checks →

Unlock stair simulation, AR visualization, and more.

Will it fit? Know before you buy.

Get a verdict in seconds — then test every angle until you’re sure.

Open the fit calculator

ItemFits

About Blog FAQ Features Pricing Work with us

Home / Container & Packing Fit

Bag Size Calculator

Get the capacity you need from your packing list, then check it against the cabin limit.

Trusted across thousands of fit checks · updated daily

400 questions answered

Room Preview·Visual Results·Simulations

Fit summaryFIG. VOLUME

Whether it fits comes down to the measurements most people skip.

A 56 x 45 x 25 cm cabin bag is about 40 to 45 liters, measured outside.

The spaceDay pack 15 to 25 liters. Weekend 30 to 40. Week 40 to 50. Multi week 55 to 70. Common cabin allowance 56 x 45 x 25 cm, about 40 to 45 liters

What decides itTotal packed volume plus 20 percent, and the longest rigid item vs. bag internal length

What to measureRolled t shirt about 1 liter. Jeans 2. Fleece 3. Shoes 3 to 4. Wash bag 3. Enter your list for an exact figure

ReferenceFurniture Dimensions

About 10 seconds · no signup

Verdicts compare the supported item orientations against the measurements shown above. See our methodology

10,000+ fit checks run·Last check: a few minutes ago

“Checked my carry-on against 3 airlines before a trip. Saved me a gate-check fee.” — Frequent flyer

Measure smart

What to measure.

Four numbers decide nearly every fit check. Get these right and the rest follows.

01The largest single item going in, at its widest points. Capacity in liters is irrelevant if one rigid item is longer than the bag
02Your total packing list by rough volume, counting a rolled t shirt at about 1 liter and a pair of shoes at 3 to 4 liters
03The cabin allowance of the airline you are flying, in all three dimensions, since these vary more than the marketing suggests
04Bag external dimensions rather than internal, because airline sizers and gate checks measure the outside including wheels, handles and straps
05Whether the bag has compression straps, which reclaim 10 to 20 percent of packed volume on soft luggage

Don't make these

Common mistakes.

Most “it didn't fit” stories trace back to one of these oversights.

⚠Buying by liters alone. A 40 liter bag shaped long and narrow will not take a 15 inch laptop that a 30 liter bag swallows easily
⚠Measuring the internal capacity and presenting it at the gate. Airline sizers measure the external dimensions including wheels and handles, which add 3 to 5 cm
⚠Assuming carry on limits are standard. They are not: common allowances run from 40 x 30 x 20 cm to 56 x 45 x 25 cm, and the small end will reject a bag the large end waves through
⚠Packing to the stated capacity. A bag filled to its nominal liters will not close comfortably, and 80 to 85 percent is the practical working figure
⚠Forgetting that a rigid item sets a floor on the bag length. Skis, a tripod, or a guitar decide the bag before capacity is even considered

Frequently asked

Questions we keep getting.

What size backpack do I need?
For a day, 15 to 25 liters. For a weekend, 30 to 40. For a week with laundry, 40 to 50. For multi week travel, 55 to 70. Those are starting points: the actual constraint is usually the longest rigid item you are carrying rather than the total volume.
01
How many liters is a carry on bag?
A bag at the common 56 x 45 x 25 cm allowance is about 40 to 45 liters. At the stricter 40 x 30 x 20 cm personal item size it is about 20 liters. Because allowances differ so widely between carriers, check the specific airline rather than buying to a generic carry on label.
02
How do I calculate what bag capacity I need?
Add up your packing list by volume. A rolled t shirt is roughly 1 liter, a pair of jeans 2, a fleece 3, a pair of shoes 3 to 4, and a wash bag 3. Add the total, then add 20 percent, because a bag packed to its stated capacity will not close comfortably.
03
Should I measure the bag inside or outside?
Both, for different reasons. Internal dimensions decide whether your items fit. External dimensions decide whether the airline accepts it, because gate sizers measure the outside including wheels, handles and any expansion. External is typically 3 to 5 cm larger per dimension.
04
Do compression straps make a real difference?
On soft luggage, yes. They recover 10 to 20 percent of packed volume by squeezing air out of clothing, which is often the difference between a bag that closes and one that does not. On anything with a rigid frame or a hard shell they achieve nothing.
05

Related fit checks.

All container fit checks →

Unlock stair simulation, AR visualization, and more.

Will it fit? Know before you buy.

Get a verdict in seconds — then test every angle until you’re sure.

Open the fit calculator