Article — Bag Capacity
A bag's capacity is the interior volume it can hold, almost always quoted in litres. The number you see on the marketing tag is the outside box volume — length times width times height — and the real usable space inside is 10 to 20 percent smaller once you account for rounded corners, padding, lining, and the wheel housing on a suitcase. For travel, the most important figures are 38.5 L (the IATA carry-on box of 55×35×20 cm), 45 L (the typical U.S. domestic carry-on ceiling), and 158 cm in linear dimensions (the worldwide checked-bag fee threshold). Get within those numbers and you fly without surcharges.
This guide walks through how bag volume is calculated, where the discrepancy between marketing litres and real-world capacity comes from, what each major airline category actually allows, and how to match a bag to the length of your trip.
What bag volume actually means
Bag volume is the product of three measurements: length, width, and height. Multiply them in centimetres and divide by 1,000 to get litres. A bag measured at 55×35×20 cm is 38,500 cubic centimetres, or 38.5 litres. The same math in inches gives 21.7×13.8×7.9, producing 2,350 cubic inches — the same volume, different unit.
The complication is that this number describes a perfect rectangular box. No real bag is a rectangular box. Hardshell suitcases have rounded corners. Soft backpacks have curved sides that flex when packed. Both have interior linings, padding, frame stays, and pockets that eat into the usable space. The American backpack manufacturer Tortuga estimates the usable interior of a 40 L pack at 32–36 L; the rest is structure.
The U.S. National Institute of Standards and Technology recognises one litre as exactly 1,000 cubic centimetres. The cubic inch on luggage tags is 16.387 cubic centimetres, so dividing cm³ by 16.387 converts directly. A 38.5 L IATA carry-on is exactly 2,350 cubic inches.
Measuring a bag the right way
For a wheeled suitcase, measure outside-to-outside including the wheels and the recessed handle — airlines measure the same way. Lay the bag on its back, measure floor-to-top for height, side-to-side for width, and front-to-back for depth. Do not subtract for the wheels: airline sizers do not.
For a backpack or duffel, measure the main compartment only. Side water-bottle pockets and detachable daypacks are excluded from the litre figure. Stand the bag upright, fully loaded, and measure the bulge at its widest point. A bag that measures 50×30×20 cm empty might balloon to 55×35×25 cm packed — and that packed figure is what an airline gate agent sees.
Most U.S. and European airlines explicitly include wheels in the published carry-on dimensions. A suitcase advertised as 56×36×23 cm "to fit overhead bins" can be 55 cm without wheels and 58 cm with them — sizing fail. Always measure the bag as the airline will: every protruding part included.
Airline limits a bag must respect
Three numbers cover most travel. The International Air Transport Association (IATA) publishes a recommended cabin baggage size of 55×35×20 cm — 38.5 L — intended to let all passengers stow a bag in the overhead bin. U.S. domestic carriers are more generous: American, Delta, United, and Southwest all allow up to 56×36×23 cm (46 L). Budget European carriers (Ryanair, Wizz Air) often charge for anything above the IATA spec.
For checked baggage, the worldwide rule is 158 cm of linear dimensions — L plus W plus H. That equals 62 linear inches and originates from older U.S. cargo-hold geometry. Most carriers also enforce a 23 kg weight limit per bag for economy, with a 32 kg ceiling beyond which manual handling is forbidden under International Labour Organization guidance. Personal items — the under-seat bag — vary more, but 40×25×20 cm is a reliable upper bound.
- IATA carry-on — 55×35×20 cm, 38.5 L, 7–10 kg
- U.S. domestic carry-on — 56×36×23 cm, 46 L, usually unweighed
- European budget carry-on — 40×20×25 cm (Ryanair priority is 55×40×20)
- Checked maximum — 158 cm linear, 23 kg economy, 32 kg first class
- Personal item — about 40×25×20 cm, under-seat fit
Bag sizes by trip length
One Bag and minimalist travel communities have built decent empirical data on the right bag size per trip length. A weekend (two nights) needs 20–30 L: one change of clothes, toiletries, a layer, and electronics. A week needs 35–45 L, which is precisely why carry-on bags cluster in this range. Two weeks needs 50–65 L — the medium checked range — and any trip longer than that benefits more from a laundry plan than from a bigger bag.
Capacity above 80 L is rarely useful for ordinary travel. Expedition packs in the 90–120 L range are designed for technical loads — ropes, climbing hardware, multi-day food caches — not for clothes. A 110 L bag stuffed with shirts is heavy, unwieldy, and often hits the 32 kg airline ceiling before it is even three-quarters full.
U.S. Customs and Border Protection lets U.S. residents bring back $800 of duty-free goods per traveller. The volume that money buys — clothes, electronics, gifts — usually fits in 10–20 L. Bag sizing for the return trip rarely needs to grow by more than that.
Packing efficiency and real capacity
How you pack changes effective capacity by 20–30 percent. The U.S. National Park Service's hiking guidance recommends rolling clothes rather than folding for the same volume reduction. Compression sacks — mesh cubes or vacuum-zip bags — squeeze soft items like down jackets and sweaters to roughly half their loose volume. Hard items (shoes, books, electronics) cannot be compressed and dictate the minimum bag size.
A common mistake is buying for the worst-case trip. A 65 L pack used for a weekend is a 65 L pack on the plane, on transit, and across cobblestone streets — even when only 25 L of contents are inside. The empty space adds weight, slack, and discomfort. Bags should size to the median trip, not the maximum.
A "40 litre" pack from one brand can hold appreciably less than a "40 litre" pack from another. American Backpacker magazine's annual gear reviews have documented variances of up to 15% between advertised and measured interior capacity. When a litre figure matters, look for the manufacturer's interior dimensions and recompute — or measure the empty bag yourself.
Common bag-buying mistakes
A bag can be 45 L and still violate a carry-on rule if its shape is wrong — long and thin instead of compact. Carry-on limits are dimensional, not volumetric. Always check L, W, and H individually against the airline's published box, not just the litre figure.
A 70 L hardshell suitcase often weighs 4–5 kg empty. Once packed to 23 kg total, you are carrying around 5 kg of bag for every 18 kg of stuff. The lightest checked-bag designs (polypropylene shells, ultralight nylon duffels) weigh under 2.5 kg for the same volume — a free 2 kg of usable allowance.
The U.S. Federal Trade Commission's mail-order rule lets you return luggage purchased online within 30 days. Use it: pack the bag with everything you actually take, weigh it on a bathroom scale, and measure the bulge. If anything fails, return and try a different size or shape before the trip.