A suitcase listed as 20 inches can sound straightforward until an airline allowance is shown as 55 x 40 x 20 cm. That is where luggage size conversion matters. Getting it right helps you compare cases properly, pack for the trip ahead and avoid the frustration of finding your bag is too large at the gate.
For most travellers, the key is to understand three things: external dimensions in centimetres, the equivalent measurement in inches, and capacity in litres. Each tells you something useful, but they are not interchangeable.
Why suitcase sizes can be confusing
Luggage brands often describe cases by their height in inches, such as 20-inch cabin luggage, 24-inch medium luggage or 28-inch large luggage. Airlines, however, usually set their limits in centimetres and give three measurements: height, width and depth.
A 20-inch case does not automatically meet every 55 cm cabin bag rule. The advertised inch size commonly refers to the main body height, while the airline measures the whole item. That includes wheels, handles, side feet and any fixed external pockets. A case can therefore be sold as 20 inches yet measure 56 cm or more from floor to top handle.
This is especially relevant for budget flights. Ryanair, Wizz Air and easyJet each have their own baggage rules, and the allowance can vary depending on whether you have booked a basic fare, a cabin bag option or checked luggage. Always compare the case's full external dimensions with your booking's current allowance before travelling.
Luggage size conversion: inches to centimetres
The basic conversion is simple: one inch equals 2.54 centimetres. To convert inches to cm, multiply the inch measurement by 2.54. To convert centimetres to inches, divide by 2.54.
In practice, suitcase sizes are rounded. A case described as 20 inches may be 50 cm, 52 cm or 55 cm high depending on the manufacturer. Use the inch label as a useful shopping category, then rely on the exact external measurements when checking airline compliance.
| Common suitcase size | Approximate height in cm | Typical use |
| --- | ---: | --- |
| 16 inches | 41 cm | Underseat bag or compact overnight trip |
| 18 inches | 46 cm | Short break or light packer |
| 20 inches | 51 cm | Cabin case for many airlines |
| 22 inches | 56 cm | Larger cabin case or compact checked bag |
| 24 inches | 61 cm | Medium checked luggage |
| 26 inches | 66 cm | Medium-to-large checked luggage |
| 28 inches | 71 cm | Large checked case for longer holidays |
| 30 inches | 76 cm | Extra-large family or extended-trip luggage |
These figures refer to height only, so they are a guide rather than an airline check. A 55 x 40 x 20 cm allowance means every measurement must fit, not just the height. If your bag is 55 cm tall but 23 cm deep, it may still be refused as a cabin case.
Measure luggage the way an airline does
Place the case upright on a flat floor and measure from the ground to its highest point. Then measure its widest point from side to side and its deepest point from front to back. Include wheels, handle housings, feet and any protruding features.
Do this with the case empty first. If you plan to use an expandable zip, remember that opening it changes the depth. An expandable case is useful for a return journey with extra shopping, but it is usually best kept closed when using it as cabin luggage.
Soft-shell luggage deserves an extra check. It can be easier to fit into a sizer when lightly packed, but overfilling exterior pockets can push it beyond the stated depth. Hard-shell cases keep their shape more consistently, which can make airline checks easier to judge.
Converting suitcase dimensions into litres
Litres describe volume, not external size. They help you estimate how much a suitcase can hold, but they cannot confirm whether it will meet an airline's baggage rules.
As a rough calculation, multiply height x width x depth in centimetres, then divide by 1,000 to get the outside volume in litres. For example, a 55 x 40 x 20 cm case equals 44 litres on paper:
55 x 40 x 20 = 44,000 cubic cm
44,000 ÷ 1,000 = 44 litres
The usable packing capacity will be lower because wheels, handle tubes, shell thickness, linings and compartments take up room. This is why two cases with similar external measurements may have different stated capacities. A 40-litre cabin case with a well-designed interior can also be more useful than a slightly larger case with awkward wheel housings or limited organisation.
As a general guide, 25 to 35 litres suits a weekend away with careful packing. Around 35 to 45 litres works well for many cabin-bag trips. Medium checked cases often sit around 60 to 80 litres, while large cases can offer 90 litres or more. Your packing style matters as much as the number. Winter clothing, walking shoes and family items take up far more space than lightweight summer clothes.
Choose the size for your journey, not just the label
For a one- or two-night city break, an underseat bag or compact cabin backpack is often the simplest choice. It can save money on basic airline fares and keeps essentials close by. The trade-off is limited space, so choose versatile clothing and keep liquids within airport security rules.
For a long weekend or a business trip, a cabin suitcase gives you more structure and room for shoes, toiletries and a change of clothes. Look for a lightweight design, smooth wheels and organised internal sections. The less the empty case weighs, the more of your allowance is available for your belongings.
A medium checked case is a practical option for a week away, particularly when sharing luggage space is not realistic. Large cases suit longer holidays and family travel, but do consider weight. It is easy to fill a 28-inch case beyond a typical 20 kg or 23 kg checked-baggage limit. A larger suitcase does not give you permission to exceed the airline's weight allowance.
For flexible travel, a 2-in-1 suitcase-backpack can be a sensible middle ground. It offers the easier mobility of a backpack when navigating stairs, platforms or uneven pavements, while still giving you the organisation of a travel case. Check its full packed dimensions carefully if you intend to use it under the seat.
Common conversion mistakes to avoid
The most common mistake is comparing an airline's full dimensions with only the named size of a suitcase. A 20-inch label is not the same as a guaranteed 55 cm cabin bag. Check height, width and depth every time.
Another is assuming litres and dimensions tell the same story. A case can have a generous capacity but still be too deep for cabin use. Equally, an airline-compliant case may hold less than expected if its interior is heavily shaped around the wheel area.
Weight is the third factor people overlook. Some airlines permit a bag that fits the dimensions but impose a strict cabin weight limit. Pack the case at home and weigh it before leaving. A small luggage scale can remove a lot of uncertainty, particularly when travelling with low-cost carriers.
Finally, do not rely on a previous flight's experience. Airline rules, fare bundles and routes can change. A bag accepted on one journey may not meet the allowance on the next, even with the same airline.
A quick check before you buy or fly
When comparing luggage, start with the airline allowance you are most likely to use. Then choose a case with external measurements at or below that limit, allowing a little room for measurement differences. Check its empty weight, capacity and interior layout before deciding whether it suits your trip.
At ATX Luggage, practical sizing is designed around real travel constraints, from compact underseat formats to dependable checked cases. The right case is not simply the biggest one you can buy. It is the one that fits your airline rules, carries what you need and lets you move through the airport with confidence.
