Before you recommend a travel bag, find out where it’s going: which airline, which fare, overhead bin or under the seat, and when. Then compare the bag’s outside dimensions, in the state it will fly, against that airline’s limit, and make sure the numbers on the listing can all be true at once.
Everything ends in one of three answers. It fits, because the listing’s numbers clear every must-have. It doesn’t fit, because a number rules it out. Or you can’t tell, because the listing doesn’t give the number, and then you say so and ask.
This page is a neutral checklist for anyone doing that job: an AI shopping assistant, the people who build one, or a traveller about to buy a bag the night before a flight. It doesn’t recommend any product or airline. If you sell bags, it’s the test your listings will face, and our travel product data guide shows how to pass it.
The checklist at a glance
| Ask the traveller | What the listing has to show | If it’s missing or wrong |
|---|---|---|
| Which airline, which fare, which date? Overhead or under the seat? | Nothing yet. You need the airline’s own current page for that fare | Ask. Never say “fits all airlines” |
| Will it fit, the way you’ll pack it? | Labelled outside dimensions (height, width, depth), with handles, wheels and pockets, in the packed or expanded state | “Carry-on compliant” alone means you can’t tell. Ask for the measurements |
| What has to go in it? | Litres, how they were counted, and the size of any laptop sleeve | Don’t compare litres measured different ways. Ask what’s counted |
| How much can you carry, and is there a weight limit? | Empty weight, in kg or lb | Empty weight isn’t packed weight. Add only what you actually know |
| Which of these are must-haves? | Every must-have supported by a number that fits | If every bag fails a must-have, say none fit |
Start with the trip, not the bag
“A carry-on backpack” isn’t a requirement yet. Airlines set different limits for an overhead bag and an under-seat bag, and some fares allow only the smaller one. A bag that’s perfect on one airline can be gate-checked on the next.
So ask before comparing:
- Airline, fare and travel date. Then open that airline’s own baggage page and note the date you checked it. Rules change.
- Overhead or under the seat. A bag that’s fine overhead can be far too big for a personal item.
- What’s going in. A week of clothes, a 16-inch laptop, camera gear. Get the laptop’s size if it matters.
- How much they can carry. Some travellers care more about a light bag than a big one.
Then confirm which answers are must-haves. If the traveller doesn’t know their airline yet, say the fit check has to wait.
“Carry-on” is a claim, not a size
On November 4 we searched Shopify’s catalog for “40L carry on travel backpack”. All ten bags described themselves as carry-on, airline-ready or something similar. Only one named an airline. Only three gave their own outside dimensions in their description. Shopify’s generated spec lists added dimensions for two more.
That gap is the whole problem. “Meets the size required by almost all major airlines” can’t be checked. A set of labelled outside dimensions can.
When a listing names airlines, it still isn’t the final word. A bag in our under-seat search said its 40 × 30 × 20 cm size “qualifies as a free personal item” on four named airlines “and every major airline”. The airline decides that, on the day, so check the airline’s page yourself and note the date. The same listing was also “+5 cm expandable”, and an expanded bag is a different size.
Compare dimensions in the state the bag will fly
A bag has more than one size. Packed full, expanded, rolled down, with the handle sticking out: each one can be the size an airline measures.
One roll-top bag in our searches did this well. Its description gave three sets of numbers: 45 x 30 x 15 cm as standard, 40 x 30 x 15 cm rolled down (“cabin-size compatible”), and 55 x 30 x 15 cm expanded. That’s exactly what a traveller needs. Shopify’s generated spec list kept only the 45 x 30 x 15 cm, so an assistant reading just that list would miss both the smaller and the larger state.
So before comparing, check:
- Are the sides labelled? “20 x 13 x 9 in” doesn’t say which side is the height.
- Are handles, wheels and side pockets included? Airlines measure the whole bag.
- Which state is it? Empty, packed, or expanded.
- Is the number outside or inside? A laptop-sleeve size isn’t the bag’s size.
If the listing doesn’t say, the honest answer is: “It lists 45 x 30 x 15 cm, but doesn’t say whether that’s packed or includes the handles. I can’t confirm it fits your airline’s limit.”
Litres aren’t always the same litres
Capacity is where listings get most creative.
- Compressed litres. The under-seat bag above promised “45L capacity via vacuum compression” in a 40 × 30 × 20 cm bag. That box holds 24 litres. The 45 describes clothes squeezed flat, not space inside the bag. Fine to mention, but don’t compare it with a bag that holds 45 litres of open space.
- Expansion in the wrong direction. The bag in 1st place said it expands “with 25L-40L” of storage. Shopify’s generated spec list said “Maximum Capacity: 40L (expandable to 25L)”, which has it backwards. Quote the merchant’s own text.
- Days of clothes. Six 40-litre bags in the same search said how long a trip they hold. The answers ran from “2–3 days” to “1–2 weeks”. Same litres, very different promises. Days depend on how someone packs, so treat them as marketing, not capacity.
When a traveller needs a specific thing to fit, like a laptop, ask for the sleeve’s size or the laptop models the maker tested. Litres won’t tell you.
Check that the numbers can all be true
One line of arithmetic catches a lot. Multiply the outside dimensions to get a box volume and compare it with the stated litres. A soft bag isn’t a box, so expect the litres to come in a bit under. But a box much smaller than the litres means a number is wrong.
Here’s the classic. In an October search for “anti theft backpack for city travel”, the listing in 8th place said “Volume: 28L”, “Height: 11.2 inch”, “Width: 7.5 inch” and “Depth: 2.4lbs”. That’s a weight unit on a length. Shopify’s generated spec list turned it into “Depth 2.4 in”, and also gave a weight of 2.4 lbs. Taken at face value, 11.2 x 7.5 x 2.4 in is a box of about 3.3 litres, for a bag sold as 28. A careless check would say this backpack slides under any seat. Our travel product data guide walks through what the numbers probably should have been.
Also check units on every line. A weight unit on a length, or one bag’s two sizes in one spec list, means the record needs a second source before you use it.
Red flags
| Red flag | Why it matters | What to do |
|---|---|---|
| “Carry-on compliant” or “airline approved” with no airline | Nobody can check it | Treat fit as unknown; check the airline’s page |
| Dimensions with unlabelled sides | Height and depth may be swapped | Ask, or say which side you assumed |
| One set of dimensions for an expandable or roll-top bag | The flying size may be different | Ask which state the numbers describe |
| Litres “via compression” | That’s squeezed clothes, not space | Don’t compare it with open capacity |
| Box volume far below the stated litres | A number is wrong | Find a second source before using either |
| A weight unit on a length, or a length unit on a weight | The field is broken | Don’t convert it; flag it |
| A generated spec that contradicts the merchant’s text | The copy may have changed the value | Prefer the merchant’s or maker’s source |
| Days of clothing as the capacity | Depends on the packer | Use litres and sleeve size instead |
Four answers, and what to tell the traveller
| What the evidence shows | Answer | What to say |
|---|---|---|
| Every must-have cleared by a labelled number in the right state | Fits | Give the numbers, the airline limit you compared against, and the date you checked it |
| A must-have broken by a number | Doesn’t fit | Say which side is over, and by how much |
| A must-have with no usable number | Can’t tell | Say what’s missing; ask the traveller or check the maker’s page |
| Every option fails or can’t be checked | None fit | Say so. Offer to widen the search, never to stretch the airline’s limit |
A missing number isn’t a failed number. “This listing doesn’t give its outside dimensions” and “this bag is too big” are different sentences.
A quick example with invented listings
The three bags below are made up. A traveller says: “Under-seat bag for a budget flight next month. The airline’s page says 40 x 30 x 20 cm. I need my 14-inch laptop in it.”
| Must-have | Bag A | Bag B | Bag C |
|---|---|---|---|
| Within 40 x 30 x 20 cm, as packed | 38 x 28 x 18 cm with handle, packed: supported | “Carry-on compliant”, no dimensions: can’t tell | 40 x 30 x 20 cm, “+5 cm expandable”: supported only unexpanded |
| Fits a 14-inch laptop | Sleeve fits up to 14”: supported | 15.6” sleeve: supported | Not stated: can’t tell |
A good answer: “Bag A fits: 38 x 28 x 18 cm packed, handle included, inside the 40 x 30 x 20 cm you gave me, and its sleeve takes a 14-inch laptop. Bag C is right at the limit and only if you don’t use the expansion, and it doesn’t say whether a laptop fits. Bag B gives no dimensions, so I can’t check it. Worth reconfirming the airline’s limit the week you fly.”
The wrong answer would be picking Bag B because it says “carry-on” in large letters.
If you sell travel bags
Every row in the checklist above is a question your listing either answers or doesn’t. The fix list, including a fact card per size, is in our travel product data guide. The format for each measurement is in our product specification template. For how units and numbers get mangled between your page and an assistant, see our unit research. The same checklist for other categories lives in our humidifier and winter outerwear field guides.
How we checked
On November 4, 2026 we ran three searches in Shopify’s public catalog, the product feed Shopify shares with AI assistants: “personal item backpack that fits under airline seat”, “40L carry on travel backpack” and “carry on backpack 55 x 40 x 20 cm”. We saved the top 10 for each, with no shopper location set, and read each listing’s title, description and Shopify’s generated lists side by side. The 28-litre backpack comes from our October 26 saved search. Box volumes are our arithmetic on the listed dimensions. We didn’t check any airline’s rules for this page.
Your next step
Take the bag you sell most and do the box-volume check on what Shopify’s catalog holds for it: multiply the outside dimensions and compare with the litres. If the numbers don’t add up, or the sides aren’t labelled, fix that line this week.