Standard carry-on dimensions by region
US majors (American, Delta, United) standardize around 22 x 14 x 9 inches for the overhead bin bag plus a personal item (backpack, purse) under the seat. Southwest allows slightly generous sizing on paper but sizers at busy gates still bite. Always check your specific airline — "regional partner" flights on smaller aircraft may have reduced bin space.
European full-service carriers (BA, Lufthansa, Air France) typically allow 55 x 40 x 23 cm carry-ons plus a small personal item. Budget carriers Ryanair and Wizz Air sell priority boarding with larger cabin bags as an add-on — the base fare often limits you to a small bag under the seat only.
Basic economy and budget fare traps
US domestic basic economy on American, Delta, and United includes a full carry-on — a policy change from earlier years when personal-item-only was common. International basic economy on United and some other carriers may still restrict carry-ons to a personal item on transatlantic fares — read fare rules at checkout.
Ultra-low-cost carriers (Spirit, Frontier, Allegiant in the US; Ryanair, Wizz, easyJet in Europe) unbundle carry-ons entirely on the cheapest fares. A $30 add-on at booking beats a $65+ gate fee when the agent tags your bag at boarding. See /guides/is-basic-economy-worth-it for fare-class context.
Personal items — what fits under the seat
Personal item limits are less standardized and less aggressively enforced than roll-aboards, but oversized backpacks get flagged at busy gates. A safe target is roughly 17 x 13 x 8 inches — large enough for a laptop bag or small backpack. Ladies' handbags and camera bags usually pass if not clearly exceeding airline published limits.
Some travellers use a maximal personal item plus a checked bag to avoid carry-on fees on budget airlines. Pack dense and heavy items in the personal item if it meets size rules — weight limits apply on some European budget carriers even for cabin bags.
Enforcement, gate checking, and liquids
Full flights trigger gate-checking of roll-aboards even when your bag is regulation size — there is simply no bin space. Gate-checked bags on US domestic flights are usually free; on budget carriers, a carry-on you paid for should still be free when gate-checked — keep your receipt.
Liquids remain 100 ml per container in a single quart/litre bag through US and EU security. Duty-free liquids in sealed STEB bags are allowed on connecting itineraries in some countries but not all — US connections often require checking duty-free liquids if you clear customs and re-screen.