The personal item is your one universal free bag
Across virtually every commercial airline in the world, your ticket includes one bag that goes in the cabin for free: the personal item. It fits under the seat in front of you, and it's the airline's polite acknowledgement that you might want to bring a wallet, a laptop, or a phone charger without paying extra.
Everything else costs money. Cabin bags (the kind that go in the overhead bin) are extra on every basic-economy fare on every US legacy carrier and on every fare on every ultra-low-cost carrier. Checked bags are extra on most domestic flights and most basic-economy international fares. The personal item is the universal floor.
Which makes it strangely important to get the dimensions right. A personal item one centimetre too tall is a $50–$100 gate-checked bag on Spirit. A personal item that doesn't fit Ryanair's specific frame at the gate is €25 minimum, often €60. The size of your under-seat bag is the difference between a cheap flight and a not-cheap flight.
US ultra-low-cost carriers
The strictest enforcers — and the airlines where personal item math matters most.
Spirit Airlines: 18 × 14 × 8 inches (45 × 35 × 20 cm). Strictly enforced via the gate sizer frame. Anything that doesn't fit is gate-checked at $79–$99 depending on the route. Wheels and handles count toward the dimensions.
Frontier Airlines: 18 × 14 × 8 inches (45 × 35 × 20 cm). Same dimensions as Spirit. Frontier added free personal item enforcement in 2018 after years of complaints; the sizer is now standard at most US gates Frontier serves.
Allegiant Air: 16 × 15 × 7 inches (40 × 38 × 17 cm). Slightly smaller than Spirit/Frontier, particularly the depth.
European low-cost carriers
The European budget airlines pioneered ultra-strict personal item enforcement and remain the toughest in the world to comply with.
Ryanair: 40 × 25 × 20 cm. Genuinely small — a typical laptop bag at the absolute upper limit. The gate sizer is rigid metal and Ryanair's gate agents check almost every passenger. Non-compliant bags cost €25–€60 at the gate, and on busy flights the agent will check them whether you protest or not.
easyJet: 45 × 36 × 20 cm. More generous than Ryanair, and easyJet enforces less aggressively at the gate, but the rule is still on the books.
Wizz Air: 40 × 30 × 20 cm. Slightly larger than Ryanair on width but tighter on depth. Enforced at the gate with a sizer.
Vueling: 40 × 20 × 30 cm. Similar to Wizz, with enforcement varying by airport.
US legacy carriers (in basic economy and standard economy)
The big three publish official dimensions but enforce loosely on standard economy and more strictly on Basic Economy where the cabin bag is excluded.
American Airlines: 18 × 14 × 8 inches. Same as Spirit/Frontier on paper. Enforcement on Main Cabin fares is light; on Basic Economy where the cabin bag is excluded, the gate may check.
Delta Air Lines: No officially published dimensions — Delta states "must fit under the seat in front of you." Enforcement is the loosest of the big three. In practice, Spirit-sized personal items pass without issue on Delta.
United Airlines: 17 × 10 × 9 inches (43 × 25 × 22 cm). Notably shallower than American, particularly on the second dimension. Basic Economy passengers occasionally get flagged for under-seat bags that easily fit on American.
JetBlue: 17 × 13 × 8 inches. Generous compared to legacy carriers and rarely enforced strictly.
Southwest: 18.5 × 8.5 × 13.5 inches. Generous, and Southwest's overall baggage policy (two free checked bags) means cabin enforcement is the lightest of any US carrier.
Alaska Airlines: Same as JetBlue — 17 × 13 × 8 inches. Loose enforcement.
European legacy and full-service carriers
European full-service carriers typically distinguish between "handbag" (personal item) and "cabin bag" (overhead). The personal item dimensions are smaller than their US equivalents.
British Airways: 40 × 30 × 15 cm. The 15 cm depth is the binding constraint — most US-sized backpacks are too deep.
Lufthansa: 40 × 30 × 10 cm. The shallowest official dimension on this list. A typical laptop fits, a typical backpack does not.
Air France / KLM: 40 × 30 × 15 cm. Same as British Airways.
Iberia: 40 × 30 × 15 cm. Aligned with the IAG group standard.
Aer Lingus: 33 × 25 × 20 cm in some fare classes — notably tighter, particularly on length. Their "Saver" fare excludes overhead access entirely.
Asian and Middle Eastern carriers
Most Asian carriers are more generous than European or US carriers on the personal item, partly because their cabin bag allowances are usually generous too.
Emirates: 40 × 30 × 20 cm for the laptop bag/handbag — and the cabin bag is also generous (55 × 38 × 20 cm), so personal item enforcement is rarely an issue.
Singapore Airlines: 40 × 30 × 15 cm. Cabin bag included separately.
Qatar Airways: 40 × 30 × 20 cm. Generous, and enforcement is light.
Cathay Pacific: 36 × 30 × 23 cm. The depth of 23 cm is generous compared to European carriers.
Air India / IndiGo: Indian carriers generally permit one handbag of 40 × 30 × 15 cm and one personal item of 30 × 20 × 10 cm. IndiGo is the strictest enforcer at the gate among Indian carriers.
How to actually pick a personal item bag
The pragmatic rule: buy a bag designed for the strictest airline you might fly. For a US traveller who might end up on Spirit, that's 18 × 14 × 8 inches. For a European traveller who might end up on Ryanair, that's 40 × 25 × 20 cm. Bags larger than these will fail at the gate on the strictest carrier; bags smaller than these will fit everywhere.
Look for bags marketed as "personal item maximizers" or "under-seat bags". Brands like Tortuga, Pakt, Wandrd, and Nomatic make backpacks specifically dimensioned to the Spirit/Frontier limit. They look small but pack surprising volume — a typical 20L personal-item bag holds 4–5 days of clothing if you pack carefully.
Avoid backpacks with prominent external pockets, expandable sections, or side handles that protrude when fully packed. The gate sizer measures the bag at its widest possible state, including all protrusions. A bag that's 18 inches measured flat might be 19 inches once stuffed — that's a $99 surprise on Spirit.
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