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Southwest is the most backpack-friendly major airline in the US. Their carry-on dimensions are the most generous (61 × 41 × 25 cm), enforcement is virtually nonexistent for reasonably-sized bags, and every ticket class includes both a carry-on and personal item with no extra fees. Add two free checked bags to that, and Southwest removes most of the anxiety that backpack travelers face on other carriers.
The unique wrinkle on Southwest is open seating. There are no assigned seats — passengers board in order (A, B, C groups) and choose their own. This means overhead bin space operates on a first-come basis, and later boarding positions face bin competition on full flights. Your carry-on strategy on Southwest is less about policy compliance and more about boarding position timing.
Last updated: May 2026
Southwest's approach is the simplest of any major US carrier:
Carry-on bag (overhead bin): One bag up to 61 × 41 × 25 cm (24 × 16 × 10 in). These are the most generous published carry-on dimensions among US airlines — 5 cm larger in each dimension than most competitors. A bag that's borderline on Delta or United is comfortably within limits on Southwest.
Personal item (under seat): One item that fits under the seat ahead of you. Southwest doesn't publish specific personal item dimensions — it's based on whether the item physically fits under the seat. In practice, this means anything up to a standard backpack or laptop bag works without issue.
No fare class restrictions: Every Southwest ticket — Wanna Get Away, Anytime, Business Select — includes the same carry-on and personal item allowance. There is no "Basic Economy" carry-on restriction on Southwest. Additionally, Southwest includes two free checked bags on all tickets, which no other major US airline matches.
Open seating impact: Since Southwest doesn't assign seats, overhead bin space is first-come-first-served during boarding. Passengers in the A group (first 60 to board) have guaranteed bin space. B group passengers usually find space. C group (last to board on full flights) may face bin shortages and be asked to gate-check. Your boarding position matters more than your bag size on Southwest.
A 25L backpack is well under Southwest's carry-on limits and can serve as either a personal item (under seat) or a small carry-on (overhead). On Southwest, where the personal item category is more flexible than other carriers, most 25L bags fit under the seat without issue on 737 aircraft. This frees up your carry-on allowance for a second bag if needed — though most travelers won't need that.
The 25L works for quick weekend trips, day-trip flights where you want everything at your feet, or as a supplement to a checked bag. Given Southwest's two free checked bags, some travelers prefer a light 25L personal item plus a checked bag rather than dealing with overhead bins at all.
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A 30L backpack is effortlessly within Southwest's carry-on limits. It fits overhead with room to spare, handles 3–5 day trips with standard packing, and never draws any attention from gate staff. Southwest operates exclusively 737 aircraft with consistent overhead bin sizes across their fleet — so a 30L bag works identically on every Southwest flight you'll take.
This is the go-to size for Southwest travelers who want to skip baggage claim without aggressive packing. Weekend getaways, 3-day work trips, and short domestic vacations all fit comfortably in 30L. Combined with Southwest's relaxed atmosphere, a 30L backpack makes the boarding-to-deplaning experience completely frictionless.
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A 35L backpack fits well within Southwest's generous carry-on dimensions and opens up 5–7 day trips without checking anything. At 35L on Southwest, you have dimensional margin to spare — unlike on other carriers where 35L starts to feel tight. This extra breathing room means bag shape matters less on Southwest; even round-profile hiking packs at 35L typically clear the limits.
For travelers doing a full week on Southwest, 35L gives you enough room for varied clothing (casual + one nice outfit), full toiletries, tech gear, and a light jacket without needing compression packing. The all-737 fleet means consistent bin size on every route.
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A 40L backpack fits comfortably within Southwest's carry-on allowance — no stress, no dimensional games, no enforcement risk. On other airlines, 40L is the maximum practical carry-on that requires careful bag selection. On Southwest, 40L is solidly mid-range relative to their 61 × 41 × 25 cm limits. Even standard-profile 40L bags (not specifically designed for airline compliance) typically pass.
This makes Southwest the best airline for one-bag travelers who use 40L packs. Week-long trips, international connections that start domestic, or any scenario where you want maximum carry-on capacity — Southwest accommodates 40L without the uncertainty you'd face on United, Delta, or especially European carriers.
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A 45L backpack is where other airlines say "check it" — but on Southwest, most 45L travel packs still fit within the published 61 × 41 × 25 cm dimensions. This is Southwest's generous limits paying dividends. A 45L bag that would be borderline-to-over on United or American is squarely within policy on Southwest.
The only consideration at 45L is overhead bin space competition on full flights. A larger bag takes more bin space, and if you're in the C boarding group, bins may already be full. The solution: check in exactly 24 hours before departure to secure an early boarding position, or purchase EarlyBird Check-In for automatic early positioning.
Southwest is the least strict major US airline for carry-on enforcement. Their culture is deliberately low-friction:
No sizer frames: Southwest does not position bag sizers at gates. There are sizers available if a dispute arises, but gate agents don't proactively ask passengers to test their bags. Visual assessment only — and even that is rare.
Focus on count, not size: Southwest gate agents focus more on passengers trying to bring three or four items onboard than on dimensional compliance. If you have one carry-on and one personal item (regardless of size), you'll almost never face any scrutiny. The "one plus one" rule is the enforcement boundary, not precise dimensions.
Full flight bin management: The only carry-on intervention you'll typically see on Southwest happens when bins fill up on packed flights. Gate agents announce that overhead space is limited and ask passengers to gate-check voluntarily. This isn't enforcement — it's a logistics courtesy. Late boarders (C group) may be directed to gate-check, but it's free.
Consistent fleet advantage: Southwest operates exclusively 737s (737-700, 737-800, 737 MAX 8). This means every flight has the same overhead bin configuration. You never face the regional jet surprise that affects other carriers — what fits on one Southwest flight fits on all of them.
Board early, not small: On Southwest, your boarding position matters more than your bag size. Check in exactly 24 hours before departure to get an A-group position. With A1–A30, you have guaranteed bin space for any carry-on size up to 45L. EarlyBird Check-In ($15–25) automates this and positions you in A16–A25 range typically.
When to use personal item only: For very short trips (1–2 nights) or when you want maximum speed through the airport. A personal item under the seat means no bin competition, no gate-check risk, and fastest deplaning. On Southwest, where bins are first-come and not assigned, keeping everything under your seat eliminates all boarding stress.
When two free checked bags changes the math: Southwest's two free checked bags fundamentally alter the carry-on calculus. If you need more than 40L of space, checking one bag on Southwest is free, fast, and removes overhead bin competition entirely. Many experienced Southwest travelers check their main bag and carry just a personal item — the opposite of what's optimal on fee-charging airlines.
Family and group travel: For families, Southwest's free checked bags mean kids' bags can go underneath while parents carry just a diaper bag or personal item onboard. This is dramatically cheaper and easier than the carry-on optimization required on airlines that charge $35+ per checked bag.
Similar size breakdowns and fit calculators for other airlines:
Delta carry-on backpack guide →
United carry-on backpack guide →
American Airlines carry-on backpack guide →