Is a 40L backpack too large for Southwest Airlines carry-on?

A 40L backpack is not too large for Southwest Airlines by volume. Southwest's carry-on limit is 24 × 16 × 10 inches — a standard 40L bag fits within that box. The limiting factor is how much gear is inside. On a 7-day standard trip in mild weather, packed volume pushes past raw capacity — compression cubes help resolve it.

Does Southwest Airlines charge extra for 40L carry-on bags?

No. Southwest Airlines does not charge for carry-on bags. Every passenger gets one carry-on in the overhead bin and one personal item under the seat at no charge, on all bookings. A 40L backpack qualifies as a carry-on on Southwest as long as it fits the 24 × 16 × 10-inch dimension limit.

Southwest Airlines Carry-On Rules — What Actually Fits

Will a 40L Backpack Fit on Southwest Airlines?

Q: Will a 40L backpack fit on Southwest Airlines?

A 40L backpack fits within Southwest Airlines' carry-on limit of 24 × 16 × 10 inches. Southwest's box is larger than the 22 × 14 × 9-inch standard used by American, Delta, and United. At 40L, the question is not whether the bag fits the box — it's whether your trip fits the bag. On a 7-day standard trip in mild weather, packed volume pushes past raw capacity — compression cubes can resolve the shortfall.

A 40L backpack fits within Southwest's 24 × 16 × 10-inch carry-on box — but on a 7-day standard trip in mild weather, packed volume pushes past raw capacity.

Compression cubes can resolve the shortfall. The bag fits the Southwest carry-on box. Whether your load fits the bag depends on trip length, packing style, and what gear you bring.

Placement: Carry-On Compression Helps

Last updated: March 2026

Southwest Airlines allows carry-on bags up to 24 × 16 × 10 inches. A 40L backpack fits that limit. On a 7-day standard trip in mild weather, packed volume pushes past raw capacity — compression cubes can resolve the shortfall. Shorter or lighter trips fit more comfortably.

Check if this will actually fit your trip →

Will This Bag Actually Work for Your Trip?

Your result depends on what you pack, not just the bag size.

Trip Setup

Trip Length

Climate

Packing Profile

Laundry Access

Gear & Footwear

Gear

Laptop Bulky Layer

Extra Packed Shoes

Bag & Airline

Bag Type

Bag Size

What do these sizes mean?

Under 25L — Small personal item / daypack
30–35L — Light travel, short trips
35–45L — Standard carry-on range
45L+ — Large carry-on or checked territory

Airline

Add personal item (up to ~25L)

Use this if you plan to bring a second under-seat item like a daypack, tote, or laptop bag.

Traveler

Clothing Size

Based on real clothing volumes and packing behavior

Packing Reality Check

When Does a 40L Bag Work on Southwest Airlines — and When Does It Need Help?

Works Well

1–7 day trips with standard or light packing
Warm or mild climate
No laptop or minimal tech
Laundry available mid-trip
With compression cubes for 7-day standard loads

Watch For

7-day standard trips — volume pushes past raw capacity, compression needed
No access to laundry on longer trips
Large clothing sizes plus tech gear
10-day or longer trips — volume exceeds compression ceiling

Published Carry-On Limits

Southwest Airlines Carry-On Rules

Max dimensions	24 × 16 × 10 inches (61.0 × 40.6 × 25.4 cm)
Weight limit	No official limit on carry-on bags
Personal item	Yes — one personal item allowed (under seat)
Carry-on fee	Included on all bookings; no additional fee
Checked bags	First two checked bags fly free on Southwest
Fit at 40L	A 40L bag fits Southwest's carry-on box — on a 7-day standard trip, volume pushes past raw capacity; compression cubes resolve it

A 40L backpack fits within Southwest Airlines' published carry-on dimensions of 24 × 16 × 10 inches. At 40L, the carry-on box is not the constraint — packed volume is. On a 7-day standard trip in mild weather, packed volume pushes past raw capacity. Compression cubes bring that within the bag's effective range.

Gear

Example Bags for This Setup

Example bags that match this setup based on real packing capacity and airline limits.

Bottom Line

A 40L bag fits within Southwest's 24 × 16 × 10-inch carry-on limit
On a 7-day standard trip in mild weather, packed volume pushes past raw capacity — compression cubes resolve it
Southwest includes carry-on at no charge — no fee risk if your bag just barely clears the box
Use the calculator above to check your trip length, climate, and gear

Other bag sizes on Southwest

Will a 25L backpack fit on Southwest? →

Will a 30L backpack fit on Southwest? →

Will a 35L backpack fit on Southwest? →

Will a 45L backpack fit on Southwest? →

Same size on other airlines

Will a 40L backpack fit on Delta? →

Will a 40L backpack fit on United? →

Will a 40L backpack fit on American? →

Will a 40L backpack fit on Ryanair? →

Will a 40L backpack fit on EasyJet? →

Packing guides

What to pack for a 3-day trip →

What to pack for a 5-day trip →

What to pack for a 7-day trip →

What to pack for a 10-day trip →

Not sure if it'll all fit? Try the packing calculator →

Notes

How Accurate Is This?

This tool reflects real-world packing conditions, not just theoretical bag sizes. Results are based on typical clothing volumes, packing efficiency, and common travel setups.

Published airline limits specify maximum external bag dimensions. A 40L backpack fits within Southwest's carry-on range. At 40L, Southwest's larger box is rarely the constraint — packed volume on longer trips is. Compression cubes can resolve loads that push moderately past raw capacity.

This is a planning tool, not a guarantee. Airline staff make the final call — packed shape, bag rigidity, and gate-day enforcement all play a role.