The answer is yes on almost every airline, and the reason is simple: no major carrier’s baggage policy says anything about what kind of bag may fill each slot. The standard allowance is one carry-on plus one personal item, described by dimensions. A backpack, a roller, a duffel and a tote are all equally eligible for either slot as long as they fit it. Two backpacks is a completely normal setup — it is, in fact, how most one-bag travelers fly.
What trips people up is not the rule, it is the shape. These allowances were written around briefcases, laptop bags and handbags, which are flat. A backpack is deep, and depth is the dimension the second slot has least of. So the question worth answering is not “am I allowed” but “how big can the second one be”, and that has a numeric answer per airline.
One thing to get out of the way first: TSA does not set any of this. TSA states plainly that it does not impose carry-on size restrictions — the airline does. TSA screens the bag; the carrier decides whether it flies in the cabin.
The published numbers, airline by airline
Every figure below is from the carrier’s own baggage page, cited and dated at the foot of this page. The second column is the one that decides your second backpack.
| Airline | Carry-on | Personal item | Two backpacks? |
|---|
| American | 22 x 14 x 9 in | 18 x 14 x 8 in | Yes |
| Delta | 22 x 14 x 9 in (45 linear in) | Must fit under the seat | Yes |
| United | 22 x 14 x 9 in | 17 x 10 x 9 in | Yes — tightest personal item here |
| Southwest | 24 x 16 x 10 in | 18.5 x 8.5 x 13.5 in | Yes — most generous carry-on |
| JetBlue | 22 x 14 x 9 in | 17 x 13 x 8 in | Yes |
| Alaska | 22 x 14 x 9 in incl. wheels and handles | Must fit under the seat | Yes |
| Spirit | 22 x 18 x 10 in (paid) | 18 x 14 x 8 in | Only if you pay for the carry-on |
| Frontier | 24 x 16 x 10 in (paid), 35 lb | 18 x 14 x 8 in, 35 lb | Only if you pay for the carry-on |
| Lufthansa | 55 x 40 x 23 cm, max 8 kg | 40 x 30 x 15 cm | No on Economy Basic (no carry-on included) |
| Emirates | One cabin bag, max 7 kg, 115 cm total | One small personal item | Yes, but the 7 kg cap governs |
Read the table for two things rather than one. The size column tells you whether the bag fits; the weightfigures in the Frontier, Lufthansa and Emirates rows tell you something the US majors never test. Two backpacks can clear every dimension and still break a 7 kg cabin allowance — see carry-on weight limits for the full set.
Where the plan actually breaks: the fare, not the airline
Nobody gets stopped for carrying two backpacks. People get stopped for carrying two bags on a fare that includes one. That is the whole risk, and it concentrates in three places:
- US budget carriers. Spirit and Frontier include a personal item of 18 x 14 x 8 in and sell the carry-on separately. Your 40L pack is the paid bag; your daypack is the free one.
- European basic fares.Lufthansa’s Economy Basic excludes the carry-on entirely and allows a 40 x 30 x 15 cm personal item only. A travel backpack does not fit that box.
- Full flights and gate checks. On a full aircraft, the overhead space runs out and gate agents start checking bags. A second backpack that lives under your feet is immune to this; one you were planning to put in the bin is not.
Which allowance your bag falls under is its own question, and the one most people get wrong. Personal item vs carry-on works through it, and the maintained table is in the carry-on size guide.
What to look for in the second backpack
Everything above is the rules. This is the part that decides whether the plan is pleasant. The second bag has one job — live in the space in front of your feet and hold what you want during the flight — and four things make it good at that job:
- Roughly 20 liters, and soft.Twenty liters is a real day bag and it fits under every seat we have figures for. The softness matters more than the number: a soft bag compresses into an awkward footwell, a structured one does not. The tightest published box here is United’s 17 x 10 x 9 in, which a soft 20L clears and a structured 20L may not.
- Low empty weight. On a route with a 7 kg cabin cap, every ounce of second bag is an ounce of clothes you cannot carry. A packable daypack at 2.5 oz costs you nothing; a padded laptop pack at 2 lb costs you a shirt and a pair of trousers.
- It should pack flat on the outbound leg. The best version of a two-backpack setup is one bag out and two bags home, with the daypack folded inside the main pack until you need it. That is what packables are for.
- Front-facing comfort. You will wear it on your chest in transit, both because that is how you keep it secure in a crowd and because it is how you carry two bags through an airport. Thin webbing straps bite when the bag is loaded.
Three second bags that work
These three cover the three sensible versions of the second slot — the one that does a real day out, the one that weighs nothing, and the one that costs nothing. Prices come from the live price layer with the date they were read.
#1Best second bag overall: 26L, expandable to 32L, Osprey's guarantee
Osprey Daylite Expandable 26+6 Travel Pack
The daypack that does a second job. 26 liters is a genuine day bag — city walking, a laptop, a jacket — and the expansion adds six more when the flight home has to swallow what the trip accumulated. It's the one pack here we'd happily use as a personal item under the seat and as the day bag once you land.
Strengths
- +26L handles a real day out, not just a water bottle and a wallet
- +The +6L expansion is there for the trip home, then zips away
- +Osprey's All Mighty Guarantee, which almost nothing else at daypack money carries
Trade-offs
- −Expanded, it stops being reliably personal-item sized — zip it down to fly
- −Costs several times what a packable stuff-sack daypack does
| Capacity | 26 L |
|---|
| Weight (empty) | Not published |
|---|
| External dimensions | Not published |
|---|
| Carry-on fit (our read) | Convertible / expandable |
|---|
| Laptop sleeve | Not published |
|---|
| Material | Not published |
|---|
| Warranty | Osprey All Mighty Guarantee |
|---|
Fit note. The '26+6' is the whole spec. Fly it at 26L, where a soft pack of that size sits inside most personal-item allowances, and treat the extra 6L as an at-destination feature — expanding it at the gate is exactly when you least want to be measured.
Specs read from the product listing, on August 11, 2026. “Not published” means the maker does not state that figure.
#2Lightest: 20L at 2.5 oz, packs to about the size of a lemon
Sea to Summit Ultra-Sil Ultralight Day Pack 20L
Twenty liters that weigh two and a half ounces and stuff into their own pocket at roughly the size of a lemon. This is the second bag you carry because you can't feel it — a beach, a market, a day hike — and it is emphatically not a bag to carry a laptop or a heavy load in.
Strengths
- +2.5 oz and packs to about 2 x 4 x 7 inches — it disappears in a main bag
- +20L is real capacity, not a token amount
- +Simple, durable siliconized nylon with nothing to break
Trade-offs
- −No padding, no frame, no laptop sleeve — thin straps bite under weight
- −Water-resistant only: unsealed seams and an exposed zipper let water in
| Capacity | 20 L |
|---|
| Weight (empty) | 0.16 lb |
|---|
| External dimensions | Not published |
|---|
| Carry-on fit (our read) | Personal-item sized |
|---|
| Laptop sleeve | Not published |
|---|
| Material | Ultra-Sil siliconized nylon |
|---|
| Warranty | Not published |
|---|
Fit note. Sea to Summit publishes the numbers that matter for a packable: 20L capacity, 2.5 oz (72 g) empty, and a packed size of about 2 x 4 x 7 in. They also state plainly that the fabric is waterproof but the pack is not — the seams aren't sealed and the zipper isn't covered. That honesty is why it's here.
Specs read from the product listing, on August 11, 2026. “Not published” means the maker does not state that figure.
#3Cheapest sensible packable: 20L, folds into its own pocket
ZOMAKE Ultra Lightweight Packable Backpack 20L
The cheapest sensible way to own a second bag. It folds into its own pocket, it's water-resistant, and it costs about what a hotel breakfast does — which is the honest argument for it. Build quality is what that price implies: fine for a few trips a year, not a bag to trust with anything heavy.
Strengths
- +Packs into its own pocket and weighs almost nothing in a main bag
- +20L stated capacity — the same as packables costing three times more
- +Water-resistant fabric handles a shower on the walk back
Trade-offs
- −Thin unpadded straps and no frame — 20 lb in it is genuinely uncomfortable
- −No published weight or dimensions, and no warranty to speak of
| Capacity | 20 L |
|---|
| Weight (empty) | Not published |
|---|
| External dimensions | Not published |
|---|
| Carry-on fit (our read) | Personal-item sized |
|---|
| Laptop sleeve | Not published |
|---|
| Material | Water-resistant nylon |
|---|
| Warranty | Not published |
|---|
Fit note. Capacity is stated, weight is not — which is the usual pattern at this price and worth knowing before you compare it to a Sea to Summit on paper. Packables live or die on strap comfort under load, and unpadded webbing is the corner cut here.
Specs read from the product listing, on August 11, 2026. “Not published” means the maker does not state that figure.
The Daylite Expandable 26+6 is the one to buy if the second bag is doing real work — a day in a city, a laptop, a jacket — and the +6 liters exist for the journey home. Fly it at 26L and zip it down at the gate; expanded, it stops being reliably personal-item sized, which is exactly when you least want to be measured. The Sea to Summit Ultra-Sil is the opposite philosophy: 20 real liters for 2.5 oz, stuffed into its own pocket, carried because you cannot feel it. Sea to Summit is also honest that the fabric is waterproof while the pack is not, because the seams are unsealed. The ZOMAKE is the cheap version of the same idea and the straps are where the money was saved — fine for a market and a beach, uncomfortable with 20 lb in it.
If the second bag needs to be secure rather than light, a 20L anti-theft daypack is the other good answer, and it has the useful property of being the bag that stays at your feet all flight. Those are on best anti-theft travel backpacks.
The setup, in one paragraph
A 35 to 40L travel backpack in the overhead bin, a soft 20L daypack under the seat, the laptop and the documents in the small one, and nothing you need in flight in the big one. That clears every allowance in the table above on a standard fare, it survives a gate check of the larger bag, and it means you are never the person unpacking an overhead bin during boarding. If your fare includes only a personal item, invert it: everything in the 20L, and pay for the carry-on only when the trip needs it.
Check your own airline and fare before you fly — these numbers are published by the carriers and they change. The dated sources are below, and the maintained table lives on the carry-on size guide.