Backpack that fits under seat with hipbelt

The Cotopaxi Allpa 28 liter has a hip belt. It’s 19 x 12 x 9. So 2 inches too tall but you might get away with it especially if you stow the hip belt and shoulder straps when boarding and carry it like a briefcase. I’ve used it as an under seat bag on US domestic trips. https://www.cotopaxi.com/products/allpa-28l-travel-pack-2?variant=41266075598909

Hip belts may not work on shorter bags because of the vertical dimension of how the bag lays against the back. Look at ecohub 30 liter underseat bag (Amazon) or Patagonia 25 liter black hole bag (Patagonia.com).

It’s really the depth and width of bag that matters when sticking underseat. A slightly longer bag will just encroach a bit on your foot space. Also, how zealous the gate luggage police are.

Check out Tom Bihn backpacks. They have multiple sizes of backpacks. There are usually some on sites like ebay that are slightly used and a little cheaper than full price. I have multiple TB bags (use a backpack every work day) and the ballistic material lasts forever and still looks brand new.

It’s really the depth and width of bag that matters when sticking
underseat. A slightly longer bag will just encroach a bit on your foot
space. Also, how zealous the gate luggage police are.

I guess if its your only carry on then it doesnt matter as long as you can cram it under the seat and as long as you dont have a seat with one of the boxes under the seat in front of you and as long as they dont change the type of aircraft. But if you have a carryon too, then you need to check the personal item size, Aer Lingus for instance is 10 x 13 x 8. Of course, if its your only carryon, you can put it in the overhead.

There is no standard underseat measurment allowance. (I think you are looking at US discount carriers.)

Most international carriers have underseat bag dimensions that are slightly lower.

Have you searched on Amazon? In just a minute, I found this one that is close to your dimensions:

https://a.co/d/9vNSPVO

Frank II, all I could think was that this was their only carryon item, which in theory could be larger than a personal item requirement for the airline (but no larger than the carry on size restriction for the airline) and no one would stop them if they wanted to cram it under the seat. Otherwise, I noticed in another thread they were flying Aer Lingus and the 17 x 13 x 9 is too large for a personal item. No idea where the "standard" underseat size came from. I think they will find that varies with the type of aircraft and maybe with the seat design used by the airline company .... no idea.

i use this one. At 12 x 16 x 7 It works for a lot of the carriers in Europe (including Aer Lingus), but not all. Some its an inch too big (i just dont over pack and it will squish down) and for some its way too big, like for Lufthansa (I have another for them). No hip belt .... its just too small the hip belt would be above my stomach. ANd its $35 (when I bought mine i think it was $24). Still its large enough for wading boots, hip waders, a couple of reels, heavy socks, long johns, and assorted fishing odds and ends. I think it works because it is so square that all the space allowed it in the bag. Going on 10 years old. Love it.

I like yours too. The first one linked is $200 which is out of my price league. But I really, really like it ... if it were smaller.

I take my Osprey daypack that I use for day hiking here at home. It has a hip belt, but not as beefy as my pack I use for overnighting in the mountains. The Osprey does not have an internal frame, so it can squish if you need to. I have been taking this one on hiking trips to Iceland and Alaska and it has worked great.

I think the issue here is the hip belt and hiking use. Osprey and Patagonia have zip-top totes with backpack straps.
Underseat bags can mean 2 things.
1). Supplemental bag
2) Sole piece of luggage around 27 liters.

Thanks everyone! I think I have figured this out. I will be flying Delta to Italy, taking a carry-on rollerbag and a backpack as my personal item to go under the seat in front of me. Delta first said it didn't have an exact size allowance for underseat items, but then came back and said the underseat allowance is 17 x 13 x 9. However, then I found something online that said underseat dimensions for most Delta aircraft are 20 x 15 x 11. For my flight from San Francisco to JFK and then on to Italy, I should be on a larger plane with these underseat dimensions.

Coming home on Aer Lingus, I will check my rollerbag, and my backpack will become my carryon, to go in the overhead bin. Aer Lingus' carryon size restriction is 21.5 in x 15.5 in x 9.5. (Aer Lingus' personal item size restriction is a shockingly small 10 x 13 x 8).

So as long as I have a daypack that's 20 x 15 x 9.5, I should be set. That gives me many more options of finding one with a hipbelt.
Special thanks to Frank II, who succeeded in finding the exact item I was first asking about!!!

Both my husband and I were looking for a new backpack for our upcoming trip to the Netherlands and Belgium where we will be doing some hiking. We both have used an Eddie Bauer 20L one in the past, but it did not have a hip belt, and after awhile it just became too uncomfortable on the back and shoulders. So he did some research and found 2 that fit the bill. Both fit into your dimension requirements and both are hydration pack ready if you want to use that feature. He is going to use this as his carry on.
He got the Gregory Miko 20L at REI and it was comfortable from the get go. Really helped transfer the weight to his hips. The hip belt has some decent sized pockets which would be pretty useful too. The inside support is somewhat stiff, which helps in the load balancing, but it definitely cannot be folded. These wider hip belts are much better than just a strap.
https://www.rei.com/product/218012/gregory-miko-20-pack-mens
The second one he liked a lot was the Patagonia Terravia Daypack 22L. It comes with a rain cover included and is on sale at Patagonia now for a good price. It does come in a S/M/L size. It is not as stiff and is somewhat foldable when empty. The hip belt pockets are a bit smaller than the Gregory.
https://www.patagonia.com/product/terravia-daypack-22-liters/196924364981.html?s_kwcid=17928&utm_source=google&utm_medium=cpc&utm_campaign=BB_Ecomm_Shopping_ALL_WBSP_SaleKWs&gad_source=1&gclid=Cj0KCQiAkoe9BhDYARIsAH85cDN2Ek2A4Ui3iW7C_LJjq9L1x5mKvAu3_XWDrHLwf1eVV_uN8ncoOaIaAqsKEALw_wcB
I ended up with the REI Flash backpack, which is incredibly lightweight and foldable. https://www.rei.com/product/227953/rei-co-op-flash-22-pack

You can check out the Daylite series of Osprey. There are several backpacks in this series that meet the airlines' underseat size requirements of 17” x 13” x 9” and come with a hipbelt. The carrying system is designed quite reasonably, which can relieve a lot of shoulder pressure when you carry it for a long time during hiking.
Also, the Speed Lite series of Deuter is worth considering. Some models in this series can also meet the size specifications you mentioned. The quality of the hipbelt is quite good, which can fix the backpack well and make it more convenient for you to move around. Meanwhile, the internal space is planned very practically, making it easy for you to pack various items needed for hiking.
In addition, some small backpacks of Gregory, such as individual models in the Zulu series, are also worthy of your attention. They can not only fit under the seat but also have a hipbelt with high comfort. It can help distribute the weight during the hiking process and is very suitable for taking on a trip for hiking.

My Mountain Smith lumbar pack has a hip belt and chest belt. It fits under the seat easily. But it isn't as large as some of the regular packs. I took it to Europe on two different hiking trips.