We bought SturdiProducts’ large-sized SturdiBag in the belief that it was just barely acceptable to most airlines. We now know that it’s too big (12Hx18Lx12W). On the bright side, its support struts are very flexible, so it does, in fact, fit under every seat we’ve encountered (and Otto the cat and Chloe the dog have both been fine — the carrier has a lot of headroom).
Also on the bright side, it’s a beautifully-made product, with great features like top ventilation panels (with privacy blinds) and a small top hatch that zips open to allow pats. It’s very light (the Large size is 2 lbs., 10 oz.; the Small size is 3 oz. lighter; and the Cube is just 1 lb., 10 oz.), which helps with the airlines’ weight limits. It also has built-in loops through which a seat belt can be buckled, making it an excellent car carrier.
The Extra Large SturdiBag (16Hx20Lx12W) is far too large for airplane use, but it’s a terrific over-the-shoulder carrier and car carrier for dogs up to the size of a Cocker Spaniel or a Sheltie.
Unusually, the SturdiBag comes with a divider in two sizes (Large and Extra Large), so you can carry two pets together, but each in their own space.
If you have a pet that fits into the small SturdiBag or the SturdiBag Cube (at 10Hx18Lx10W and 10Hx10Lx10W, they’re both pretty close to the major airlines’ maximums), I recommend them unhesitatingly. We love the Large size, and it’s fit under every seat we’ve been in, but it’s enough over the airline’s maximum sizes that you’ll have some anxious moments talking to ticket agents. So far, showing them how flexible its sides are has been effective — they’ve been willing to let the flight crew make the call, and no flight attendant has flickered an eyebrow at it.