The Journey bag will fit in most places most of the time. The soft sides allow you to get away with a lot. Plus, you shouldn't pack it super full, so it will squish if needed. AND, of course, a key feature is to take the day pack off and take it with you to your seat and leave the travel pack in the overhead bin.
Still, we wanted to give you a list of airlines and their 'acceptance' guidelines. Here's a batch of the major ones:
The airline does not release exact measurements for the space below seats, but to make sure your items fit, aim for less than 11” tall by 17” wide by 18-20” long. The maximum size for carry-on luggage is 24 inches (including wheels and handles) by 17 inches by 10 inches. The seat width is approximately 17 inches.
According to airline policy, any items to be stowed beneath seats should not exceed 18 inches by 14 inches by 8 inches.
China Eastern Airlines
Each piece of hand baggage must not exceed: Maximum weight – 10 kg. Maximum size: Domestic flights – 55 x 40 x 20 cm (21.6 x 15.8 x 7.9 inches).
Delta has a maximum carry-on size of 22 inches by 14 inches by 9 inches. No bag may exceed 45 linear inches combined. The space underneath the seat is 20 inches wide, 17 inches long and 11 inches tall. (Expect less space in the window seat.)
Cannot exceed 115 cm/45 ins and which does not weigh more than 7 kg/15 lbs. REMINDER: Bulkhead seats do not have underseat storage during take-off and landing.
Under-the-seat baggage can not exceed 17 inches by 13 inches by 8 inches on all JetBlue flights. The airline's fleet is made up of Airbus A321s, which is why the size restriction is uniform.
Economy Class on European and intercontinental flights:
1 item of hand baggage, max. 55 x 35 x 25 cm (21,5 x 13,5 x 10 inch), 1 accessory, e.g. a handbag, briefcase or laptop, max. 40 x 30 x 15 cm (16 x 12 x 6 inch). Total weight max. 12 kg (26 lbs).
carry-on baggage size is 55 x 40 x 23 cm/ 21 x 15 x 9 inches; or 57 x 54 x 15 cm/ 22 x 21 x 5.9 inches for foldable garment bags. The permitted dimensions for the personal item are max. 40 x 30 x 10 cm/ 15 x 11 x 3.9 inches.
Carry-ons must fit under the seat, and not exceed 1 x 45in in total, or 22in + 9in + 14in
The maximum size for a personal item on United is 9” by 10” by 17”. But if you forgo a larger carry-on, you can bring a larger item to stow under the seat, which is 17” deep, about 10” tall, and 20” wide (on a 757).
No personal item can exceed 36 linear inches, or 17 inches by 10 inches by 9 inches. Carry-ons cannot be larger than 50 linear inches, 24 inches by 16 inches by 10 inches. According to Dog Jaunt, the airline's A320s have a fixed under-seat height of 8.5” (which isn't much), and the aisle and middle seat are 18” wide. The window is 17” wide.
Southwest’s Boeing 737s have an underseat space of 19” long by 14” wide by 8.25” high. Middle seats are the same, except for a 19” width.