The bag has to match a number the airline printed. Aer publishes 21.5 x 13.5 x 9 in. TSA publishes the 3-1-1 liquids tray.
Carry-on only is a geometry brief. The bag has to fit the bin the airline published. The liquids have to fit the tray the checkpoint published. Everything else is taste: who is walking the terminal, how long the trip is, and whether the bag opens like a suitcase or a backpack.
We copy the numbers. We do not invent a “most airlines” average and hope. United’s bags page is the airline carry-on page we cite for a common U.S. domestic story: 22 x 14 x 9 in, including handles and wheels on the typical United template. Confirm the live page for your ticket. Basic Economy and partner-operated flights can change what boards.
Published cabin geometry
The Aer Travel Pack 4 35L is the current Aer carry-on on our list. Aer publishes 35 L, 3.90 lb for the ballistic nylon 35 L, and 21.5 x 13.5 x 9 in (54.5 x 34 x 23 cm). That geometry sits inside the 22 x 14 x 9 in story United prints. Aer also lists a $259 list for the ballistic nylon 35 L, a 16 in suspended laptop sleeve, 1680D Cordura, YKK zippers, and Duraflex hardware. Those are Aer’s bullets.
The Cotopaxi Allpa 35L publishes 22 x 12 x 10 in (56 x 30 x 25 cm) and 35 L. The extra inch in depth is the clamshell. Confirm it against the airline you booked, not against a memory of “carry-on size.”
The Osprey Farpoint 40 publishes 22 x 14 x 9 in (55 x 35 x 23 cm) and 40 L. Osprey’s table is the one we quote. Farpoint is the walker: frame, torso, hipbelt you can stow. Aer is the organizer. Allpa is the drawer.
A solo backpacker on a carry-on flight can pick any of the three from the journey page. The 35 L Aer is the overhead bag on that walk when the laptop is the day. Farpoint 40 is the step if a shell and a knit will not fit 35 L. Allpa 35 is the bag when you want clothes in mesh compartments after the bin.
A business traveler on rail still cares about cabin geometry on the flights that bookend the train. The same published inches apply. A young family that flies one leg treats the adult clothes bag as a carry-on and the stroller as a separate airline rule.
The liquids tray is 3-1-1
The TSA 3-1-1 liquids rule is the official U.S. checkpoint page. Each traveler may bring a quart-sized bag of liquids, aerosols, gels, creams, and pastes. Containers are limited to 3.4 oz (100 ml) or less per item. Larger containers go in checked baggage. Medications and infant nourishments have published exemptions on that same page.
People say “FAA 3-1-1.” The liquids tray at a U.S. airport is TSA’s rule. We cite the TSA page because that is the page that prints 3.4 oz / 100 ml and the quart-sized bag.
The Humangear GoToob+ publishes 50 ml, 75 ml, and 100 ml sizes. The 100 ml tube is the one that matches the tray. The Matador FlatPak Toiletry Bottle publishes a 90 ml / 3 oz class bottle, the number that matters at screening. Cap them as the maker describes. The travel bottles field note is the hub.
Empty the water bottle. The tray is not the place to discover a 32 oz flask still full.
One bag, then a personal item if you need it
Carry-on only can still mean a personal item under the seat: a daypack, a workpack, or the snack bag. The carry-on backpacks field note is the method for the main bag. The daypacks field note is the method for the bag that stays at your feet.
Do not buy a second 35 L pack as the personal item. That is two carry-ons dressed as one trip.
Plan the trip, then pick the inches
Open the Trip Planner and name the traveler and the mode. The solo backpacker carry-on journey is the walk that already maps Aer, Allpa, and Farpoint to a cabin. This note is the geometry chapter. The airline page and the TSA page are the numbers. The bag you can lift into the bin is the one that matches both.
