Does anyone know what size a pack can be to still fit within the airline carry on parameters? 50 liter? 30? 60?

FAA allows each carrier to set limits. a quick google search Best to search with your carrier

"United Carry On bag Size. The maximum dimensions for a carry-on bag are 9 inches x 14 inches x 22 inches (22 cm x 35 cm x 56 cm), including handles and wheels."

Delta "Baggage must meet carry on size limits (approximately 56 x 35 x 23 cm). Due to limited overhead space, Delta Air Lines passengers travelling on flights with 50 seats or less are only permitted to carry one personal item on board the aircraft."

A 30 Liter pack would most likely exceed the limits.

Thanks Dan I am need of a new backpack for an upcoming trip I went to your site and see the explorer is just a tad over-sized but might be my best option

"Edited to say bushcrafter not explorer"

This was covered in a recent thread here:

Carry on

I’m a big fan of the rucksack for my carry on bag.

tarnkt: "This was covered in a recent thread here:

Carry on

I’m a big fan of the rucksack for my carry on bag."

Thanks that link was very helpful

Here is the mathematical answer:

Using United's slightly smaller dimensions 22 cm x 35 cm x 56 cm, the volume would be 43,120 cubic cm. 1 cubic cm = 1 mL and it takes 100 mL to make 1 L.

So you could potentially carry on a 43 L bag as long as it wasn't strangely shaped.

straighthairedcurly you are correct. The trick is meeting the shape.
I have seen many items brought on a plane that do not fall within the carriers size limits, or more items than the carriers permit.

I have an Osprey 35 travel backpack specifically designed to be carry-on friendly. It's divided, and has a laptop sleeve and a separate tablet sleeve, so it's not a true rucksack with vast expanse of volume. It easily comes in under the standard carry on dimensions, in fact it comfortably fits under the seat and allows for a carry-on suitcase to be stowed above, if you're on a plane/airline that limits overhead storage to one item per passenger (as they should... and should *enforce* such.)

Backpacks aren't as strictly enforced as rolling suitcases because they are moldable and can be mashed up somewhat to fill a smaller space than a suitcase takes up.

Osprey has some travel backpacks that are 60L and still come in under the standard carry-on limits.

As Newman would say," crimp, fold and cram".

I have used my CCS Pioneer pack and my CCS Ruck.