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For carry-on you usually have to be at 40L or less.
I have the Women’s Deuter Trail 29L and I’ve traveled with mine for years.
The cool thing is I’ve never had to pay for a carry-on because if I had to it would fit under my seat.
It is my backpack for the weekend or two weeks.
I just do laundry if I have to.
rei.com
Deuter Trail 28 SL Pack - Women's | REI Co-op
Offering optimal organization and load stability on technical terrain, the women's Deuter Trail 28 SL pack is made for hikers who want a pack that sits close to the body with plenty of ventilation.