Small backpacks will fit under an airplane seat easily, like children’s backpacks, slim laptop backpacks, ultralight/slim backpacks, and small daypacks.
Check the dimensions on your backpack; if it’s smaller than 18”H, 14”W, and 9”D, it may be able to fit under an airplane seat. Dimensions vary by airline, so check your airline’s website for personal item rules to be sure.