Yes, a backpacking backpack can be a personal item—but only if it fits the sizer.

You can absolutely use a backpacking backpack as a personal item on a budget airline if you don’t overpack it, but the only number that matters is whether the actual pack fits in the airline’s sizer, not the liter volume.

Budget airlines give you a tiny box for a free personal item—typically around 40 x 30 x 20 cm (Ryanair) or 45 x 36 x 20 cm (easyJet, Spirit, Frontier). A 40-liter pack might sound small, but “liters” measure volume, not height. The problem is usually the frame. True backpacking packs have metal stays or a plastic framesheet that won’t bend, and they’re often 70–80 cm tall before you even cinch anything down. That won’t fit in a 40 cm slot no matter how empty the bag is.

So check the pack’s external dimensions, especially height, not its capacity. If your pack has a removable frame or is soft-walled with no internal frame, you can compress it down significantly. But if it’s got a proper suspension system, it’s probably going to be too tall, even when loose. Also watch for the hip belt and top lid—they add bulk that gets caught in the sizer.

My advice: before you fly, measure the empty pack at its most compressed. If it’s close, you can get away with it by packing light and tamping it into the box at the gate. Just be ready to pay the fee if it doesn’t.

The sizer is the only judge that matters.