Your hiking backpack is already a carry-on

For most people, a regular hiking backpack can absolutely be your carry-on, as long as it compresses down to airline size.

The main thing is dimensions, not volume. A 40–50 liter hiking pack will often squeeze into the 22x14x9 inch limit if you use its compression straps and don’t pack it to the brim. The problem is usually the frame. If your pack has an internal frame or a rigid back panel, check its total length first. You can’t compress titanium stays or a plastic framesheet. If the frame is too tall, it won’t fit in the sizer no matter how much you cinch it down.

Dedicated travel packs are more convenient, not more necessary. They give you a suitcase-style opening, a laptop sleeve that doesn’t bury everything, and a way to hide the hip belt and shoulder straps so they don’t get ripped off in baggage handling. Those are real niceties. But if you’re already hiking with a decent pack and you don’t need to pull out a laptop mid-flight, a regular backpack is fine. Use packing cubes, keep heavy stuff low, and wear it loose when boarding so it sits lower in the overhead bin.

Don’t buy a new bag until the airline actually turns you away.