There’s no single “correct” backpack size for covering 25 kilometers in a day — it really comes down to a few key variables: how long you’ll be out, what the weather’s doing, how much gear you’re bringing, and what feels comfortable on your own back.
That said, most hikers find that something in the 30–40 liter range hits the sweet spot for a demanding day hike like this. That’s typically enough room for the basics: water, snacks or meals, a first aid kit, a map or GPS device, and a layer or two for changing conditions.
Planning to camp overnight or cook along the way? Then you’re in different territory. Once you start adding a tent, sleeping bag, stove, and food supplies, a 30–40L pack won’t cut it. For multi-day treks or gear-heavy outings, look toward the 50–70 liter range instead.
One rule holds no matter which size you land on: pack smart, not heavy. Overpacking — even in a bigger bag — puts unnecessary strain on your shoulders and back over the course of a long hike. Stick to what you’ll actually use, and your body will thank you by kilometer 20.