A common guideline is that a fully loaded backpacking pack, including food, water and fuel, should weigh no more than about 20 percent of your body weight. For a 150-pound hiker, that means about 30 pounds or less. For comparison, a loaded day-hiking pack should stay under about 10 percent of body weight, or about 15 pounds for the same person.
This is a rough ceiling, not a strict rule. Very small backpackers often end up above 20 percent, and longer trips, cold weather and extra comfort gear all add weight. Backpackers often track base weight, which is the pack weight without food, water and fuel; under 20 pounds is generally considered lightweight and under 10 pounds ultralight.
Source: REI Expert Advice, “How Much Should Your Pack Weigh?”
Image: Backpackers in the Mountains, Glacier National Park (NPS photo), public domain, via Wikimedia Commons.