The answer is: not more than 20% of your body weight, and ideally, it'll be about 10% of your body weight. If you weigh 200 lbs, you'd be fine with carrying 40 lbs for a day hike. If you're 160 lbs, you'd be okay with a ~30 lbs pack.
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A 50L+ backpack is a great friend to those travelling for a few weeks. These bags are spacious and can accommodate a number of outfit changes before you ever have to visit a laundromat. But don't be tempted to fill your pack to its capacity before you've even left - remember to save space for souvenirs!
Trips of 5 days or more usually call for packs of 70 liters or more. These are also usually the preferred choice for winter treks lasting more than 1 night.
A 30L – 50L backpack will be just the ticket. It will be able to hold all your sleeping, cooking, and camping gear you need for trips up to 5 or 6 days.
Size Matters (but Bigger Isn't Better)Like a few other things in life, size matters when it comes to choosing a backpack — but not in the way you might think. Based on many years of dragging excessively large backpacks around the world, I have to say bigger is not better. You do not need an 80 or 90 litre pack.
2-3 Day Packs: 35-50 litersA weekend backpack like a standard 40L backpack is big enough to carry a backpacking sleeping bag, small tent, and pad, as well as extra clothing and layers.
Their capacity lets you carry an extra layer, food and gear for day trips. 21–35 liters: This is the sweet spot for most hiking and travel daypacks. There's enough capacity to hold food, clothing and some extras, like a camera and a book.
Pack Weight for Backpacking and HikingA loaded backpacking pack should not weigh more than about 20 percent of your body weight. (If you weigh 150 pounds, your pack should not exceed 30 pounds for backpacking.) A loaded day hiking pack should not weigh more than about 10 percent of your body weight.
70L is definitely too big. I believe the 55L is a 40L + a 15L daypack. This will probably work, but double check the length, width, and height, and make sure none of those dimensions are larger than allowed for a carry on.