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Multiday (3-5 nights; 50-80 liters) These are the most popular backpacking packs sold at REI and they're an excellent choice for warm-weather trips lasting 3 or more days. 50-80 liter packs are also used for backcountry skiing, for day trips, overnighters and sometimes 2-night trips.

Most daypacks range from 20 liters to 35 liters on the high-end. While a smaller sized backpack is usually sufficient for 1/2 day hikes, you'll want a larger pack in the 35-liter range for all-day hikes so you can carry extra water, food, clothing, and the 10 essentials.

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. 36 50 liters: These larger packs are ideal for trips that require additional clothing and gear, such as climbing, mountaineering or non-summer hiking.

Multiday (3-5 nights; 50-80 liters) These are the most popular backpacking packs sold at REI and they're an excellent choice for warm-weather trips lasting 3 or more days. 50-80 liter packs are also used for backcountry skiing, for day trips, overnighters and sometimes 2-night trips.

A 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.

How much food should you take backpacking? It depends, but the short answer is about 2,500 to 4,500 calories of food per person per day.

A 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.

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. 36 50 liters: These larger packs are ideal for trips that require additional clothing and gear, such as climbing, mountaineering or non-summer hiking.

You should carry 1 liter of water for every 2 hours of backpacking you have ahead of you. Of course, adjust this for your specific situation. Consider your age, the intensity of the hike, your personal sweat rate, body type, duration of the hike, the weather, and the distance.

A good general recommendation is about a half liter of water per hour of moderate activity in moderate temperatures. You may need to increase how much you drink as the temperature and intensity of the activity rise. For example, strenuous hiking in high heat may require that you drink 1 liter of water or more per hour.