The hiking backpack for a day between 20 and 30 liters must contain: food, water, waterproof clothing, warm technical clothing in case of sudden weather changes, a small kit help for you or if you meet someone who needs it, a knife, your sunglasses, a compass, a whistle... The 3 or 4 day hiking backpack between 40 and 50 liters must contain the same thing plus the tent, the sleeping bag, a change of clothes, stove, cup, hat or cap, socks, survival kit. Avoid superfluous priority must be given to water and food. For more than a week choose a bag of 60 to 70 liters or even more if you need to be completely autonomous in remote places.