First of all, you have to decide how big to buy. The capacity of the bag generally ranges from 40-120 liters. For the average traveler, a 65-75 liter bag is suitable for most long-distance travel. If you need to camp in the wild for a long time, or when you go out often like moving, you may need 120 liters, and the average person will not need more than 100 liters of bags. Moreover, in theory, each person's back weight should be at most one-third of his own weight. When you have too many things, you only know how much you have when you back. So, packlightly.
If you are just taking part in a general urban tour, a one-day hike, or a one-day suburban climbing event, you don't need a tent sleeping bag or something. Buying a day backpack is enough. The capacity ranges from 10 liters to 40 liters, and the gap between brands is small. These bags are similar to the shoulders used by students, and the multi-functional bag also has a belt. When you buy, pay attention to the work of the shoulder strap. The zipper is preferably a coarse-toothed YKK.
In any case, it is best to bring your usual equipment to the store to try when buying a bag. Be sure to be picky, unsuitable and uncomfortable.
Before choosing a bag, you should ask your family and friends to help you measure your torsolength with your ruler (preferably with a soft tape measure for tailoring). This torso length refers to the distance from your seventh vertebra (the most prominent bone at the junction of your neck and shoulders) to the vertebrae parallel to your tibia. The length of this torso is the same as your inner frame. Don't think that you should carry a large bag of 1 meter. Some people have long legs and short legs, and some people have short legs.
In general, if your torso is less than 45 cm in length, you should buy a small bag. If your torso is between 45-52 cm long, you should choose a medium bag. If your torso is more than 52 cm in length, you should pick a large bag.
Good manufacturers offer extra large (56 cm or more) or extra small (40 cm or less) bags, and can also be tailor-made. The sizes of the major manufacturers vary, and even the same manufacturer's different series of package sizes are not the same. You'd better ask, or find some information, just don't measure the length of the inner frame. In addition to the inner frame mentioned above, the carrying system also has a shoulder belt (ShoulderBelt), a belt (HipBelt), and a chest strap (ChestBelt).
Good manufacturers and dealers can change the shoulder strap and belt according to your body shape. In addition, because of the different shapes of men and women, good manufacturers have designed some styles of backpacks for girls. The carrying system of these bags is specially made according to the physical characteristics of women.
The size of each manufacturer is different. Generally speaking, the size of the belt is as follows:
If your waist circumference (around the cheekbones) is below 68 cm, choose a small trumpet.
If your waist is 68-78 cm, choose the small one.
If your waist is 78-92 cm, check the number.
If your waist is 92-106 cm, choose a large size.
If your waist is more than 106 cm, choose extra large.
As for the shoulder strap, there is no gap between the adjustment and your shoulder blade (except for the hunchback). A good bag can adjust the weight distribution on the shoulders at will, you can adjust it according to your physical strength and environment.
In short, the good carrying system is properly adjusted, and most of the weight (more than 60%) can be transferred to the ankle and hips, so that your shoulders will be much easier and more flexible. You will probably have enough energy to enjoy nature by then.