Consider the number of days of travelling and the type of activities: first, determine the number of days you are travelling and the type of main activities you will be doing. If you're only travelling for one or two days for short trips or light outdoor activities such as hiking, picnics, etc., a smaller capacity rucksack may be sufficient. Generally speaking, a 20-30 litre backpack can meet these needs. For longer multi-day trips or activities that require you to carry more gear, such as camping, hiking, etc., you may need a larger capacity backpack, usually 40 litres or more.

Make a list of what you need: Before choosing the capacity of your rucksack, make a list of all the items you plan to take with you, including clothing, shoes, sleeping bag, tent, cooking utensils, food, water, etc. Make sure you take into account the volume and weight of each item. This will help you estimate the required backpacking capacity more accurately.

Consider personal comfort and carrying capacity: In addition to the volume of items, consider your personal comfort and carrying capacity. A backpack that is too large or heavy may put an extra burden on your body, leading to fatigue and discomfort. Make sure you choose a backpack that you can carry comfortably and that doesn't exceed your physical strength and capacity.

Leave some space: even if you have listed all your belongings, leave some space for possible extras or souvenirs. In addition, leaving some space will make the items in your backpack more organised and easier to find and access.

Try out backpacks of different capacities: Before making a purchase, try out backpacks of different capacities to get a feel for their weight and comfort. Some outdoor equipment shops offer backpack trial services, so you can take this opportunity to find the best backpack capacity for you.

Consider the function and design of the backpack: Apart from the capacity, you should also consider the function and design of the backpack. For example, some backpacks have special compartments for large items such as sleeping bags and tents, while some focus more on comfort and the design of the backpacking system.

Adjust to the conditions: Finally, adjust to the actual conditions. If you anticipate bad weather or need to carry more warm clothing, you may need to increase the capacity of your pack. Similarly, if you plan to travel light and reduce unnecessary items, you may opt for a smaller capacity backpack.