To measure the size of a backpack in liters, you can use the following formula:
Length x Width x Height = Volume (in cubic centimeters)
Divide the volume in cubic centimeters by 1000 to get the volume in liters.
For example, if a backpack is 18 inches long, 12 inches wide, and 6 inches high, its volume would be 216 cubic inches. To convert this to liters, you would divide by 61.024
216 cubic inches / 61.024 = 3.54 liters
This means that the backpack has a capacity of 3.54 liters.
Here are some tips for measuring the size of a backpack in liters:
Use a tape measure to measure the length, width, and height of the backpack.
Measure the backpack in its fully packed state. This will give you the most accurate measurement of its capacity.
If the backpack has external pockets, be sure to measure those as well.
Some backpacks have a volume listed on their product label. If this is the case, you can use this number instead of measuring the backpack yourself.
Here is a table of common backpack sizes in liters:
| Size | Capacity (liters) |
| Small | 10-20 |
| Medium | 20-30 |
| Large | 30-40 |
| Extra large | 40-50 |
The size of the backpack that you need will depend on your specific needs. For example, if you are only carrying a few items, such as a laptop and a book, then a small backpack will be sufficient. However, if you are carrying a lot of gear, such as hiking or camping equipment, then you will need a larger backpack.