Measuring the volume of a backpack is actually very easy, no calculation or complicated formula is needed.
Some people would use the Volume = Length x Width x Height formula, but that is actually incorrect. It is because this formula is for measuring the volume of a box. However, most backpacks are not in the shape of a box. Instead, backpacks usually come in irregular shape. More importantly, the size of the backpack is not our concern. What we want to know is the storage volume of the backpack. You could have a very big backpack which is heavily padded all around, which leaves only very little storage space inside. In that case, shall we consider it as a "Big" backpack or a "Small" backpack?
So, to make it clear, what concerns us should be the storage volume, which is the capacity inside the backpack that is available for your to put things. Make sense?
The method I am going to show you below is very easy to use and most backpack and outdoor gear manufacturers use this method to measure their backpack volume.
1) First, you need lots of little balls or beans
We use plastic balls with 10mm diameter, but you can use anything available such as Ping Pong balls or dried beans. You need lots of them.
2) Fill the backpack using a measuring cup
Fill the backpack with balls/beans until it is completely full.
Some would fill the main compartment only, and others would also include the outside pockets as well. There isn't really a standard. In my opinion, I would include the outside pockets as well because they also contribute storage capacity.
3) Just count how many cups of balls/beans you have filled
You can get the accurate volume of the backpack by checking the amount of balls/beans you have filled in.
For example, if you are using a 1L measuring cup, and the backpack can take 20 cups of ball/beans, then the volume of the backpack is simply 20L. This is straight forward, right?
I hope that the information above is useful. Please feel free to leave your comments if you have any.