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 here. 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 should concern you is the storage volume, which is the capacity inside the backpack that is available for your to put things. Make sense?
The method to measure storage volume we are using and most backpack and outdoor gear manufacturers use is the following: