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:
- 1- liter measuring cup
- A lot of plastic balls 10 – 20 mm diameter, Ping Pong balls or dried beans.
- Reference standard ASTM F2153 – 07 (2012)
Determine capacity as follows:
Step 1: Remove all hangtags and packaging.
Step 2: Loosen all straps to eliminate any constrictions so the pack may achieve its largest capacity.
Step 3: Fill the backpack using the 1- liter measuring cup. Fill the main compartment with the balls or dried beans, making sure the spheres are occupying all the corners. Continue filling and tamping until the main compartment is full in accordance with the definition of extended capacity.
Step 4: Continue to fill all enclosed compartments to their maximum capacity, making sure the compartments are filled and closed. Include the outside pockets as well because they also contribute storage capacity.
Step 5: Tamp and massage the pockets to remove areas not filled with balls and continue until full.
Step 6: Calculation: 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 hold 20 cups of ball/beans, then the volume of the backpack is 20L.