Your volume is calculated based on the real volume of your products. The volume occupied by products staying in the warehouse for less than 2 weeks is free.
The dimensions of each product are taken when a product arrives at the warehouse for the first time.
For example, for a book that measures 24 cm x 32 cm x 2 cm, the volume of each product is 0.001536 m3.
Check [How are storage fees computed] to know more about how storage volume is billed