Difference beween two periods of inventory

[Chisk220]

How do I calculate the difference between between two periods of an inventory?

The period is in months (1-12)

1. I want to calculate the changes in inventory between eg period 1 and 2. Which formular should i use?

2. How can I use this number to calculate the inventory turnover rate? I only have the data of the inventory in each period.

Thanks!