How do I calculate the value of stock in my shop?

For calculating zakat on your inventory you have to consider the following items and then proceed. 

• Is the inventory marked for sale? If it is, then zakat is only due on sellable inventory. 
• The inventory has been available for sale for one lunar year. The same SKU or products. It can be general or specific if you like. Example, phone cases or a specific phone case. It can become really complex the more specific you get so to generalize inventory items together might be easier to manage in the calculation. 
• Inventory price can be selected as either the wholesales price you would see the inventory for or the retail price you have it marked for. Our opinion is that the wholesale prices would suffice.