Most sellers calculate profit in their head. Price minus cost minus FBA fee — “seems okay.” Then they wonder why their bank account does not match their spreadsheet.

A proper profit calculator is not a spreadsheet. It is a formula that accounts for every cost line item, updated with live data, and applied consistently across every platform you sell on. Here is how to build one — and why the platform-specific differences matter more than most sellers realize.

The Full Cost Formula

For a product sold on Amazon via FBA, the actual cost per unit is:

``` Unit Cost = Product Cost (1688 ex-factory price + shipping)

• Amazon Commission (15% for most categories, varies by category)
• FBA Fulfillment Fee (weight and size tier)
• Monthly Storage Fee (per cubic foot, accrues daily)
• Aged Inventory Surcharge (after 181 days)
• Advertising Cost of Sale (total ad spend ÷ units sold)