How can stock be a calculation rather than a number?
Because everything it depends on is already recorded. The quantity on each purchase order adds up to Qty Purchased, the quantity on each sales order adds up to Qty Sold, and Qty On Hand subtracts one from the other. No screen asks anyone to maintain it, which means no screen offers anyone the chance to get it wrong, and correcting a mistyped order quantity corrects the stock figure in the same motion.
Can a total be built on top of another calculation?
Yes, and this build leans on it twice. Line Total is worked out per order from quantity and price, and both Total Spent on the supplier and Total Purchased on the customer add those calculated figures up. Neither level is stored anywhere, so a corrected price flows through the line and into every total that reads it without anyone recalculating a thing.
What happens when a physical count disagrees with the system?
You reconcile by fixing the orders rather than overwriting the total, which is the point. If ten units went out unrecorded, the missing sales order is the thing that is wrong, and adding it corrects the stock figure, the customer's lifetime value and the revenue reporting together. A system that lets you type over the stock number lets you hide the missing order forever.
Does this handle a product supplied by more than one supplier?
Yes. Purchase orders link to a supplier and to a product independently, so the same SKU can arrive from several sources and every one of those receipts still counts toward the same Qty Purchased. Each supplier keeps their own Total Spent, so you can see who you actually buy from without that splitting the product record in two.
Is this an ERP?
No, and a distributor who needs a general ledger, landed costs or warehouse management should buy those. What this is: the order-and-stock spine, modelled so the numbers derive from the transactions, working this afternoon and changeable by you on a Tuesday. Plenty of businesses spend two years implementing an ERP to reach a worse version of exactly this.
What happens to the sample data?
It is real, editable data that exists so the workspace demonstrates itself rather than opening empty - thirty-six records here, including orders whose quantities are what make the stock calculation show a believable number on the very first screen. Delete it in bulk when you are ready for your own, or edit the rows into your first real orders.