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Reorder point (ROP)
The reorder point is the stock level that tells you to place a new order. ROP = Average daily sales × Lead time (days) + Safety stock.
Safety stock
Safety stock protects you when sales spike or the supplier is late. A common formula is Z × σ × √Lead time, where σ is the standard deviation of daily sales and Z depends on the service level you want (1.65 for 95%, 2.33 for 99%). A simpler rule: (Max daily sales × Max lead time) − (Average daily sales × Average lead time).
Economic order quantity (EOQ)
EOQ = √(2 × Annual demand × Cost per order ÷ Holding cost per unit per year). It balances ordering often (more ordering cost) against holding too much (more storage cost, tied-up cash, expiry).
Days of stock
Current stock ÷ Average daily sales. If this is less than your lead time, you are already late.
Doing this for hundreds of products by hand is impossible — inventory software tracks sales per product and warns you when stock hits the reorder point.