What perpetual inventory means
In a perpetual inventory system, every transaction that changes stock—a sale, a purchase-order receipt, a return, a transfer between locations, a manual adjustment—updates the recorded quantity immediately. At any moment, the system is meant to show what is actually on the shelf, not an estimate from the last count.
This is the model behind barcode-scanning inventory software: scanning an item at receiving increases the count, scanning it at a sale or pick decreases it, and the on-hand number in the system stays current between physical counts rather than only being accurate right after one.
How periodic inventory differs
A periodic inventory system does not track quantity continuously. Instead, stock is counted physically at set intervals, and the value of goods sold in between is calculated using the accounting formula: beginning inventory + purchases − ending inventory (from the count) = cost of goods sold. Between counts, the business is effectively working from an estimate, not a live number.
Periodic counting requires less day-to-day data entry, which is why some very small, low-SKU operations still use it. But it means stockouts, overstock, and shrinkage go undetected until the next count—sometimes months later—and reorder decisions are made on assumptions rather than current data.
- Perpetual: quantity updates on every transaction, always current
- Periodic: quantity is only accurate right after a physical count
- Perpetual needs scanning/POS discipline at each movement to stay accurate
- Periodic needs a full physical count on a schedule, but less daily data entry
Which one fits your business
Any business selling across multiple channels, holding more than a handful of SKUs, or needing to catch low stock before it becomes a stockout benefits from perpetual tracking—the whole point of a reorder point or low-stock alert depends on the system knowing current quantity, which periodic counting cannot provide between counts.
Periodic counting still has a role even inside a perpetual system: physical counts (cycle counts or a full annual count) remain the way you verify that the perpetual record is actually correct and catch shrinkage. The two are not mutually exclusive—most real operations run perpetual tracking day to day and periodic counts as an accuracy check.