perpetual inventory system
Imagine a supermarket where every time the cashier beeps an item across the scanner, a computer instantly subtracts that item from the store's stock count. At any moment of the day, the manager can ask 'how many cartons of milk do we have left?' and get an exact answer without anyone walking to the fridge to count. That always-up-to-date approach is the spirit of a perpetual inventory system.
Technically, a perpetual inventory system updates the inventory account continuously, with every purchase and every sale. When goods come in, inventory rises; the instant goods are sold, the system records the sale and also moves the cost of those goods from inventory to cost of goods sold, item by item. Because the books always show a current figure for both inventory on hand and cost of goods sold, you do not have to wait until period-end to know your gross profit. Modern barcode scanners and point-of-sale software made this practical for ordinary shops.
It matters because it gives managers real-time control: they can spot shrinkage (theft, spoilage, errors) by comparing what the books say is on hand against a physical count, and they can reorder before running out. The catch is that even a perpetual system still needs at least one physical count to confirm reality — the computer can be wrong if items are stolen, miscounted, or mis-scanned. The difference between book inventory and the count is recorded as an inventory shrinkage adjustment.
An electronics store sells a phone that cost 300. At the moment of sale its software does two things at once: it records 500 of sales revenue, and it reduces inventory by 300 while raising cost of goods sold by 300. The owner can check at lunchtime exactly how many phones remain and how much gross profit the day has produced so far.
Every sale instantly updates both inventory and cost of goods sold.
Perpetual records are not magically accurate — a physical count is still needed to catch theft and errors, and the gap is recorded as shrinkage. 'Perpetual' means continuously updated, not perfectly correct.