Perpetual vs Periodic Inventory System: Which to Use

What is a perpetual inventory system?

A perpetual inventory system keeps a running record of every item: each receipt adds to it and each issue or sale takes from it, so the quantity and value on hand are known at any moment.

The moment a strip of tablets is billed at a pharmacy or a carton leaves a warehouse, the records reduce the stock, move its cost into cost of goods sold and update the margin. Open the books at three in the afternoon on a Tuesday and the closing stock on screen is meant to be right as of that moment. Item-wise accounting in most current accounting and ERP software works this way once inventory tracking is switched on.

What makes it perpetual is the second posting. Each sale records revenue and, at the same time, takes the cost of the item sold out of the inventory account. A business can invoice through its software and still not be perpetual if that second posting never happens.

What is a periodic inventory system?

A periodic system records purchases as they happen, usually in a purchases account, but does not touch the stock account when goods are sold. Revenue is booked; the cost side waits. At the end of the month, quarter or year the stock on hand is counted and valued, and only then is cost of goods sold worked out:

The system is cheap and simple because nothing is tracked item by item during the period. The price is blindness: between two counts nobody knows the true stock or the true margin. Whatever was stolen, broken, issued wrongly or given away as samples is simply part of the formula's answer, and it surfaces only as a margin that looks worse than expected.

Difference between perpetual and periodic inventory

Both systems answer the same question, closing stock and cost of goods sold, but at different moments and with different discipline.

The line that matters most is the one about losses. Under perpetual records the difference between the book quantity and the counted quantity is a number you can see, investigate and book as a write-off with a reason. Under periodic records the same loss inflates cost of goods sold and is never isolated.

How a perpetual inventory system works step by step

1. Receipt. A purchase or goods-received note increases the item's quantity and the inventory value in the ledger.
2. Sale or issue. The invoice or issue slip books revenue (for a sale) and, in the same step, moves the item's cost from inventory to cost of goods sold or to production.
3. Return, scrap or sample. A sales return adds stock back; damaged, expired or sampled goods are taken out with their own entry, so the record still matches what should be on the shelf.
4. Running balance. Quantity and value are available on demand, feeding reorder levels and margin reports.
5. Count and adjust. A physical count, all at once or item groups in rotation, is compared with the book balance, and the difference is investigated before it is written off.

What is the journal entry for perpetual inventory?

This is where the two systems visibly diverge. For example, take a trader who sells goods for ₹50,000 (illustrative, excluding GST) that cost ₹32,000 to buy.

Worked example: a perpetual stock card and its entries

A stock card that moves with every transaction shows the system at work. This illustrative card follows one product through July on a first-in first-out basis.

1. 1 July, opening: 100 units at ₹200, a stock value of ₹20,000 in this example.
2. 6 July, purchase: 150 units at ₹220 come in; the example balance is 250 units worth ₹53,000.
3. 14 July, sale of 120 units: first-in first-out issues the 100 older units at ₹200 and 20 newer ones at ₹220, a cost of ₹24,400 in this example; 130 units worth ₹28,600 remain.
4. 22 July, purchase: 100 units at ₹240; the example balance is 230 units worth ₹52,600.
5. 28 July, sale of 90 units: all from the ₹220 layer, a cost of ₹19,800 in this example; 140 units worth ₹32,800 remain (40 at ₹220 and 100 at ₹240).

Each sale carries its own cost entry; for the 14 July sale in this example it is debit Cost of goods sold ₹24,400, credit Inventory ₹24,400. The card shows 140 units at the month end, and a count should find 140. If it finds 136, the four missing units are the loss the periodic system would have hidden. The choice of cost formula behind these numbers is compared in the retail inventory method vs the cost method; LIFO is not available under Indian accounting standards.

Do most companies use periodic or perpetual inventory?

Any business of scale that bills through accounting or ERP software with inventory tracking switched on is running perpetual records, whether it set out to or not: supermarkets scanning barcodes, fulfilment centres, pharmacies, distributors and manufacturers. Periodic records survive in small operations, a stationery shop or a low-value spares store, where tracking every item costs more than it saves.

GST law pushes in the same direction. Rule 56(2) of the CGST Rules requires every registered person other than a composition taxpayer to keep accounts of stock for goods received and supplied, showing the opening balance, receipts, supplies, goods lost, stolen, destroyed, written off or given away as gifts or free samples, and the balance of stock, including raw materials, finished goods, scrap and wastage. A stock account kept movement by movement is, in practice, a perpetual record.

Why choose perpetual over periodic (and the disadvantages)

• Control. Losses become visible within days rather than at the next count, while the trail is still warm.
  Perpetual: advantage
• Live figures. Stock and margin are known at any moment, reorder levels can trigger automatically, and slow-moving items show up in an ageing report.
  Perpetual: advantage
• Discipline. Every issue, return, sample and scrap has to be entered, or the book figure becomes fiction. It needs set-up, trained staff and software.
  Perpetual: disadvantage
• False confidence. A clean-looking book balance tempts people to skip the count that keeps it honest.
  Perpetual: disadvantage
• Cheap and simple. Nothing to post through the period beyond purchases and sales.
  Periodic: advantage
• Blind between counts. No stock figure for a lender or a manager until the next count, and losses never separated from cost.
  Periodic: disadvantage

Choosing and running the right system

The decision is rarely about accounting theory. It turns on a few practical questions:

1. How much is each item worth, and how fast does it move? High-value or fast-moving stock almost always justifies perpetual records; a small, slow, low-value store may reasonably stay periodic.
2. Are you registered under GST outside the composition scheme? Then you already owe a stock account of receipts and supplies, and perpetual records are the natural way to keep it.
3. Does someone need a stock figure between counts? A lender asking for stock statements, a manager setting reorder levels or a buyer checking margins all need a figure a periodic system cannot give without estimating.
4. How many locations hold stock? Each extra site multiplies what a periodic count has to cover at once.
5. Would a mix work? Some businesses keep perpetual records for the few items that carry most of the value and count the long tail of cheap items periodically.

Whichever you choose, the count stays. Closing stock in the accounts has to be stock that exists, and a count is how that is shown. If the book balance and the shelves have drifted apart and nobody can explain why, or you are about to switch systems and the opening figure is uncertain, an independent stock audit gives you a verified starting point.