---
title: "Periodic Inventory System vs Perpetual: Which to Use"
description: "A periodic inventory system updates stock only at a physical count, so COGS is a plug figure. See one month booked both ways and where shrinkage hides."
canonical: https://luckpanda.app/blog/periodic-inventory-system
author: "Dhrumil Barot"
author_bio: "Dhrumil is a small-business operations and compliance expert. Dhrumil built LuckPanda so small businesses can run invoicing, inventory, accounts and compliance in one place."
published: 2026-11-11
updated: 2026-11-11
category: "Purchasing and inventory"
keywords: "periodic inventory system"
language: en
---

# Periodic Inventory System vs Perpetual: Which to Use

A periodic inventory system updates stock only at a physical count, so COGS is a plug figure. One month booked both ways shows where shrinkage hides.

By [Dhrumil Barot](https://luckpanda.app/about#founder), Founder. Published 11 Nov 2026. Updated 11 Nov 2026. 7 min read.

**TL;DR:** A periodic inventory system updates the stock balance only when you count, and works out cost of goods sold (COGS) afterwards as opening stock plus purchases minus closing stock. A perpetual inventory system records every receipt and sale as it happens, so you always have a book balance to compare with the count. Both give the same profit for the month; only perpetual tells you how much of your COGS was actually lost stock.

A periodic inventory system is a method of inventory accounting in which purchases go to a Purchases account during the period, sales record only revenue, and the inventory balance is updated once, after a physical count at the end of the month, quarter or year. COGS is then calculated as opening inventory + purchases - closing inventory. A perpetual inventory system instead updates the inventory account with every purchase, sale, return and adjustment. Small shops with few items and owners who see every shelf can live with periodic. Anyone with several locations, lenders who ask for stock statements, or stock that walks out of the door needs perpetual, because periodic folds theft, breakage and counting errors into COGS where nobody can see them.

## Periodic vs perpetual inventory at a glance

| Criterion | Periodic inventory system | Perpetual inventory system |
| --- | --- | --- |
| When the stock balance updates | Only after a physical count | With every receipt, sale, return and adjustment |
| Where purchases are recorded | Purchases account | Inventory account |
| Entries per sale | One (revenue) | Two (revenue, and COGS out of inventory) |
| How COGS is found | Opening + purchases - closing count | Summed from each sale as it happens |
| Shrinkage (theft, damage, errors) | Hidden inside COGS | Shown as the gap between book and count |
| Stock level between counts | Unknown | Known from the books |
| Setup and running effort | Low: a count sheet and a ledger | Higher: item master, every movement recorded |
| Physical counts still needed | Yes, they produce the numbers | Yes, to check the book (IRS and auditors expect it) |
| Fits | One location, few SKUs, low value per unit | Several locations, many SKUs, lender or audit reporting |

## What is a periodic inventory system?

A periodic inventory system is one where the books do not track stock between counts. During the period you record purchases in a Purchases expense account and record each sale only as revenue. You do not know the cost of what you sold until you count what is left.

![Under the periodic system the inventory balance only moves at the count, then COGS is worked out](https://cdn.sanity.io/images/pcmcvz9y/production/46732e93e4c351eb3fd58def4bbfe384dbf8f1f9-1344x326.png?w=1200&auto=format&fit=max)

*Nothing touches the inventory account until the count.*

The cycle runs like this:

1. Bring last period's closing stock forward as this period's opening stock.
2. Record every purchase in the Purchases account (debit Purchases, credit cash or the supplier).
3. Record every sale as revenue only (debit cash or the customer, credit Sales).
4. Count the stock at period end and value it with your cost formula (FIFO or weighted average).
5. Work out COGS, then close the Purchases account to zero.

## What is a perpetual inventory system?

A perpetual inventory system is one where the inventory account is updated with every movement. A purchase debits Inventory directly. A sale makes two entries: revenue, and a transfer of the item's cost from Inventory to COGS. At any moment the book balance tells you what should be on the shelf.

That book balance is a claim, not a fact. The IRS says it plainly in [Publication 538](https://www.irs.gov/publications/p538): if you keep a perpetual or book inventory, "you must take a physical inventory at reasonable intervals and the book amount for inventory must be adjusted to agree with the actual inventory." Perpetual does not remove counting. It gives the count something to be compared against.

## How is COGS calculated under periodic?

COGS under the periodic system is calculated once per period with this formula:

COGS = opening inventory + net purchases - closing inventory (from the physical count)

Net purchases means purchases plus freight-in, minus purchase returns and discounts. The closing figure is the count valued at cost under your chosen formula. Our [COGS formula guide](https://luckpanda.app/blog/cogs-formula) walks through each part, and [how to calculate ending inventory](https://luckpanda.app/blog/how-to-calculate-ending-inventory) covers valuing the count.

## One month booked both ways: where shrinkage hides

Here is the same month at a small hardware shop selling one item, a box of wall screws, recorded under each system. Cost is ₹50 a box all month to keep the arithmetic clean; price changes are what [FIFO vs LIFO](https://luckpanda.app/blog/fifo-vs-lifo) and weighted average deal with.

- Opening stock: 200 boxes at ₹50 = ₹10,000
- Purchases during the month: 600 boxes at ₹50 = ₹30,000
- Sales: 650 boxes at ₹80 = ₹52,000
- Unknown to the owner: 20 boxes were damaged in a leak and thrown away without being recorded

The count at month end finds 130 boxes, worth ₹6,500.

**Periodic.** The owner books ₹30,000 of purchases to Purchases and ₹52,000 of sales. At the count:

COGS = ₹10,000 + ₹30,000 - ₹6,500 = ₹33,500 Gross profit = ₹52,000 - ₹33,500 = ₹18,500

**Perpetual.** Each sale moved ₹50 a box from Inventory to COGS, so COGS from sales is 650 x ₹50 = ₹32,500. The book says 200 + 600 - 650 = 150 boxes, ₹7,500. The count finds 130. The 20-box gap is posted as an inventory adjustment of ₹1,000.

COGS from sales = ₹32,500 Shrinkage adjustment = ₹1,000 Gross profit = ₹52,000 - ₹32,500 - ₹1,000 = ₹18,500

![The same month gives the same profit, but only perpetual separates the 20 lost boxes](https://cdn.sanity.io/images/pcmcvz9y/production/ba51f6a1f0d729aba2e09e13d11dbe5a1dbc4bac-1344x548.png?w=1200&auto=format&fit=max)

*Profit is identical; only perpetual names the Rs 1,000 loss.*

The profit is identical. What differs is what the owner learns. Under periodic, the ₹1,000 leak is inside COGS, indistinguishable from boxes that were sold. If the leak happens every month, COGS just looks a little high and margins look a little thin, and nobody asks why. Under perpetual, a ₹1,000 shrinkage line appears the first month, and the owner can fix the roof.

This is the angle most comparisons skip: periodic does not make shrinkage smaller, it makes it invisible. The IRS treats it the same way: a theft or casualty loss of inventory is normally claimed "through the increase in the cost of goods sold by properly reporting your opening and closing inventories" (Publication 538). The deduction is there either way; the information is not.

## How do the journal entries differ?

The journal entries differ at three points: purchases, sales and the period end. Using the example above:

| Event | Periodic entry | Perpetual entry |
| --- | --- | --- |
| Buy 600 boxes | Dr Purchases 30,000 / Cr Supplier 30,000 | Dr Inventory 30,000 / Cr Supplier 30,000 |
| Sell 650 boxes | Dr Cash 52,000 / Cr Sales 52,000 | Dr Cash 52,000 / Cr Sales 52,000 |
| Cost of those sales | No entry | Dr COGS 32,500 / Cr Inventory 32,500 |
| Count finds 20 short | No separate entry | Dr Shrinkage 1,000 / Cr Inventory 1,000 |
| Period end | Close Purchases and adjust Inventory from 10,000 to 6,500, COGS 33,500 | No closing entry needed |

[Inventory Systems: Perpetual vs Periodic](https://www.youtube.com/watch?v=tAJ8mRfffJ8&t=145s) (Accounting Stuff)

James from Accounting Stuff books one month of a bookshop's purchases and sales under both systems, showing the Purchases account that only exists under periodic and why a periodic sale makes only one journal entry.

- [2:25](https://www.youtube.com/watch?v=tAJ8mRfffJ8&t=145s) The bookshop example: 300 books on hand, 500 bought, 450 sold
- [4:33](https://www.youtube.com/watch?v=tAJ8mRfffJ8&t=273s) Step one: closing stock becomes opening stock under both systems
- [5:26](https://www.youtube.com/watch?v=tAJ8mRfffJ8&t=326s) Step two: purchases go to Inventory under perpetual, Purchases under periodic
- [7:09](https://www.youtube.com/watch?v=tAJ8mRfffJ8&t=429s) Step three: perpetual records revenue and COGS at each sale
- [8:55](https://www.youtube.com/watch?v=tAJ8mRfffJ8&t=535s) Periodic records only revenue, because COGS is unknown until the count

## Which businesses should use periodic vs perpetual?

Use periodic when all of these are true: one location, a short item list, low value per unit, and an owner who is on the floor every day. A tea stall, a small bakery or a single-counter stationery shop fits. The count is quick and a monthly COGS figure is enough.

Use perpetual when any of these is true:

- You have more than one shop or warehouse and need to know where stock sits.
- You reorder from data. A [reorder point](https://luckpanda.app/blog/reorder-point-formula) needs a stock level today, not at the last count.
- A bank lends against stock. In India, under CARO 2020 clause 3(ii)(b), auditors of companies with working capital limits above ₹5 crore secured on current assets report whether the quarterly statements filed with banks agree with the books.
- You are audited. CARO 2020 clause 3(ii)(a) asks the auditor whether the coverage and procedure of physical verification are appropriate and whether discrepancies of 10% or more in aggregate for any class of inventory were noticed and properly dealt with. You can only report a discrepancy if you have a book figure to compare with.
- You sell online and offline from the same stock, where overselling costs you customers.

## Cost, setup and accuracy, criterion by criterion

**Setup.** Periodic needs a count sheet and a purchases ledger. Perpetual needs an item master (each SKU defined once with its unit and cost) and the discipline to record every receipt, sale, transfer and write-off.

**Accuracy between counts.** Periodic has none: the stock level between counts is a guess. Perpetual is as accurate as the recording. Unrecorded damage, theft and receiving errors make the book drift, which is why counts still matter.

**Valuation rules.** Both systems use the same cost formulas. Under [IAS 2](https://www.ifrs.org/issued-standards/list-of-standards/ias-2-inventories/) (and India's Ind AS 2, which follows it), inventory is measured at the lower of cost and net realisable value, using specific identification, FIFO or weighted average; LIFO is not allowed. US GAAP and US tax rules still permit LIFO. IAS 2 also says all losses of inventories are recognised as an expense in the period the loss occurs, which is easier to follow when the loss has its own line.

**Tax.** In the US, small business taxpayers (average annual gross receipts of $26 million or less over the 3 prior tax years, indexed for inflation, and not a tax shelter) have simpler inventory options under Publication 538. Changing your inventory accounting method needs Form 3115.

**Reporting.** Periodic gives you a reliable COGS once per period. Perpetual gives a running COGS, gross margin by item and stock ratios such as [average inventory](https://luckpanda.app/blog/average-inventory) at any date.

## Common mistakes with each system

- Periodic: counting on a different date from the purchase cut-off, so goods received but not yet booked (or booked but not received) distort COGS.
- Periodic: treating "COGS looks high this month" as a pricing problem when it is shrinkage.
- Perpetual: never counting, then trusting a book figure that has drifted for a year.
- Perpetual: posting count differences straight to COGS with no note, which throws away the very information perpetual exists to give.
- Both: switching cost formulas mid-year. IAS 2 and the IRS both expect consistency.

## Moving from periodic to perpetual

The switch is mostly about recording movements, not about software. Take a full count to set a reliable opening balance, set up each item once, and from that date record every receipt against a purchase document and every issue against a sale. Count a few items each week (cycle counting) instead of everything once a year, and post each difference as an adjustment with a reason.

If you want that record without a spreadsheet, LuckPanda's [Inventory module](https://luckpanda.app/modules/inventory) keeps one item list for buying and selling, holds stock by warehouse, and logs each receipt, delivery, transfer and adjustment as a stock movement tied to the document that caused it, so a count difference shows up as its own line instead of disappearing into COGS.

## Questions

### What is a periodic inventory system?

A periodic inventory system records purchases in a Purchases account and sales as revenue only, then updates the inventory balance after a physical count at the end of each period. COGS is calculated as opening inventory plus purchases minus closing inventory.

### How is COGS calculated under the periodic inventory system?

COGS equals opening inventory plus net purchases minus closing inventory from the physical count. In the example above that is ₹10,000 + ₹30,000 - ₹6,500 = ₹33,500, which includes ₹1,000 of unrecorded damaged stock.

### Which businesses should use periodic vs perpetual inventory?

Periodic suits a single location with few low-value items and an owner who sees all the stock. Perpetual suits businesses with several locations, many SKUs, stock-based bank borrowing, statutory audits or online sales from shared stock.

### Does a perpetual inventory system remove the need for physical counts?

No. The IRS requires a physical inventory at reasonable intervals with the book adjusted to match, and in India CARO 2020 asks auditors to report on physical verification and discrepancies of 10% or more. Perpetual makes counts more useful, not optional.

### Do periodic and perpetual give different profit?

With the same cost formula and the same closing count, gross profit is the same. With FIFO the results match; with weighted average or LIFO the timing of the cost calculation can make them differ slightly, because perpetual averages after each purchase and periodic averages once for the whole period.

### Is the periodic inventory system allowed under GAAP and IFRS?

Yes. Both frameworks set how inventory is measured (cost formulas, lower of cost and net realisable value), not whether you track it continuously. Periodic is acceptable as long as the closing count and valuation are reliable.

## More guides

- [Bar Inventory Template: Columns, Formulas and Free Sheets](https://luckpanda.app/blog/bar-inventory-template)
- [Supplier Performance Metrics: 10 KPIs, Formulas and a Worked Example](https://luckpanda.app/blog/supplier-performance-metrics)
- [Source to Contract vs Procure to Pay: Where Each Process Starts and Ends](https://luckpanda.app/blog/source-to-contract-vs-procure-to-pay)

## Related tools

- [Invoice generator](https://luckpanda.app/tools/invoice-generator): Bill a client in any currency, with tax worked out as you type.

## About the author

**Written by [Dhrumil Barot](https://luckpanda.app/about#founder)**, Founder. Dhrumil is a small-business operations and compliance expert. Dhrumil built LuckPanda so small businesses can run invoicing, inventory, accounts and compliance in one place. [LinkedIn](https://www.linkedin.com/in/bdhrumil) · [X](https://x.com/fyidhrumil)