Periodic vs. Perpetual Inventory System in-depth
Inventory Record Systems
For any business, tracking inventory (stock) is crucial. You need to know the physical quantity and monetary value of the items you have sold and the items still in hand. There are two main systems to do this: the Periodic Inventory System and the Perpetual Inventory System.1
This guide will break down both systems to help you understand how they work, their pros and cons, and which one to use.
1. The Periodic (Physical) Inventory System
The Periodic Inventory System is a method where you determine your inventory value by doing a full physical count (or measurement/weight) of all your items. This count is typically done periodically—usually just once at the end of the financial year.2
How It Works
Under this system, the business does not track the exact inventory level or Cost of Goods Sold (COGS) during the year. Instead, it uses a simple formula after the physical count to figure out the COGS as a residual (leftover) number.
Formula for Cost of Goods Sold (COGS):
COGS = Opening Inventory + Purchases – Closing Inventory3
Opening Inventory: You know this from the last year’s count.
Purchases: You know this from your purchase records.
Closing Inventory: This is the only number you get from your big physical count at the end of the year.
Benefits of the Periodic System
Simple to Use: It’s very easy to manage, especially for small businesses.
Less Expensive: It is the cheapest system to use. It does not require any special software, scanners, or costly training.
Limitations of the Periodic System
Includes Lost/Stolen Goods: The biggest drawback. The COGS formula assumes that any item not in the closing count must have been sold. This means that items lost to theft, waste, or damage are improperly included in the Cost of Goods Sold, hiding your losses.
Disrupts Business: The physical count requires you to stop all normal business operations for several days, which can be costly.4
No Real-Time Data: You have no idea what your inventory level is until the end of the year. This makes it impossible to know if you are “out of stock” of an item.
No Inventory Control: Because you only count once a year, it’s impossible to identify when items went missing, making it difficult to control losses.5
2. The Perpetual Inventory System
The Perpetual Inventory System is a more advanced method that involves continuously recording inventory balances every time an item is received (a purchase) or issued (a sale).
How It Works
A separate record (like a stock card or database entry) is kept for each type of inventory. This record is updated immediately.
When goods are bought, the inventory account is debited.
When goods are sold, two entries are made: one to record the sale, and one to directly calculate the COGS and reduce the inventory account.
Under this system, the Cost of Goods Sold is determined directly, and the Closing Inventory is the residual figure left in the records. Regular physical checks are still done to compare the record balance to the actual stock and identify any discrepancies (like theft).
Benefits of the Perpetual System
No Business Disruption: You don’t need to shut down the business for a massive year-end count.
Real-Time Data: You always know your exact stock level, which helps prevent “out of stock” situations and keeps customers happy.
Strong Inventory Control: Because you have a continuous record, you can easily spot losses from theft or damage as soon as they happen and take corrective action.
Faster Reporting: Financial statements can be prepared quickly (e.g., monthly or quarterly) because the inventory and COGS figures are always available.
Limitations of the Perpetual System
Expensive: This system is costly. It requires significant setup costs, including inventory software, point-of-sale (POS) scanners, and other equipment.
Higher Labor Cost: It requires more labor, as every single item must be entered into the system.
Can Be Misleading: The system is only as good as the data entered. If employees make mistakes (e.g., scan the wrong item or enter the wrong quantity), the records will be inaccurate.
Requires Monitoring: Because of potential employee errors or customer theft, this system requires extra security monitoring, which adds to the cost.6
3. Periodic vs. Perpetual: Key Differences
| Basis for Comparison | Periodic Inventory System | Perpetual Inventory System |
| Meaning | Based on a physical count at the end of the period. | Based on continuous, real-time record-keeping. |
| How COGS is Found | COGS is the residual figure (calculated at the end). | COGS is determined directly at the time of each sale. |
| How Inventory is Found | Closing Inventory is determined directly (by counting). | Closing Inventory is the residual figure (from the records). |
| Business Operations | Requires closure of business for stock-taking. | Does not affect daily operations. |
| Inventory Control | Not possible. Losses are hidden in COGS. | Provides strong control. Discrepancies are easily found. |
| Cost | Simple and inexpensive. | Costly and complex (requires software and training). |
| Best Suited For | Small businesses | Large enterprises |
4. What is Stock Taking? (Adjusting for Dates)
Often, it is not possible to do the full physical stock count on the exact closing date of the financial year (e.g., March 31st). The count might be done a few days before or a few days after.
When this happens, you must adjust the value of the stock you counted to find the true value that existed on the closing date.
How to Calculate Stock on the Closing Date
Example: Your financial year ends on March 31st, but you do the physical count on April 7th.
You must start with the stock value you counted on April 7th and reverse all transactions that happened in the “gap” (April 1st – April 7th).
Here is the calculation:
| Particulars | Amount (₹) |
| Value of stock (as per physical count on April 7th) | XXX |
| Add: Cost of Goods Sold (Sales) during the gap (April 1 – 7) | + XXX |
| (Why? These items were sold and gone by April 7th, but they were in stock on March 31st, so we must add them back.) | |
| Less: Purchases (less returns) during the gap (April 1 – 7) | – XXX |
| (Why? These items were in the count on April 7th, but they had not arrived yet on March 31st, so we must remove them.) | |
| = Value of Stock on March 31st | XXX |
(Note: If you count before the closing date, you reverse this logic—add purchases and subtract the cost of sales made during the gap.)
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