A company can account for changes in inventory using either periodic inventory system or perpetual inventory system.

In the perpetual system, the company maintains a continuous record of inventory changes. All the purchases and sales of inventory are directly recorded in the inventory account. For every sale, the COGS is debited and inventory is credited.

In the periodic system, the company updates inventory records only periodically. All inventory purchases are debited to purchase account. The cost of goods sold and inventory values are determined at the end of the period. At the end of the period, we add purchases to the beginning inventory to arrive at the cost of goods available for sale. Then we subtract ending inventory to calculate COGS.

Under FIFO and specific identification methods, the values for COGS and ending inventory will be same in both perpetual and periodic inventory system. However, they will differ in LIFO and average cost methods.

Example

We will use the same example of SuperMart.

DatePurchases (Sales)Balance
1Beginning inventory (@ $3.80)500
2Purchased 1,500 units (@ $4.00)2000
14Purchased 6,000 units (@ $4.40)8000
20Sold 4,000 units4000
30Purchased 2,000 units (@ $4.75)6000

Our earlier calculations of COGS and Ending inventory were based on periodic inventory system. We will now recalculate the same under perpetual inventory system using FIFO and LIFO methods.

FIFO Perpetual System

The COGS and Ending inventory are the same in FIFO perpetual system as in FIFO periodic system.

On June 20, 4000 units were sold. The COGS will be calculated as follows:

UnitsFrom Cost
500June 1 Beginning inventory500 units * $3.80$1,900
1,500June 2 purchase1,500 units * 4.00$6,000
2,000June 14 purchase2,000 units * $4.40$8,800
FIFO COGS$16,700

On June 30, there are 6,000 units in inventory. The ending inventory will be calculated as follows:

UnitsFrom Cost
4000June 14 purchase4,000 units * $4.40$17,600
2,000June 30 purchase2,000 units * $.75$9,500
FIFO Ending Inventory$27,100

LIFO Perpetual System

On June 20, 4000 units were sold. The COGS will be calculated as follows:

UnitsFrom Cost
4000June 14 purchase4000 units * $4.40$17,600
LIFO COGS$17,600

On June 30, there are 6,000 units in inventory. The ending inventory will be calculated as follows:

UnitsFrom Cost
500June 1 Beginning inventory500 units * $3.80$1,900
1,500June 2 purchase1,500 units * 4.00$6,000
2000June 14 purchase2,000 units * 4.40$8,800
2000June 30 purchase2,000 units * $.75$9,500
LIFO Ending Inventory$26,200

Note that the COGS and Ending inventory under Perpetual LIFO are different from periodic LIFO.