Inventory: Perpetual vs Periodic
Know what you have, when you have it.
Why This Matters
You own a sneaker store. A customer asks: "Do you have Air Jordan 4s in size 10?"
Store A
"Let me check the computer... Yes, we have 3 pairs in the back."
Store B
"I have no idea. We'll count everything at the end of the month."
Both systems are used in real businesses. But choosing the wrong one can mean lost sales, overstocking, or theft you never catch.
The Core Difference
Perpetual System
"Always Current"
- Sale happens → Inventory updates immediately
- Purchase arrives → Inventory updates immediately
- Return processed → Inventory updates immediately
You ALWAYS know:
✓ How many units you have
✓ What they cost
✓ What's been sold
Periodic System
"Check Later"
- Sale happens → No inventory update
- Purchase arrives → Goes to "Purchases" account
- Return processed → Goes to "Purchase Returns"
You DON'T know until you:
✗ Stop operations
✗ Physically count everything
✗ Calculate what was sold
Visual: The Warehouse Shelf
Imagine a shelf with 10 boxes initially. Watch how each system tracks events:
📦📦📦📦📦📦📦📦📦📦
Both Systems: 10 units
PERPETUAL
✓ Updates to 7 units
✓ Records $300 COGS
PERIODIC
✗ Still shows 10 units
✗ No COGS recorded yet
PERPETUAL
✓ Updates to 12 units
✓ Inventory account updated
PERIODIC
✗ Records "Purchases"
✗ Inventory unchanged (still 10)
PERPETUAL
✓ Expected 12, found 11
✓ Shrinkage detected: 1 unit
PERIODIC
✗ Just now calculates ending inventory
✗ Shrinkage hidden in COGS
Key insight: The perpetual system catches the missing unit. The periodic system buries it in Cost of Goods Sold—you'd never know it was stolen.
How Each System Works (Entries)
Perpetual Inventory System
The Inventory account is updated with every transaction.
1. Purchase 100 units at $50 each ($5,000):
Inventory5,000
Accounts Payable5,000
2. Sell 30 units for $100 each ($3,000 rev):
Cash (or AR)3,000
Sales Revenue3,000
And immediately record the cost (30 × $50 = $1,500):
Cost of Goods Sold1,500
Inventory1,500
Periodic Inventory System
Purchases go to a temporary account. COGS calculated at period end.
1. Purchase 100 units at $50 each ($5,000):
Purchases5,000
Accounts Payable5,000
Notice: Goes to "Purchases", NOT "Inventory"
2. Sell 30 units for $100 each ($3,000 rev):
Cash (or AR)3,000
Sales Revenue3,000
Notice: NO Cost of Goods Sold entry! We don't know the cost yet.
3. End of Period COGS Calculation
Beg. Inventory $ 0
+ Purchases $5,000
= Goods Available $5,000
- End Inv (counted) $3,500 (70 units × $50)
= Cost of Goods Sold $1,500
Side-by-Side Comparison
| Feature | Perpetual | Periodic |
|---|---|---|
| When inventory updates | Every transaction | End of period only |
| COGS recorded | At time of each sale | End of period (calculated) |
| Know current inventory? | Yes, always | No, until you count |
| Detects theft/shrinkage? | Yes (discrepancies visible) | No (hidden in COGS) |
| Best for | High-value, retail, e-commerce | Low-value items, small biz |
When to Use Each System
Use Perpetual When:
- You sell high-value items (electronics, vehicles)
- You need real-time data (e-commerce, retail)
- Theft/shrinkage is a concern
- You have POS systems and scanners
Use Periodic When:
- You sell low-value, high-volume items (nails, candy)
- Tracking cost exceeds tracking benefit
- You're a small business with simple operations
- Items are hard to track individually
Key Takeaway
Perpetual inventory systems update with every transaction—you always know what you have. Periodic systems update only at period-end through physical counts.
Perpetual catches theft and errors immediately; periodic buries them in COGS. Most modern businesses use perpetual because technology made it affordable and customers expect real-time availability. The system you choose affects your journal entries, your COGS timing, and your ability to control inventory.
Test Your Understanding
See if you've got the basics down. Click each option and check your answer.
Question 1: In a perpetual inventory system, when is Cost of Goods Sold recorded?
Question 2: Which account is used in a periodic system but NOT in a perpetual system?
Question 3: A store's perpetual system shows 50 units in stock. A physical count reveals only 47 units. What happened?
Question 4: A small hardware store sells thousands of individual screws, bolts, and nails daily. Which system makes more sense for these items?
Question 5: Under a periodic system, which formula calculates Cost of Goods Sold?
Ready to Practice?
You now understand how perpetual and periodic systems differ. The Practice Lab puts you in control: record transactions under both systems, calculate periodic COGS, and detect shrinkage.
Try the Practice LabWhat's Next?
Now that you understand when inventory is tracked, the next question is how to value it when costs change. That's where FIFO, LIFO, and Weighted Average come in.