Perpetual vs Periodic Inventory: Which Method Should You Use?

Perpetual vs periodic inventory is the choice between counting your stock continuously and counting it on a schedule. That one decision shapes how you value stock, when you know your true cost of goods sold, and how fast you catch shrinkage. Pick wrong and you either drown in overhead you don't need or fly blind between counts. This walks through both methods with a worked example, a formula, and a plain rule for choosing.

Perpetual vs periodic inventory: the core difference

A perpetual system updates stock levels after every single transaction. Sell three units, the on-hand count drops by three and cost of goods sold (COGS) ticks up immediately. Receive a purchase order, the balance rises the moment you scan it in. You always have a live number.

A periodic system does none of that in real time. Purchases pile into a temporary account, and you only calculate what sold after a physical count. Between counts, your books don't know how much you have. You infer it.

That's the whole split. Everything else (accuracy, cost, tooling, accounting entries) flows from whether the count is continuous or scheduled.

How periodic inventory works

Periodic inventory leans on one formula, run at the end of each accounting period:

COGS = Beginning Inventory + Purchases − Ending Inventory

You know beginning inventory (it's last period's ending number). You know purchases from invoices. Ending inventory comes from physically counting what's left on the shelf. Rearrange those and COGS falls out.

A worked example

Say you run a small parts distributor. At the start of the quarter you had $20,000 in stock. Over three months you bought $50,000 more. At quarter-end, your team counts the shelves and finds $15,000 of stock remaining.

COGS = 20,000 + 50,000 − 15,000
COGS = 55,000

So $55,000 of product moved this quarter. Simple. But notice what you did not have: on day 40 of the quarter, you had no reliable idea what was on hand or what your margins were. You waited until the count to find out. And if $2,000 of that stock was stolen or broken, the formula quietly rolls that loss into COGS. It looks like you sold it. Shrinkage hides inside the number instead of showing up as a variance you can investigate.

Periodic is cheap to run and needs almost no technology. That's its entire appeal.

How perpetual inventory works

Perpetual keeps a running balance per SKU. Every receipt, sale, transfer, and adjustment writes to the record the instant it happens. COGS is calculated transaction by transaction, so it's always current.

The practical upside is that you can answer three questions at any moment:

• How many of SKU-1042 do I have right now?
• What did the units I just sold actually cost me?
• Does my system count match the physical shelf (and if not, by how much)?

That third one matters. Because perpetual tracks a book quantity continuously, a physical count becomes a check, not the source of truth. The gap between book and physical is your shrinkage, and you can see it, quantify it, and chase it.

Perpetual needs infrastructure: barcode or RFID scanning, a point-of-sale or warehouse system that logs movements, and integrations so sales and receipts hit the ledger automatically. Modern systems handle this over an API, which is why perpetual has become the default for anyone selling across multiple channels.

Side-by-side comparison

Factor	Periodic	Perpetual
Stock update timing	End of period only	Real time, every transaction
COGS known	After a physical count	Continuously
Shrinkage visibility	Buried in COGS	Shows as book vs physical variance
Tech required	Minimal	Scanning, software, integrations
Cost to run	Low	Higher upfront, lower long term
Best fit	Low SKU count, low volume	Higher volume, multi-location, multi-channel
Physical count	The source of truth	A verification step

Which one should you use

Run the checklist. Lean periodic if most of these are true:

1. You carry a small number of SKUs.
2. Order volume is low and predictable.
3. Individual unit cost is low, so a miscount barely dents the P&L.
4. You genuinely can't justify software or scanning yet.

Lean perpetual if any of these are true:

1. You sell across more than one channel or location.
2. Stockouts or overselling cost you real money or customer trust.
3. You need margins per order, not per quarter.
4. Shrinkage, theft, or spoilage is a live concern.
5. You're planning to grow and don't want to re-platform later.

Most businesses past the hobby stage land on perpetual. The reason is simple: the cost of software keeps dropping while the cost of not knowing your stock stays high.

One accounting wrinkle worth knowing

Your cost flow assumption (FIFO, LIFO, or weighted average) interacts with the method. Under FIFO, periodic and perpetual produce the same COGS. Under LIFO or weighted average, they can diverge, because perpetual recalculates the cost layer at each sale while periodic settles it once at period-end. If your accountant uses weighted average, expect the two systems to report slightly different numbers for the same physical reality. Neither is wrong; they just measure at different moments.

Where Inventoros fits

If you want perpetual tracking without paying a per-seat SaaS bill forever, this is the gap Inventoros closes. It runs perpetual, multi-location stock out of the box, updates balances in real time over a REST and GraphQL API plus webhooks, and it's self-hosted and MIT licensed, so your inventory data stays on your own server. You can install it on cPanel, a VPS, or Docker, then wire it into your storefront or POS. The documentation covers setup and the movement model end to end.

FAQ

Is perpetual or periodic inventory better for a small business?

It depends on volume, not size. A small shop with a dozen low-cost SKUs can run periodic fine. A small shop selling on three channels needs perpetual so it doesn't oversell. The trigger is transaction complexity, not headcount or revenue.

Does periodic inventory track cost of goods sold?

Yes, but only at the end of each period, and only after a physical count. It uses Beginning Inventory + Purchases − Ending Inventory. Between counts you have no live COGS figure, and any shrinkage gets absorbed into that number instead of being flagged separately.

Can you switch from periodic to perpetual inventory?

You can, and most growing businesses do. Start with an accurate physical count to set your opening balances, load those into your inventory software, then log every movement from that point forward. The clean opening count is the part people skip, and it's the part that makes or breaks the transition.

Do you still need physical counts with a perpetual system?

Yes, just far less often and for a different reason. Under perpetual, a count verifies the book figure rather than creating it. Many operations use cycle counting, checking a small slice of SKUs each week, so the whole catalog gets validated over time without a full annual shutdown.