Periodic inventory system: Definition, formula, and example

A periodic inventory system means you physically count stock at set intervals, such as monthly or quarterly, and calculate cost of goods sold (COGS) from that count. The formula is:

COGS = Beginning inventory + Purchases – Ending inventory

With $20,000 in beginning inventory, $35,000 in purchases, and $15,000 counted at period end, your COGS is $40,000. The periodic inventory method fits small businesses with few stock-keeping units (SKUs) and low transaction volume.

Your COGS is only as accurate as your count.

What is a periodic inventory system?

A periodic inventory system is an inventory accounting method that updates your inventory balance and COGS only when you complete a physical count. Small retailers, e-commerce sellers, restaurants, and other businesses that can count stock by hand tend to use it. You'll also hear it called the periodic stock system or the periodic method.

Between counts, you record sales revenue as usual and post inventory you buy to a Purchases account. Your Inventory balance stays at the last count's figure, and you don't record COGS until the next count. Schedule counts around the dates you need reliable margins, such as lender reporting or year-end taxes.

How a periodic inventory system works

A periodic inventory system runs on a repeating four-step cycle that ends with a count.

1. Start with beginning inventory: Use the dollar value from your last physical count.
2. Record purchases as you buy: Post inventory purchases to a Purchases account and leave the Inventory account alone.
3. Count stock at period end: Count what's on hand and value it at cost.
4. Calculate COGS and adjust the books: Apply the formula, then post one adjusting entry that updates Inventory and closes Purchases.

The count drives every number that follows, so give it some structure. Pick a date at least 1 week out so you can schedule staff and warn suppliers about the receiving freeze.

• Count outside business hours: Count after close or on a weekend so sales don't move stock mid-count.
• Freeze receiving and shipping: Hold deliveries and outgoing orders until you finish, or log them on a separate sheet.
• Count by location or SKU: Work shelf by shelf or bin by bin, and have a second person recount high-value items.
• Use count sheets: List each SKU, its location, and the counted quantity, and have each counter initial their sheets.
• Value each item at cost: Multiply counted units by what you paid, using your cost-flow method, such as first in, first out (FIFO) or weighted average.

Your ending count becomes next period's beginning inventory. An overcount lowers this period's COGS and raises next period's by the same amount.

How to calculate COGS under a periodic inventory system

Under the periodic inventory method, you calculate COGS once per period: beginning inventory plus net purchases, minus ending inventory. The result captures everything that left your shelves, whether you sold it or lost it.

1. Start with beginning inventory

Your beginning inventory is the dollar value of last period's ending count. Say you run a boutique that counts stock every quarter, and last quarter's count came to $20,000. That figure should match the Inventory balance on your balance sheet, so fix any gap before you go further.

2. Add purchases for the period

Use net purchases: gross purchases minus returns and purchase discounts, plus freight-in. If the boutique bought $36,500 of merchandise, returned $1,000, took $700 in early-payment discounts, and paid $200 in freight-in, net purchases come to $35,000. Add that to beginning inventory, and your cost of goods available for sale is $55,000.

Keep shipping costs to customers out of this number, since they're a selling expense.

3. Subtract ending inventory from the physical count

The quarter-end count values the boutique's remaining stock at $15,000. That gives you:

COGS = $20,000 + $35,000 – $15,000 = $40,000

Every dollar of overcounted stock understates cost of goods sold by a dollar. Recount your highest-value SKUs before you lock in the number.

4. Record the period-end adjusting entry

The count reaches your books through one adjusting entry. You remove the old beginning inventory balance, record the new ending balance, close out Purchases, and record COGS.

If your count comes in lower than purchase and sales records suggest, the gap is shrinkage, and a periodic system only surfaces it at count time. Unless you track it separately, shrinkage sits inside your $40,000 COGS figure.

Post this entry before you run period-end financials, or your balance sheet will still show last period's inventory.

Periodic vs. perpetual inventory system

A periodic system updates inventory and COGS at set intervals after a count, while a perpetual system updates both with every purchase and sale. Periodic fits if you carry under a few hundred SKUs and sell through one channel. Move to perpetual when you sell across several channels or need real-time stock levels to reorder.

Perpetual systems still need physical counts because theft and data entry errors pull records away from what's on the shelf. If stockouts or guesswork on reorders already cost you sales, it's time to move to perpetual.

Advantages and disadvantages of a periodic inventory system

A periodic system saves you money and daily bookkeeping time, and it costs you visibility between counts.

Advantages

• Low cost: You skip inventory software and scanner costs.
• Less day-to-day bookkeeping: You post purchases to one account and skip per-sale inventory entries.
• Works with tools you already have: A spreadsheet and printed count sheets handle the whole process.

If your stock fits on a few count sheets, those savings are real.

Disadvantages

• No real-time stock levels: You don't know what's on the shelf between counts, which raises stockout risk.
• Hidden shrinkage: Theft, damage, and errors stay invisible until the count.
• Estimated interim COGS: Monthly statements between counts rely on an estimate, usually based on historical gross margin.
• Time-consuming counts: Counts take staff hours and can pause sales and receiving.
• Harder to scale: Each new SKU and sales channel adds count time and room for error.

Once you pass a few hundred SKUs or sell in more than one channel, these costs usually outweigh the savings.

Tools that help with a periodic inventory system

Counting and tracking stock happen in inventory-specific tools.

• Inventory management software: Stores count results by SKU and location, and gives you a path to perpetual tracking later
• Barcode scanners: Speed up the count and cut transcription errors on count sheets
• Point-of-sale (POS) systems: Record each sale and its revenue, which keeps your sales records current between counts

Once you finish the count, those totals still have to reach your books as an adjusting entry. Pick tools that export count totals by SKU to a spreadsheet so you can value them at cost.

Turn your period-end count into accurate books with Ramp Stack

After the count, someone still has to compare counted inventory to purchase records, calculate COGS, and post the adjusting entry. For most owners and bookkeepers, that work happens after hours in a spreadsheet, where one transposed number throws off COGS.

Ramp Stack gives you agents that pull purchase activity from your accounting system and bills, then reconcile it against your counted ending inventory. The agent prepares the period-end adjusting journal entry in a formula-backed workbook you can open and check.

You review and approve every entry yourself:

• Check every number: Formulas and source data stay visible, so you can trace COGS back to each bill and your count.
• Review the full record: Each session has a Session Audit Log you can review and export.
• Approve before anything posts: Nothing posts to QuickBooks, NetSuite, or Sage Intacct until you approve it.
• Start without a Ramp card: You can use Stack on its own, with no Ramp card or credit application.
• Get running in an hour: You can have Stack set up and running within 1 hour.

Try Stack for free to see how agents prepare your period-end inventory entries and why more than 85,000 businesses use Ramp to save time and money.