Inventory accounting is how you record the stock you own and the cost of what you sell. Inventory is treated as an asset on your balance sheet when you buy it, valued at landed cost, and becomes an expense, cost of goods sold, only when the unit sells. This timing, capitalizing inventory and expensing it on sale, is the core of inventory accounting and what makes an inventory business's profit meaningful month to month.
When does inventory become an expense?
When it sells, not when you pay for it. Buying inventory converts cash into an asset of equal value, so it does not hit your profit and loss. When a unit sells, its landed cost moves from the inventory asset to cost of goods sold on the P&L. Expensing inventory when you pay the supplier instead is the most common inventory-accounting mistake and it distorts every monthly profit figure.
Andrew Erickson is the founder of Inventory Hero. He has spent years working with Amazon FBA sellers on demand forecasting, restock planning, and the cash flow side of running a private-label brand. Inventory Hero exists because every spreadsheet-based inventory system he tried eventually broke — usually right before Q4.
What is the difference between perpetual and periodic inventory?
Perpetual inventory updates your inventory and COGS continuously as units move, so you always have a current figure. Periodic inventory updates only at intervals via a physical count, computing COGS at period end. FBA sellers generally need perpetual (usually via software) because the volume and the pace of sales make periodic counting too coarse for good decisions.
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Inventory accounting is the discipline of recording the stock you own and the cost of what you sell, and its central rule is one of timing: inventory is an asset from the day you buy it and becomes an expense only when it sells. The short version: buying inventory does not reduce your profit (it converts cash to an asset), selling it does (its cost moves to COGS), and getting that timing right is what makes an inventory business's monthly numbers honest. This is a practical overview, not tax advice. Below is the core flow, perpetual versus periodic, and why it matters for FBA.
You buy it. Your cash (or accounts payable) goes down, and your inventory asset goes up by the unit's landed cost. Profit is unchanged, because you swapped one asset for another of equal value.
You sell it. Revenue is recognized, and the unit's landed cost moves from the inventory asset to cost of goods sold on your profit and loss. Now, and only now, does the cost hit your profit.
That is the whole engine. Everything else in inventory accounting is bookkeeping around these two events, and the discipline is refusing to let inventory touch your P&L until it sells. The actual debits and credits that record them are the inventory journal entries.
Put numbers on it. You buy 500 units at a 9 dollar landed cost, then over the next two months sell 300 of them at 25 dollars:
Your gross profit for the period is 7,500 minus 2,700, or 4,800 dollars. Notice the 4,500 dollar purchase never appears as an expense; only the 2,700 dollars that actually sold does. That is the timing that keeps the P&L honest.
Get the timing wrong, by expensing inventory when you pay for it, and your books lie:
The month of a big buy shows a large loss that is not real; you converted cash to stock, you did not lose money.
The month you sell that stock shows inflated profit, because the cost was already taken earlier.
Neither number helps you make a decision, and both mislead a lender or a buyer. Correct timing, cost recognized against the sale it belongs to, is what lets your monthly profit reflect what actually happened, which is the entire point of keeping books. It is also why Amazon seller accounting leans on accrual rather than cash, and why the cost tied up in inventory is the inventory leg of your cash conversion cycle.
To do this you need a consistent cost per unit, which is your landed cost, the factory price plus freight, duties, and inbound, not the bare invoice; the landed cost calculator helps you build that per-unit number. When units of the same SKU were bought at different costs, you need a valuation method (FIFO or weighted average) to decide which cost flows to COGS on each sale. Consistency is what matters: pick a method and apply it the same way every period, or your margins are not comparable.
For the unit side of the count, your source is Seller Central: the Manage FBA Inventory dashboard shows current on-hand, and the Inventory Ledger report gives historical positions for a period close. Pair those unit counts with your landed cost per SKU and you have the inventory value the books need.
Perpetual updates inventory and COGS continuously as units move, so you always have a current figure. It needs a system, but it gives you real-time inventory value and margins.
Periodic updates only at intervals, using a physical count to compute ending inventory and back into COGS at period end.
FBA sellers generally need perpetual, because the volume and pace of Amazon sales make periodic counting too coarse for good reorder and pricing decisions. You still count periodically to reconcile, but you manage on a continuous figure.
Your ending inventory (what you still hold, at cost) and your COGS (what sold, at cost) are two sides of the same coin, which is why calculating ending inventory accurately is what makes your COGS, and therefore your profit, correct. Get ending inventory wrong and COGS is wrong by the same amount, straight into your bottom line. Put dollars on it: a 900-dollar error in ending inventory is a 900-dollar error in COGS, which is a 900-dollar swing in profit and a real change in the tax you owe. That is why this is a number to get right, not estimate.
Inventory accounting is the timing discipline of holding stock as an asset until it sells and expensing it as COGS only then. Value units at landed cost, apply one valuation method consistently, track perpetually because FBA volume demands it, and reconcile ending inventory so COGS is right. Done well, your P&L shows the cost of exactly what sold, no more, which is what makes your net profit margin real. For the wider picture, see Amazon seller accounting; for the method choice, inventory valuation.