Inventory & COGS

~7 min read

Background

Why it matters

Inventory is the line item most likely to be wrong on the balance sheet and the one that most directly damages earnings when it is. It is also the clearest example in the subject of an accounting choice (FIFO, LIFO, average cost) that changes reported profit with no change in economics, and of the COGS ↔ inventory loop that makes gross margin move in ways you must understand rather than misread. This matters most for retail, consumer, pharma, and semiconductor roles; less so for banks or asset-light software.

Core concepts

Inventory is an asset until it’s sold, then it becomes COGS. The moment of sale is the moment the cost moves from the balance sheet to the income statement. That’s why inventory builds suppress COGS and drawdowns inflate it — and why a company can temporarily boost gross margin by slowing restocking, then pay for it later with writedowns.
The COGS loop. COGS = Beginning inventory + Purchases − Ending inventory. Because COGS is a plug that depends on the inventory balance, anything that changes estimated inventory value changes reported gross profit one-for-one. If inventory is written down by $X, COGS rises by $X and gross profit falls by $X immediately.
The three cost-flow methods (US GAAP allows all three; IFRS forbids LIFO):
- FIFO — first in, first out. In a rising-cost environment, COGS reflects older, cheaper costs, so reported gross margin is higher.
- LIFO — last in, first out. COGS reflects recent, higher costs, so reported gross margin is lower — a deliberate conservative choice, and one that creates a LIFO reserve on the balance sheet when prices fall.
- Average cost — smooths across the period; margin moves with the average of old and new costs.
- The economics are identical; only the reported COGS, gross margin, and inventory balance differ. In an inflationary period, FIFO also produces a lower taxable income, which is why the LIFO vs. FIFO choice often flips for tax reasons. (Note: the balance-sheet inventory and the tax basis can differ, giving you a deferred-tax line.)
Inventory is reported at the lower of cost and net realizable value (LCNRV). If inventory is worth less than it cost — because it’s obsolete, damaged, or selling below cost — it must be written down. Writedowns are the loudest early-warning signal in retail, and they are frequently below the operating line, so a company can show a “great” operating margin while quietly destroying inventory value.
Working capital is the strategic story. Inventory days (DIO) is a direct cash commitment, and a company that lengthens DIO is funding its suppliers. Too little inventory risks stockouts and lost sales; too much ties up cash and raises obsolescence risk. The right level is a competitive decision, and the wrong one shows up in both the cash flow statement and the gross margin.
“Inventory” is not homogeneous. Finished goods, work-in-progress, and raw materials have different turnover, different obsolescence risk, and — under LIFO — different cost layers. A company shifting toward older, slower-moving SKUs (a common retail trick when new product disappoints) quietly changes the mix and hides markdowns. Always check the inventory note and, where disclosed, reserves against slow-moving stock.

Mental model

  THE COGS ↔ INVENTORY LOOP

  Beginning inventory
    +  Purchases
    −  Ending inventory
    =  COGS  →  Revenue − COGS = Gross profit
      ↑              ↓
      └──── writedowns flow straight through here

  CONSEQUENCE: if you estimate inventory $50M too HIGH,
  COGS is understated by $50M and gross profit is OVERSTATED by $50M.

  ─────────────────────────────────────────────
  INFLATING PRICES:  costs 10 → 12 → 14
  FIFO:  COGS uses  10, 12  →  gross margin HIGH
  LIFO:  COGS uses  12, 14  →  gross margin LOW (same cash, same business)
  ─────────────────────────────────────────────

  STRATEGIC LEVER:  Slow restocking → lower COGS → higher margin NOW,
  but less product on hand → lost sales, and a future writedown.

Interview questions

A retailer’s gross margin expanded 200bps while revenue fell 3%. Why is that suspicious? Answer: Falling revenue with rising margin is a hard combination to explain operationally, so I’d look for: (a) LIFO or a rising-cost environment inflating the gap, (b) a change in product mix toward higher-margin items, (c) inventory drawdown boosting COGS comparability, or (d) inventory writedowns taken in a prior year making this year’s cost basis artificially low — the classic “taking the hit early to set up easy comps.” Ask whether inventory levels and the LIFO reserve moved, and whether the writedown history is a pattern. A genuine margin expansion on falling sales is possible (price increases, cost deflation) but is the exception.
Explain the “inventory reserve” and why a company might take a large writedown in a weak quarter. Answer: The reserve (or allowance) reduces inventory to net realizable value. Companies take large writedowns in weak periods to “clean up” the balance sheet, front-load future costs, and set a lower base for future COGS — which mechanically raises future gross margins. It’s an earnings-management pattern, and it’s why analysts watch the inventory balance and writedown history rather than just the margin. If a company writes down inventory every 2–3 years, treat the “adjusted” gross margin as the real one and ignore the reported margin in the “bad” year.
What is the difference between LIFO reserve and LIFO liquidation, and why does the second matter more? Answer: A LIFO reserve is the gap between inventory on a LIFO basis and on a FIFO basis — a permanent balance sheet reconciliation, growing as prices rise. LIFO liquidation is when a company sells more than it replaces during a period, so older, cheaper cost layers flow into COGS. That produces a temporary spike in gross margin that is purely mechanical and non-repeatable. A company that liquidates LIFO in a year of good margins is borrowing profit from the past — and the margin will reverse once it must buy at current higher prices. Watch for this specifically in industrials and chemicals.
A company has inventory of $800M on $5B of revenue. Days inventory is ~58. Its peers are at 90. What questions do you ask? Answer: First: is 58 days good? Maybe — a business with shorter, more predictable demand (groceries, quick-turn manufacturing) legitimately runs lean. Second: is inventory valued aggressively? A low DIO with a big writedown history suggests they sold through excess stock and are now short. Third: is it a mix effect — is a high-margin, fast-moving SKU shrinking? Fourth: is the cost flow assumption (LIFO vs. FIFO) flattering the number? And fifth: does the low DIO come with high stockout risk? I’d want to see inventory by category and the 3-year trend before concluding it’s efficiency.
Why is COGS sometimes disclosed below the operating line? Answer: Because companies can classify certain costs as “other operating expenses” rather than COGS, or present a combined “cost of revenue” line that includes items you wouldn’t predict. The effect: gross margin becomes non-comparable across companies, and “operating income” stays the robust number (see Expenses & matching). If you’re comparing gross margins, check the policy note and confirm both companies classify costs the same way — a 500bp “gross margin improvement” that is entirely a reclassification is a reporting artifact.
How does inventory affect the cash flow statement, and why can a company “grow” its CFO by liquidating inventory? Answer: Inventory is an operating asset: an increase is a use of cash (negative CFO), a decrease is a source (positive CFO). So a company can generate cash in a tough year by selling down inventory — accepting lost future sales to fund current obligations. That’s a real, and sometimes necessary, tactic, but it’s finite: inventory can only be drawn down once, and after that the CFO benefit reverses while the lost-margin cost persists. This is why “CFO improved” must always be checked against inventory and receivables movements, not read as pure operational strength.

Watch

INVENTORY & COST OF GOODS SOLD — Accounting Stuff. The link between inventory on the balance sheet and revenue/COGS on the income statement, with a worked example and manufacturing-vs-merchandising framing.
The Essential Guide to Inventory in Accounting — Accounting Stuff. A longer compilation covering perpetual vs. periodic systems and all three cost-flow methods (FIFO, LIFO, average cost). Heavily timestamped if you only need one section.
FINANCIAL RATIOS: How to Analyze Financial Statements — Accounting Stuff. The efficiency ratios — inventory turnover, DSI, DIO, DPO, and the cash conversion cycle — run from roughly the 10:29 mark.