A change in inventory is the difference between your ending inventory and your beginning inventory for a period. The formula is straightforward: ending inventory minus beginning inventory. A positive number means stock grew; a negative number means stock shrank. That single figure moves cost of goods sold, net income, cash from operations, and several ratios lenders watch, which is why it gets attention well out of proportion to the arithmetic behind it.
How to Calculate the Change
Take the inventory balance at the end of the period and subtract the balance at the start. Both figures come from the same books, valued using the same cost flow method (more on that below). For a manufacturer, the total spans raw materials, work-in-process, and finished goods; for a retailer or wholesaler, it’s usually just finished goods.
A positive change often reflects a planned buildup ahead of a busy season or an investment in future sales. It can also signal trouble. If inventory keeps climbing while revenue stays flat, that points to overstocking, slowing demand, or obsolescence risk.
A negative change usually means sales drew down existing stock faster than replenishment, which is generally healthy. Sustained drawdowns without matching revenue growth can indicate supply problems or deliberate liquidation.
Effect on Cost of Goods Sold and Net Income
The inventory change is the bridge between what you spent on goods and what shows up as an expense. Cost of goods sold equals beginning inventory plus purchases, minus ending inventory. Every dollar sitting in ending inventory is a dollar that does not reduce profit for the period.
When ending inventory rises above beginning inventory, cost of goods sold shrinks. Lower COGS means higher gross profit and, other things equal, higher net income. The extra stock stays on the balance sheet as an asset instead of running through the income statement.
A negative change does the opposite. More goods flowed out than came in, so COGS climbs, gross margin compresses, and net income falls. That’s normal when a company intentionally works down stock, and worth a closer look when the drawdown wasn’t planned.
Effect on the Cash Flow Statement
Under the indirect method, the cash flow statement starts with net income and adjusts for items where accrual accounting and cash movement diverge. Inventory is one of them, because COGS is an accrual figure that doesn’t line up perfectly with cash spent on stock.
An increase in inventory is subtracted from net income. You spent cash to acquire or produce those goods, but the outflow isn’t yet reflected in COGS because the goods haven’t been sold. Subtracting captures the cash tied up in unsold stock.
A decrease in inventory is added back to net income. You sold goods that were already on the shelf from a prior period, so the cash from those sales sits in revenue while the original cash outlay happened earlier. Adding the decrease back prevents double-counting.
This is why cash flow from operations can look very different from net income. A company posting strong profits while rapidly building inventory may actually be burning cash, and lenders pay close attention to that gap when they evaluate a borrower’s ability to service debt.
How Your Cost Flow Method Changes the Number
The dollar value of the change depends on how you assign costs to units. Under U.S. GAAP, four methods are acceptable: First-In, First-Out (FIFO), Last-In, First-Out (LIFO), weighted average cost, and specific identification. Whichever you pick, use it consistently. Switching methods between periods breaks year-over-year comparisons and requires disclosure. For tax purposes, inventory must be taken on a basis that conforms to best accounting practices in the trade or business and clearly reflects income.1Office of the Law Revision Counsel. 26 USC 471 – General Rule for Inventories
FIFO and LIFO
FIFO assumes the oldest items sell first. When input costs are rising, FIFO pushes cheaper older costs into COGS and leaves newer, higher costs in ending inventory. The result is a larger, more positive inventory change and higher reported profit.
LIFO flips this. Newer items are assumed sold first, so during inflation the more expensive recent purchases hit COGS and cheaper old costs stay on the balance sheet. Ending inventory is smaller, the change is smaller, and reported profit is lower. Many companies choose LIFO for exactly that reason: lower reported income means a lower tax bill during inflationary periods.
When costs are falling, these effects reverse. FIFO produces a smaller ending inventory value and LIFO a larger one.
LIFO carries a specific risk. If you dip into old inventory layers, the costs assigned to COGS can be years or decades out of date. Selling through those layers assigns artificially low costs to COGS, inflating both reported income and taxable income. This LIFO liquidation catches businesses off guard during supply shortages or deliberate stock reductions, and the tax hit can be steep.
Federal law also requires any company using LIFO for tax purposes to use LIFO for financial reporting to shareholders and creditors.2Office of the Law Revision Counsel. 26 USC 472 – Last-In, First-Out Inventories The implementing regulation reinforces this by barring the use of any other inventory method for credit or shareholder reporting purposes.3eCFR. 26 CFR 1.472-2 – Requirements Incident to Adoption and Use of LIFO Inventory Method LIFO’s effects can’t be hidden by using a different method in outside reports.
Weighted Average and Specific Identification
Weighted average cost blends all unit costs, smoothing out price swings. It usually lands between FIFO and LIFO for both ending inventory and COGS.
Specific identification tracks the actual cost of each individual item. It fits high-value, distinguishable goods like vehicles or custom equipment, and it’s impractical for high-volume, interchangeable products.
Write-Downs and Shrinkage: Changes That Aren’t Sales
Not every change in the inventory balance comes from buying or selling. Two other sources can move the number, and both flow through the income statement.
Under GAAP, inventory measured using FIFO or weighted average cost must be carried at the lower of its cost or its net realizable value, meaning the estimated selling price minus predictable costs to complete and sell the goods.4Financial Accounting Standards Board. Accounting Standards Update 2015-11 – Inventory (Topic 330) When net realizable value drops below cost, you record a write-down. The inventory balance falls, and a loss is recognized in the period the decline is identified, either through COGS or as a separate line item. Under U.S. GAAP, write-downs are one-way; once marked down, inventory cannot be written back up if the market recovers. Companies using LIFO or the retail inventory method follow the older lower-of-cost-or-market framework, which uses replacement cost bounded by a ceiling and floor, with a similar practical outcome.
Shrinkage is the second source. It’s the gap between recorded inventory and what a physical count actually finds, caused by theft, damage, administrative errors, and vendor fraud. When a count comes in low, the inventory account is reduced and a matching expense is booked, either inside COGS or as a separate shrinkage expense. A company showing $100,000 on the books but $95,000 on the shelves records a $5,000 reduction to inventory and a $5,000 expense. Federal tax rules allow the use of shrinkage estimates between physical counts, as long as the business performs regular counts at each location and adjusts its estimates when actual shrinkage differs from projections.1Office of the Law Revision Counsel. 26 USC 471 – General Rule for Inventories Relying on estimates without ever reconciling to a physical count won’t pass muster with auditors or the IRS.
Ratios That Move With Inventory
The change in inventory ripples into the ratios analysts and lenders use to size up a business.
The current ratio (current assets divided by current liabilities) rises when inventory grows, since inventory is a current asset. That can mislead. Inventory is the least liquid current asset, and a current ratio pushed up by inventory growth doesn’t mean short-term obligations are easily covered.
Inventory turnover equals cost of goods sold divided by average inventory, and it measures how quickly stock converts into sales. A rising inventory balance without a matching rise in COGS drags turnover down and points to slower-moving goods. A declining balance paired with steady or rising COGS lifts turnover, which suggests efficient stock management. Lenders watch this closely because slow-turning inventory ties up cash and raises obsolescence risk.
Gross profit margin moves directly with the change in inventory, because the change determines COGS. A positive change lowers COGS and lifts the margin; a negative change does the opposite. Reading gross margin trends alongside turnover often reveals whether a margin improvement reflects genuine pricing power or just goods piling up on the shelves.