Inventory affects your profit and loss statement through a single channel: cost of goods sold. The stock sitting in your warehouse at the close of a period is money kept off your current expenses, and the stock that moved out the door is money added to them. How much you hold, how you value it, and what adjustments you take against it all push the profit figure on your P&L up or down.
How Inventory Reaches Your P&L
Inventory itself is a balance sheet asset. It only touches the P&L when it converts into cost of goods sold (COGS), and the conversion follows one formula: beginning inventory, plus purchases or production during the period, minus ending inventory. What’s left is the cost assigned to whatever you actually sold.
This is the matching principle at work. Expenses belong in the same period as the revenue they produced. While a product sits unsold, the money you spent on it stays parked on the balance sheet. The moment it sells, that cost jumps to the COGS line and reduces your gross profit.
The IRS builds its rules around the same framework. Businesses required to keep inventories must value them at both the beginning and end of each tax year to calculate COGS on their return.1Internal Revenue Service. Publication 538 – Accounting Periods and Methods For a retailer, the costs flowing through the calculation are mostly purchase prices and freight. For a manufacturer, direct labor, raw materials, and factory overhead fold in as well.
Why Higher Ending Inventory Produces Higher Reported Profit
The formula creates a mechanical result that catches many owners off guard. Because ending inventory is subtracted from total available goods, a larger ending balance means a smaller COGS. A smaller COGS means higher gross profit, even if you didn’t sell an extra unit.
Picture two quarters with identical revenue and identical purchases. In Quarter 1 you sold through most of your stock. In Quarter 2 a big shipment arrived late and sat on the shelf. Quarter 2 will report a higher profit, not because the business did better, but because more costs got deferred. Those costs haven’t gone away. They will hit the P&L whenever the goods finally sell or get written down.
Lenders and investors read inventory trends across periods for this reason. A business steadily building inventory can be reporting rising profits on paper while struggling to move product. A business drawing inventory down can look less profitable in the short term even as cash flow improves. The P&L reports what the accounting rules say happened. The inventory balance hints at what’s really going on.
How Valuation Methods Change the Profit Number
Two businesses can buy the same goods at the same prices and report different profits, because the method they use to assign costs to sold versus unsold units determines which dollars move to COGS first.
First-In, First-Out
FIFO assumes the oldest costs are expensed first. When prices rise, the older, cheaper costs move to COGS while the newer, pricier goods remain on the balance sheet. Reported profit is higher and the inventory value on the balance sheet is higher. The tax bill follows.
Last-In, First-Out
LIFO reverses the assumption. The most recently purchased costs hit COGS first. In an inflationary period, the most expensive costs are matched to current sales, reported profit drops, and tax liability drops with it. The trade-off is a balance sheet inventory value that can grow stale, reflecting costs from many years back. LIFO is permitted under U.S. tax law.2Office of the Law Revision Counsel. 26 USC 472 – Last-in, First-out Inventories If you elect it for taxes, you have to use it for financial statements too, so you can’t show FIFO profits to investors while claiming LIFO income to the IRS.
Weighted Average Cost
This method blends the cost of all units available for sale during a period and assigns that average to each unit sold. It smooths out price swings and produces profit figures between what FIFO and LIFO would show. Businesses with high volumes of interchangeable goods often prefer it because tracking individual unit costs isn’t practical.
Specific Identification
When every item is unique and high-value, you track the actual cost of each unit and expense exactly that cost when it sells. Car dealerships, art galleries, and custom manufacturers use this approach because each item has a meaningfully different cost basis. It produces the most precise COGS figure but only makes sense for low-volume, high-dollar inventory.
Sticking With Your Method
Whichever method you choose, the IRS expects consistency. Section 446 requires taxpayers to use a consistent accounting method and to get IRS consent before switching.3Office of the Law Revision Counsel. 26 US Code 446 – General Rule for Methods of Accounting This prevents businesses from toggling methods to cherry-pick the most favorable tax result each year.
Write-Downs, Shrinkage, and Impairment
Inventory doesn’t always hold its value. Products go out of style, food expires, components become obsolete, and theft happens. When value drops below what you paid, you have to adjust the books, and the adjustment lands on the P&L as an expense with no offsetting revenue.
Under current GAAP, businesses using FIFO or weighted average cost must measure inventory at the lower of original cost or net realizable value, which is what you’d expect to receive from selling it minus the costs to complete and sell it. GAAP replaced the older “lower of cost or market” test for these methods through ASU 2015-11. The market-based test still applies to LIFO inventory. The IRS has its own rules for recognizing inventory losses, and the two systems don’t always line up.
Shrinkage is the other common source of write-downs. The gap between what your books say you have and what a physical count reveals is recorded as an expense. Theft, shipping errors, storage damage, and simple miscounting all contribute. These adjustments erode profit margin without adding any revenue. A business that hasn’t reconciled physical inventory in a long time can face a painful correction when the true count finally happens.
Costs You Have to Capitalize Into Inventory
Purchase price is only the starting point. Section 263A of the Internal Revenue Code, known as the uniform capitalization or UNICAP rules, requires businesses to fold both direct costs and a proper share of indirect costs into inventory value.4Office of the Law Revision Counsel. 26 USC 263A – Capitalization and Inclusion in Inventory Costs of Certain Expenses Factory rent, production equipment depreciation, warehouse utilities, quality control labor: none of these get deducted immediately. They’re baked into inventory cost and only reach the P&L when the goods sell.
For manufacturers the effect is pronounced. Raw materials, production wages, and allocated factory overhead all become part of inventory. A manufacturer building up finished goods is simultaneously deferring the recognition of all those production costs. The P&L won’t show them until the products ship, which can open a wide gap between when cash leaves the business and when the expense appears on the income statement.
Resellers face a lighter version of the same rules. They don’t have production costs to capitalize, but they still need to include freight, warehousing, and purchasing department costs in inventory value under Section 263A if they exceed the small business threshold.
The Small Business Exemption
Not every business has to deal with the full weight of UNICAP or the standard inventory rules. Section 471(c) exempts businesses that meet the gross receipts test under Section 448(c).5Office of the Law Revision Counsel. 26 USC 471 – General Rule for Inventories For tax years beginning in 2026, that threshold is $32 million in average annual gross receipts over the prior three years.6Internal Revenue Service. Rev. Proc. 2025-32
Qualifying businesses can treat inventory as non-incidental materials and supplies. That means they deduct the cost when they use or sell the items, without running the full beginning-plus-purchases-minus-ending calculation. They’re also exempt from Section 263A. This is a meaningful break for smaller retailers, restaurants, and tradespeople who would otherwise have to track and capitalize indirect costs that barely move their tax result. Taxpayers using the LIFO method under this small business exception must switch to FIFO, specific identification, or average cost, because the simplified approach doesn’t accommodate LIFO.7eCFR. 26 CFR 1.471-1 – Need for Inventories
Physical Counts and Record Accuracy
Your P&L is only as accurate as your inventory records. Federal rules require businesses to take physical inventory at reasonable intervals and adjust book figures to match what’s actually on the shelves.1Internal Revenue Service. Publication 538 – Accounting Periods and Methods The tax code allows estimates of shrinkage between counts, but only if you count physically on a regular basis and correct your estimates once the actual numbers come in.5Office of the Law Revision Counsel. 26 USC 471 – General Rule for Inventories
Every discrepancy between book count and physical count is an adjustment that flows through to profit. If your books show $500,000 in inventory and the count reveals $470,000, the $30,000 gap is an expense. Perpetual inventory systems left unverified accumulate these discrepancies silently until year-end or an audit forces a reckoning. The longer between counts, the larger the potential correction.
Changing Methods
Switching from one inventory method to another isn’t a matter of picking differently on next year’s return. The IRS treats it as a formal change in method of accounting, which requires filing Form 3115 and obtaining consent before the new method applies.8Internal Revenue Service. Instructions for Form 3115 Many inventory changes qualify for automatic consent, but some require a separate application and a user fee.
The reason for the control is the same reason consistency matters: changing methods changes reported profit. Moving from LIFO to FIFO in an inflationary period would immediately boost reported income. The IRS requires a Section 481(a) adjustment when you change methods, spreading the cumulative difference between the old and new method over a set number of years so the transition doesn’t create either a windfall deduction or a spike in taxable income in one year.3Office of the Law Revision Counsel. 26 US Code 446 – General Rule for Methods of Accounting Getting it wrong can trigger penalties, so most businesses run the change past a tax professional before filing.