Work in process inventory is the accumulated cost of goods that have started production but aren’t yet finished — the direct materials, direct labor, and manufacturing overhead absorbed by units still moving through the factory. It sits on the balance sheet as a current asset, feeds directly into taxable income when goods are sold, and is governed by both GAAP measurement rules and specific provisions of the Internal Revenue Code. Getting the number right matters because errors distort reported profit and can expose you to accuracy-related tax penalties.
What Goes Into the WIP Number
Three cost categories make up every dollar of WIP value, and the boundaries between them matter.
Direct materials are the physical inputs being shaped or assembled into the product itself: steel, resin, lumber, electronic components. If you can trace a material to a specific unit, it belongs here. General shop supplies like lubricants and cleaning solvents don’t; they fall under overhead.
Direct labor covers the wages, payroll taxes, and benefits of workers who physically handle the product — machine operators, welders, assembly technicians. A supervisor’s salary can’t be traced to a single unit, so it moves to overhead.
Manufacturing overhead is every production cost that isn’t direct materials or direct labor: equipment depreciation, factory rent, utilities, maintenance, quality control. These indirect costs get allocated to WIP through a predetermined overhead rate, usually based on machine hours or direct labor hours. Underallocating understates inventory on the balance sheet; overallocating inflates it.
The WIP Inventory Formula
The core calculation is straightforward:
Ending WIP = Beginning WIP + Total Manufacturing Costs − Cost of Goods Manufactured
Beginning WIP is the ending balance from the prior period, carried forward. Total manufacturing costs are everything added during the current period: direct materials used, direct labor incurred, and manufacturing overhead applied. Cost of goods manufactured (COGM) is the total cost of units actually completed and transferred to the finished goods warehouse during the period. Subtracting COGM from the sum of beginning WIP and current-period costs leaves the value of units still in production.
Most errors enter through overhead allocation. If your predetermined overhead rate was set at the start of the year using estimates, actual overhead almost certainly diverges. The gap between applied and actual overhead has to be reconciled at period end, either adjusted into cost of goods sold or prorated across WIP, finished goods, and cost of goods sold.
Costing Partially Completed Units
Half-finished products can’t be counted the same way as completed goods. If you have 1,000 units that are 40% complete, you effectively have 400 equivalent units of production for cost-assignment purposes. Multiply physical units by their percentage of completion and you have the figure that costs get spread across.
The complication is that materials, labor, and overhead often enter at different points. Raw materials may be added entirely at the start while labor and overhead accumulate gradually. That means equivalent units get calculated separately for each cost component. A unit that’s 100% complete for materials but 60% complete for labor has different equivalent-unit figures in each category.
Two methods handle this differently. The weighted-average method lumps together all costs from beginning inventory and the current period, then spreads them across all equivalent units — transferred-out units plus ending WIP. FIFO separates work done in the current period from work carried over in beginning inventory, giving a cleaner picture of current-period efficiency. FIFO takes more work but produces more useful data for performance evaluation.
Valuation Methods and the Tax Consequences
The valuation method you choose determines which costs land in WIP and finished goods versus cost of goods sold. In the U.S., the two primary options are FIFO and LIFO, and the choice has real tax effects.
FIFO (first-in, first-out) assumes the oldest costs flow to cost of goods sold first. When prices are rising, lower costs hit the income statement, producing higher reported profit and higher taxable income. The inventory left on the balance sheet reflects more recent costs.
LIFO (last-in, first-out) assumes the newest costs flow out first. When prices are rising, LIFO pushes higher costs into cost of goods sold, reducing reported profit and lowering the tax bill. The tradeoff is that balance sheet inventory may reflect costs from years or even decades earlier.
LIFO comes with a significant compliance requirement. Under federal tax law, if you elect LIFO for tax purposes, you must also use LIFO in any financial statements you provide to shareholders, creditors, or other outside parties.1Office of the Law Revision Counsel. 26 USC 472 – Last-in, First-out Inventories You can’t report lower profits to the IRS under LIFO while showing investors rosier numbers under FIFO. To elect LIFO, file Form 970 with your tax return for the first year you adopt it.2Internal Revenue Service. About Form 970, Application to Use LIFO Inventory Method Any later change requires IRS approval through Form 3115.
One LIFO-specific risk: if you sell more inventory than you purchase in a period, you start liquidating older, lower-cost layers. This “LIFO liquidation” can trigger an unexpectedly large tax bill because those older costs produce artificially high profits. If you later abandon LIFO entirely, recapture rules require adding the difference between LIFO and FIFO inventory values back into taxable income, typically spread over four years.
Lower of Cost or Net Realizable Value
Whichever non-LIFO method you use, GAAP requires you to measure inventory at the lower of recorded cost or net realizable value. Net realizable value is what you expect to sell the finished product for, minus remaining costs to complete, sell, and ship it. When evidence shows net realizable value has dropped below cost through damage, obsolescence, or falling market prices, you write the inventory down and recognize the loss in the current period.3FASB. Accounting Standards Update 2015-11, Inventory (Topic 330) For manufacturers with volatile commodity prices or fast-changing product lines, this becomes a recurring exercise at each reporting period.
Normal and Abnormal Spoilage
Not every unit that enters production makes it out the other side, and how you handle losses affects WIP valuation.
Normal spoilage is the waste you expect under efficient operating conditions — a predictable reject rate built into the process. These costs get absorbed into the cost of the good units produced, raising the per-unit cost of remaining WIP and finished goods. That treatment is correct because normal spoilage is inherent to manufacturing.
Abnormal spoilage is anything above the expected level: equipment malfunctions, operator errors, contaminated batches. Under both GAAP and international standards, abnormal spoilage must be expensed immediately as a period cost rather than folded into inventory. This keeps you from inflating reported inventory value with costs that reflect inefficiency. Calculate it by subtracting normal spoilage units from total spoilage units, then multiplying the abnormal units by the cost per equivalent unit.
IRS Rules for Inventory Accounting
Federal tax law requires businesses to account for inventory using a method that clearly reflects income. Section 471 gives the IRS authority to prescribe inventory methods that conform to best accounting practices in your trade or business.4Office of the Law Revision Counsel. 26 USC 471 – General Rule for Inventories
Uniform Capitalization (UNICAP)
Section 263A requires manufacturers to capitalize both direct costs and a proper share of indirect costs into inventory, including WIP. You can’t immediately deduct overhead items like factory rent or equipment depreciation; they must be absorbed into inventory value and recognized only when the goods are sold.5Office of the Law Revision Counsel. 26 USC 263A – Capitalization and Inclusion in Inventory Costs The allocation rules require detailed tracking of indirect costs, which is where much of the complexity lives.
The Small Business Exemption
The Tax Cuts and Jobs Act carved out a meaningful exception. If your average annual gross receipts over the prior three tax years don’t exceed the inflation-adjusted threshold — $32 million for tax years beginning in 2026 — you’re exempt from both the Section 471 general inventory rules and the Section 263A capitalization requirements.6Internal Revenue Service. Revenue Procedure 2025-325Office of the Law Revision Counsel. 26 USC 263A – Capitalization and Inclusion in Inventory Costs
Qualifying small businesses can choose between two simplified approaches: treat inventory as non-incidental materials and supplies (deducting costs when the materials are first used in production), or follow the inventory method used on their financial statements or internal books.4Office of the Law Revision Counsel. 26 USC 471 – General Rule for Inventories For a small manufacturer, that can eliminate complex cost-allocation calculations entirely. Tax shelters are excluded from this exemption regardless of gross receipts.
Penalties for Misstatement
Inventory errors that cause tax underpayment carry real consequences. An accuracy-related underpayment from negligence or a substantial misstatement of income triggers a penalty equal to 20% of the underpaid amount.7Office of the Law Revision Counsel. 26 USC 6662 – Imposition of Accuracy-Related Penalty on Underpayments If the IRS determines the misstatement was fraudulent, the penalty rises to 75% of the portion attributable to fraud.8Office of the Law Revision Counsel. 26 USC 6663 – Imposition of Fraud Penalty These aren’t inventory-specific, but inventory manipulation is a common route to them.
Where WIP Sits on the Balance Sheet
Under U.S. GAAP, WIP inventory is a current asset, grouped with raw materials and finished goods. That classification feeds directly into your current ratio and working capital calculations, both of which lenders and investors watch. Large WIP balances relative to finished goods can signal production bottlenecks — capital tied up in half-built products isn’t generating revenue.
Both U.S. GAAP under ASC 330 and International Financial Reporting Standards under IAS 2 define inventory to include goods in the process of production for sale.9KPMG. Inventory Accounting: IFRS Standards vs US GAAP Broad objectives are similar, but differences in measurement and disclosure can affect comparability for companies reporting under both frameworks.
When units are completed, a journal entry moves their accumulated cost from WIP to finished goods. When a customer buys the product, another entry shifts that cost to cost of goods sold on the income statement. This chain keeps expenses recognized in the same period as related revenue, which is a foundational requirement of accrual accounting.
Job Order Costing or Process Costing
The right tracking system depends on what you make and how.
Job order costing assigns costs to each individual job, batch, or production run. If you build custom machinery, fabricate specialized parts to order, or produce anything where each unit or small batch is meaningfully different, this is your system. Every job gets its own cost sheet tracking specific materials, labor hours, and overhead. You gain precision — you know what each job costs and can identify unprofitable work — at the cost of administrative weight, since someone has to record every material requisition and time entry against the right job number.
Process costing averages costs across large volumes of identical or near-identical units. Chemical plants, food processors, paper mills, and continuous-flow operations typically use this approach. Instead of tracking individual units, you track costs by department or production stage and divide total costs by equivalent units of output. Simpler, but with less visibility into unit-level cost variations.
Hybrid approaches are common. An electronics company might use process costing for circuit board fabrication and job order costing for final assembly of customized systems. Whatever the setup, the documentation has to be detailed enough for auditors to trace costs from raw material purchases through WIP to finished goods and eventually to cost of goods sold.
Keeping the Number Accurate
WIP is uniquely vulnerable to errors and fraud because it’s in constant motion. Units move between workstations, materials get consumed and scrapped, and costs accumulate across multiple accounts at once. Without solid controls, your reported WIP can drift significantly from reality.
Separation of duties is the most basic protection. The person who physically handles inventory shouldn’t be the same one who maintains the accounting records for it. When one employee’s work checks another’s, hiding stolen materials by altering records becomes much harder. Rotating job assignments discourages long-running schemes because a new person is more likely to notice discrepancies the previous holder created or concealed.
Physical counts remain essential, but shutting down a factory for a wall-to-wall count is expensive. Cycle counting offers an alternative: count a small, targeted subset of inventory on a rotating schedule and compare to system records. Over time, every item gets counted at least once a year without halting production. When a cycle count reveals a discrepancy, investigate immediately; it could be a data entry error, an unrecorded scrap event, or something worse.
Tracking shrinkage by production zone or material category rather than as a single company-wide figure helps pinpoint problems. A 2% overall shrinkage rate might mask a 15% rate at one workstation, and that’s where the fix lives. Serialized purchase orders, material requisitions, and receiving reports add another layer; periodically checking the numerical sequence against accounting records catches fictitious transactions and duplicate payments before they compound.