To calculate work-in-process inventory in manufacturing, take your beginning WIP balance, add every dollar spent on production during the period, and subtract the cost of goods that finished production and moved to the finished goods warehouse. Written as a formula: Ending WIP = Beginning WIP + Total Manufacturing Costs − Cost of Goods Manufactured. The result is the value of everything still sitting on your production floor in an unfinished state, and it appears as a current asset on your balance sheet.
The Three Inputs to the Formula
Every WIP calculation rests on three numbers. Get any one of them wrong and the ending balance is wrong.
Beginning WIP inventory is the dollar value of unfinished goods carried over from the prior period. It comes straight from last period’s balance sheet and must match that period’s ending WIP figure exactly. If the two don’t tie, everything downstream is off.
Total manufacturing costs are what you spent on production during the current period: direct materials, direct labor, and manufacturing overhead. This is the fresh investment flowing onto the floor.
Cost of goods manufactured (COGM) is the total value of items that crossed the finish line and moved into finished goods. You pull this from completed work orders and production transfer records.
Adding beginning WIP to total manufacturing costs gives you the maximum value of everything that touched the floor during the period. Subtracting COGM removes what got finished. What’s left is your ending WIP, which then rolls forward as next period’s beginning balance.
Building the Total Manufacturing Costs Figure
Total manufacturing costs split into three buckets. Each has its own tracking method and its own tax treatment.
Direct Materials
Direct materials are the physical components that become part of the finished product: lumber in furniture, steel in auto parts, fabric in clothing. Track them through material requisition forms that document what left the raw materials warehouse for the production floor. Under federal tax rules, these costs must be capitalized into inventory rather than deducted immediately. The regulations under Section 263A require manufacturers to include all direct costs in their inventory valuation.1eCFR. 26 CFR 1.263A-1
Direct Labor
Direct labor is wages and benefits for employees who physically transform materials into finished goods. Include hourly pay, overtime, and the employer’s share of Social Security and Medicare taxes. The employer portion runs 6.2% for Social Security (on wages up to $184,500 in 2026) plus 1.45% for Medicare, totaling 7.65% on most production wages.2Social Security Administration. Contribution and Benefit Base Only production hours count. Payroll for office staff, sales teams, and other non-production employees stays out.
Manufacturing Overhead
Overhead is the indirect production cost that can’t be traced to a single unit: factory rent, utilities, equipment depreciation, and salaries for floor supervisors and maintenance crews. Equipment depreciation is typically calculated using the Modified Accelerated Cost Recovery System (MACRS), which spreads the cost over the asset’s class life using declining percentages.3Internal Revenue Service. Depreciation Frequently Asked Questions
Because overhead doesn’t attach to individual products, most manufacturers allocate it using a predetermined rate tied to a measurable driver like machine hours or direct labor hours. If you estimate $300,000 in annual overhead and 10,000 machine hours, your rate is $30 per machine hour, and every job gets charged $30 for each machine hour it consumes. The estimate rarely matches actual spending, which creates variances that have to be cleaned up later.
A Worked Example
Say your furniture operation starts the month with $50,000 in beginning WIP. During the month you use $120,000 in lumber and hardware, pay $80,000 in production wages and employer payroll taxes, and apply $60,000 in factory overhead. Total manufacturing costs for the month: $260,000.
Production records show that $240,000 worth of furniture completed assembly, finishing, and quality check and moved to the finished goods warehouse. That’s your COGM.
The math: $50,000 + $260,000 − $240,000 = $70,000 ending WIP. That $70,000 represents partially assembled furniture still on the floor at month-end, and it becomes next month’s beginning balance.
If that number seems high relative to your output, it may signal a production bottleneck or stalled jobs piling up. If it seems unusually low, costs may not be flowing into WIP correctly. Either way, the number deserves scrutiny.
Valuing Partially Completed Units
A half-finished unit doesn’t carry the same cost as a completed one, but it isn’t worthless either. Equivalent units convert partially completed goods into the number of fully completed units they represent. Two hundred units that are 40% complete equal 80 equivalent units.
This matters most when you produce identical units in a continuous flow and can’t trace costs to specific jobs. The weighted-average method is the common approach: take total costs available (beginning WIP costs plus current period costs) and divide by total equivalent units (units transferred out plus equivalent units in ending WIP). Multiply the resulting cost per equivalent unit by the equivalent units in ending WIP to get its dollar value.
Completion percentages are the tricky piece. Materials might be 100% added at the start of a process while labor and overhead accumulate gradually. A unit that’s 60% complete for conversion costs but 100% complete for materials has to be split into separate equivalent-unit calculations for each cost category. Auditors watch these estimates closely because small percentage errors get multiplied across thousands of units.
How Your Costing Choices Change the Result
The same production activity can produce different WIP balances depending on which costing framework you use. Three choices matter.
Job order vs. process costing. Job order costing suits distinct products or batches — custom furniture, construction, aircraft. Every job carries its own cost sheet and materials and labor trace directly to it. Process costing suits identical units in continuous flow — cereal, paint, chemicals — where costs accumulate by department and spread across units. The choice isn’t optional in any real sense; your production process dictates it. Using the wrong system produces WIP figures that don’t reflect reality.
Standard vs. actual costing. Standard costing books every WIP transaction at a predetermined rate based on engineering estimates and historical data, then reconciles variances at period-end. It’s fast, but any gap between expected and actual spending gets written off later. Actual costing charges WIP with real costs as they hit, which is more accurate but harder to run in real time, especially for overhead. Most large manufacturers use standard costing and adjust at period-end.
FIFO vs. LIFO. FIFO (first-in, first-out) assumes the oldest costs flow out first, so during inflation lower costs move to cost of goods sold and higher costs stay in WIP and ending inventory. LIFO (last-in, first-out) does the opposite: newer, higher costs go to cost of goods sold, reducing taxable income, while WIP reflects older, lower costs. Federal tax rules permit both, but switching requires IRS approval and LIFO carries its own conformity requirements.4Internal Revenue Service. Publication 538 – Accounting Periods and Methods
Adjustments That Correct the Balance
Overhead Variances
Because overhead gets applied using a predetermined rate, the amount charged to WIP almost never matches actual overhead spending. Applied more than incurred means overhead is overapplied and WIP-related costs are inflated. Applied less means underapplied, and WIP is understated.
At year-end the variance has to clear. For small amounts, most companies just adjust cost of goods sold: underapplied overhead debits cost of goods sold, overapplied credits it. When the variance is large enough to distort the financials materially, allocate it proportionally across WIP, finished goods, and cost of goods sold. Skip the step and both the inventory balance and the income statement come out slightly off, which auditors will flag.
Normal and Abnormal Spoilage
Production waste splits into two categories with very different treatments. Normal spoilage, the routine kind, gets absorbed into inventory costs. If 2% of raw material typically gets scrapped during cutting, that waste is a production cost baked into the surviving units. Abnormal spoilage from equipment failures, operator errors, or other unusual events gets expensed immediately as a period cost and never enters WIP. Capitalizing abnormal spoilage would overstate inventory and understate current expenses.
Physical Counts and Cycle Counting
The formula is only as reliable as the data feeding it. Physical counts serve as the reality check. Most manufacturers run a full physical count at least annually, often at year-end when production is lowest, and external auditors generally treat that count as essential to financial statement reliability.
Between full counts, cycle counting keeps the numbers honest by counting a rotating subset of items on a regular schedule. Some companies cycle-count high-value WIP monthly and lower-value items quarterly. When the physical count doesn’t match the book balance, WIP gets adjusted through a shrinkage entry. Common causes include unrecorded scrap, miscounted material requisitions, and data entry errors in production reporting. Rely solely on the formula without verifying the floor and you accumulate phantom inventory: balances that look fine on paper but don’t correspond to anything real.
Federal Tax Rules That Constrain the Calculation
Two Internal Revenue Code provisions shape how manufacturers must handle WIP for tax purposes.
Section 471 requires any taxpayer whose income depends on producing or selling merchandise to maintain inventories using a method that clearly reflects income.5Office of the Law Revision Counsel. 26 USC 471 – General Rule for Inventories You can’t simply deduct production costs in the year incurred; they flow through inventory and hit cost of goods sold when the finished goods are sold. The IRS requires that your valuation method conform to generally accepted accounting principles and remain consistent from year to year.4Internal Revenue Service. Publication 538 – Accounting Periods and Methods
Section 263A, the Uniform Capitalization (UNICAP) rules, goes further. It requires manufacturers to capitalize not only direct materials and labor but also a proper share of indirect costs into inventory. Factory utilities, equipment depreciation, and portions of administrative costs that support production must be included in WIP and finished goods rather than deducted immediately.6Office of the Law Revision Counsel. 26 USC 263A – Capitalization and Inclusion in Inventory Costs of Certain Expenses More costs get trapped in inventory, WIP balances go up, and tax deductions get deferred until the goods are sold.
The Small Business Exemption
Not every manufacturer has to run this full apparatus. If your average annual gross receipts over the prior three tax years don’t exceed $32 million (the threshold for tax years beginning in 2026), you qualify as a small business taxpayer and can opt out of traditional inventory accounting under Section 471(c).7Internal Revenue Service. Rev. Proc. 2025-32
Qualifying businesses get two simplified options. You can treat inventory as non-incidental materials and supplies, deducting costs when the materials are used or consumed rather than tracking them through WIP. Or you can follow the inventory method reflected in your applicable financial statements, or in your internal books and records if you don’t have audited financials.5Office of the Law Revision Counsel. 26 USC 471 – General Rule for Inventories Either approach cuts the bookkeeping burden substantially. Switching is treated as a change in accounting method, so you file Form 3115 to make the transition.