To calculate absorption costing, add the direct materials, direct labor, and variable manufacturing overhead assigned to each unit, then add a share of fixed manufacturing overhead found by dividing total fixed overhead by the number of units produced. The result is the full cost of one unit, and it becomes the value that unit carries on your balance sheet until it sells. This method, also called full costing, is the only inventory valuation approach that satisfies Generally Accepted Accounting Principles for external financial reporting, and the IRS requires inventory methods to conform to GAAP for tax purposes.1Internal Revenue Service. Publication 538 (01/2022), Accounting Periods and Methods
Sort Your Costs First
Before any arithmetic, split your expenses into two buckets. Product costs attach to inventory. Period costs hit the income statement in the period you incur them and never touch a unit’s cost.
Four categories count as product costs under absorption costing:
- Direct materials — raw inputs physically incorporated into the finished product, such as steel, plastic, fabric, or electronic components.
- Direct labor — wages and benefits for employees who physically manufacture or assemble the goods.
- Variable manufacturing overhead — factory costs that rise and fall with production volume, including electricity for machinery, production supplies, and usage-based equipment maintenance.
- Fixed manufacturing overhead — factory costs that stay stable regardless of output, such as building rent, property insurance, equipment depreciation, and production supervisors’ salaries.
Everything outside the factory walls is a period cost. Corporate executive salaries, marketing, non-manufacturing office rent, sales commissions, and shipping to customers all belong on the income statement, not in inventory. The most common mistake in absorption costing is letting a selling or administrative expense slip into the overhead pool, which overstates inventory and understates current-period expenses.
If you run dedicated support departments like maintenance or quality control that exist to serve production, their costs feed into the manufacturing overhead pool before you compute a per-unit rate. Several allocation methods exist for pushing those costs to the production floor, ranging from a simple direct allocation to a reciprocal method that accounts for services support departments provide to each other.
Set an Overhead Rate
Fixed manufacturing overhead arrives as a lump sum. To turn it into a per-unit figure you need an allocation base: a measurable activity that stands in for how much overhead each product consumes. The most common bases are direct labor hours, machine hours, and direct labor dollars.
Pick the base that actually drives your factory costs. Hand-assembly operations usually track well with direct labor hours because more labor time correlates with more supervision, more facility usage, and more wear on tools. Automated plants track better with machine hours because depreciation, electricity, and maintenance rise with how long machines run. A quick test: if doubling the base would roughly double overhead consumption, the proxy is reasonable. A factory that runs three shifts of machines but one shift of labor would pile all its overhead onto the single labor shift if you picked labor hours, understating the cost of anything produced on the other two shifts.
Most companies then set a predetermined overhead rate at the start of the year using budgeted figures rather than waiting for actual results. The formula:
Predetermined overhead rate = Estimated total fixed manufacturing overhead ÷ Estimated total activity base
A factory that budgets $500,000 in annual fixed overhead and expects 25,000 machine hours works out to $20 per machine hour. A product that spends two machine hours in production picks up $40 in fixed overhead. This approach, called normal costing, lets you value inventory and price products throughout the year instead of waiting until year-end to compute actuals. You reconcile the estimate against reality when the books close.
Apply the Formula
With cost categories sorted and an overhead rate in hand, the per-unit calculation follows one sequence:
Total unit cost = Direct materials per unit + Direct labor per unit + Variable overhead per unit + (Total fixed manufacturing overhead ÷ Units produced)
A worked example. A company produces 10,000 units during the period with these costs:
- Direct materials: $10 per unit
- Direct labor: $15 per unit
- Variable manufacturing overhead: $5 per unit
- Total fixed manufacturing overhead: $100,000 for the period
Fixed overhead per unit is $100,000 ÷ 10,000 = $10. Add the four components: $10 + $15 + $5 + $10 = $40 per unit. That $40 is the absorption cost, and it becomes the value each unit carries on the balance sheet until it sells.
Only the fixed overhead component moves when production volume changes. Doubling output to 20,000 units cuts fixed overhead per unit to $5, pulling total unit cost down to $35. The first three components stay flat because they scale with output naturally.
What to Do About Idle Capacity
Dividing fixed overhead by actual units produced works cleanly at normal output. It breaks down when a plant runs well below capacity. A factory that can produce 20,000 units but only makes 8,000 would show fixed overhead of $12.50 per unit instead of the $5 it would show at full capacity. That inflated figure distorts pricing and makes inventory look more expensive than it should.
The accepted treatment uses “normal capacity” as the denominator instead of actual production. Normal capacity is the output you would expect over several periods under typical conditions, smoothed for seasonal swings and one-off disruptions. Fixed overhead per unit stays stable, and the unabsorbed portion representing idle capacity is expensed directly on the income statement as a period cost rather than loaded onto the units you did produce.
Turn Unit Cost Into COGS and Ending Inventory
Once you have the absorption cost per unit, two financial statement figures fall out of it. Cost of goods sold equals per-unit cost multiplied by units sold. Ending inventory equals per-unit cost multiplied by units still on hand.
Back to the earlier example at $40 per unit. If the company produced 10,000 units and sold 8,000, cost of goods sold is $320,000 and ending inventory is $80,000. The $320,000 appears on the income statement, subtracted from revenue to show gross profit. Selling and general administrative costs come out below that line as operating expenses. The $80,000 sits on the balance sheet as a current asset.
These figures have direct tax consequences. Overstating ending inventory understates cost of goods sold, which inflates taxable income and results in overpaying. Understating ending inventory reduces taxable income, which can trigger an accuracy-related penalty of 20% on the resulting underpayment if the IRS determines the error stemmed from negligence or disregard of the rules.2Internal Revenue Service. Accuracy-Related Penalty
Reconcile the Estimate to Actual Overhead
Because the predetermined rate uses budgeted figures, actual overhead almost never lands where you estimated. The gap shows up as a balance in the manufacturing overhead account at year-end. Applied less than you actually incurred, and overhead is underapplied. Applied more, and it is overapplied.
For small variances, close the whole difference into cost of goods sold. An underapplied balance means COGS was understated during the year, so you add the difference. Overapplied means COGS was overstated, so you subtract. One entry corrects the income statement.
Larger variances need proration. Instead of dropping the entire difference into COGS, spread it across work-in-process inventory, finished goods inventory, and cost of goods sold in proportion to their balances. Auditors generally push for proration when the variance is material relative to total production costs.
Compare Cost to Market Before You Report
Calculating absorption cost is not the last step. Both GAAP and the IRS require you to compare the calculated cost of each inventory item against its current market value and report the lower of the two. If market conditions have dropped the value of your finished goods below what it cost to produce them, carrying them at full absorption cost overstates your assets.
For tax purposes, “cost” here means exactly what absorption costing produces: direct materials, direct labor, and properly allocated overhead, including any additional costs required under the uniform capitalization rules. Compare that against current replacement cost or net realizable value, and use the lower. Damaged, obsolete, or out-of-season goods must be valued at their actual selling price minus disposition costs if they have been offered at that price within 30 days of the inventory date.3Internal Revenue Service. Lower of Cost or Market (LCM) An absorption cost of $40 per unit means little if the product can only sell for $30. The write-down reduces inventory on the balance sheet and books the loss in the current period.
The Tax Layer: Section 263A
Absorption costing for financial reporting captures factory-level costs. The IRS goes further. Under the uniform capitalization rules of Internal Revenue Code Section 263A, manufacturers must capitalize not just direct production costs and factory overhead but also a broader set of indirect costs that GAAP-based absorption costing might treat as period expenses.4Office of the Law Revision Counsel. 26 USC 263A – Capitalization and Inclusion in Inventory Costs of Certain Expenses
Those additional costs include purchasing department expenses, warehousing and storage, handling, and portions of officers’ compensation and employee benefits allocable to production activities.5Internal Revenue Service. Section 263A Costs for Self-Constructed Assets A manufacturer’s tax-basis inventory value is often higher than its book-basis value because of these extra capitalized costs, and the difference creates a permanent tracking obligation.
Small businesses get relief. If your average annual gross receipts for the three preceding tax years are at or below the inflation-adjusted threshold ($31 million for tax years beginning in 2025), you are exempt from Section 263A.6Internal Revenue Service. Revenue Procedure 2024-40 The threshold adjusts annually, so check the current year’s revenue procedure. Exempt businesses can use simpler inventory methods without the extra capitalization layer, though they still need GAAP-compliant absorption costing for financial reporting.1Internal Revenue Service. Publication 538 (01/2022), Accounting Periods and Methods