Source: Cost Accounting, Chapter 9 (The Ohio State University)
Tags: variable costing, absorption costing, direct costing, inventoriable costs, period costs, fixed manufacturing overhead, production volume variance, operating income differences, GAAP costing methods
Difficulty: Intermediate | Prerequisites: Understanding of product vs. period costs, manufacturing cost categories (Ch. 2–4), and contribution margin income statements.
This chapter tackles a question that trips up a lot of accounting students: when a company makes more units than it sells (or fewer), why does reported profit change depending on which costing method you use? The answer comes down to one thing, fixed manufacturing overhead, and whether you attach it to inventory or expense it immediately. Variable costing and absorption costing are two ways of handling that decision, and the difference has real consequences for income statements. You need a solid grasp of how costs are classified (variable vs. fixed, manufacturing vs. non-manufacturing) before this will click.
Variable costing treats fixed manufacturing overhead as a period expense. Absorption costing bundles it into inventory. The method you choose does not change cash flow, but it changes when costs hit the income statement, which means reported operating income differs whenever production and sales volumes are not equal.
Variable costing
A costing method that includes only variable manufacturing costs (direct materials, direct labour, variable manufacturing overhead) as inventoriable product costs. All fixed manufacturing costs are expensed in the period they are incurred.
In simple terms, this means: if the cost does not go up when you make one more unit, it stays off the product cost and goes straight to the income statement.
Absorption costing
A costing method that includes all manufacturing costs, both variable and fixed, as inventoriable product costs. The inventory "absorbs" every manufacturing cost.
Think of it as: the product soaks up its share of everything spent in the factory, whether or not that cost changes with volume.
Direct costing
An alternative name for variable costing, used in older textbooks and some industry settings.
In simple terms, this means: same concept as variable costing, just a different label.
Inventoriable costs (product costs)
Costs that attach to units of product and sit on the balance sheet as inventory until the units are sold, at which point they become cost of goods sold.
Think of it as: costs that "travel with" the product through the warehouse and only hit the income statement when the product leaves.
Period costs
Costs expensed in the accounting period in which they are incurred, regardless of whether any units are produced or sold.
In simple terms, this means: these costs show up on the income statement right away, no matter what happens in the factory.
Production volume variance
The difference between budgeted fixed manufacturing overhead and the amount allocated to production. It exists only under absorption costing because that method assigns a fixed overhead rate per unit.
Think of it as: the gap that appears when you planned to spread your fixed factory costs over X units but you produced Y units instead.
All variable manufacturing costs (direct materials, direct labour, variable overhead) are inventoriable.
All fixed manufacturing costs are treated as period costs, expensed in full during the period incurred.
Variable non-manufacturing costs (e.g. sales commissions) are also period costs.
All manufacturing costs, variable and fixed, are inventoriable.
Inventory absorbs every factory cost, so per-unit product cost is higher than under variable costing.
A production volume variance arises because fixed overhead is allocated at a predetermined rate.
The choice of denominator (practical capacity, normal capacity, budgeted output) affects the size of this variance.
Required under GAAP and IFRS for external financial reporting.
The only difference between the two methods is the treatment of fixed manufacturing overhead.
Variable costing: fixed manufacturing overhead is expensed immediately (period cost).
Absorption costing: fixed manufacturing overhead is inventoriable (product cost).
Variable non-manufacturing costs and fixed non-manufacturing costs are period costs under both methods.
Production = Sales: Operating income is the same under both methods. No inventory change means no fixed overhead is deferred or released.
Production > Sales (inventory increases): Absorption costing reports higher operating income. Some fixed overhead is "parked" in ending inventory on the balance sheet rather than being expensed.
Production < Sales (inventory decreases): Variable costing reports higher operating income. Under absorption, fixed overhead from a prior period's inventory is released into cost of goods sold, increasing expenses.
Difference in operating income:
Absorption OI – Variable OI = Fixed manufacturing overhead rate × Change in inventory (units)
If inventory rises, the difference is positive (absorption higher). If inventory falls, the difference is negative (variable higher).
Managers sometimes face pressure to overproduce near the end of a reporting period because, under absorption costing, building inventory pushes fixed overhead onto the balance sheet and inflates short-term profit. This is one reason regulators and analysts look at cash flow alongside reported earnings, and it is why some companies use variable costing internally even though absorption is required for external reports.
Students often assume variable costing and absorption costing differ in how they treat variable costs. They do not. Both methods treat variable manufacturing costs identically as inventoriable. The entire difference is about fixed manufacturing overhead.
Students sometimes think absorption costing creates profit "out of thin air." It does not change total profit over the life of the product. It only shifts when that profit is recognised. Over multiple periods, total operating income under both methods converges.
A common mistake is forgetting that non-manufacturing costs (selling, general, and administrative) are period costs under both methods. The variable-vs-absorption distinction applies only to manufacturing costs.
Students occasionally confuse the production volume variance with other overhead variances (spending or efficiency). The production volume variance exists only under absorption costing and is driven by the gap between actual production volume and the denominator volume used to set the fixed overhead rate.
⚠️ Expect questions that give you production and sales data and ask you to compute operating income under both methods, then explain the difference.
⚠️ Know the direction of the income difference cold: production > sales means absorption income is higher. This is tested in multiple-choice form constantly.
⚠️ Be ready to explain why GAAP requires absorption costing for external reporting (matching principle: all manufacturing costs should attach to the product).
⚠️ Understand the managerial incentive problem: absorption costing can tempt managers to overproduce to boost short-term profit, even when demand does not justify it.
True or False: Under variable costing, fixed manufacturing overhead is included in the per-unit product cost.
Fill in the blank: When production exceeds sales, __________ costing reports higher operating income.
True or False: Variable costing is acceptable under GAAP for external financial reporting.
Fill in the blank: The production volume variance exists only under __________ costing.
True or False: If production equals sales for the period, operating income is the same under both methods.
Answers: 1. False 2. Absorption 3. False 4. Absorption 5. True
Q: A company produces 10,000 units and sells 8,000. Fixed manufacturing overhead is £200,000. Under absorption costing, how much fixed overhead remains in ending inventory?
A: The fixed overhead rate is £200,000 ÷ 10,000 = £20 per unit. Ending inventory is 2,000 units, so £20 × 2,000 = £40,000 of fixed overhead sits in inventory rather than being expensed.
Q: Why does absorption costing report higher operating income than variable costing when inventory levels increase?
A: Because a portion of the period's fixed manufacturing overhead is allocated to the unsold units and deferred on the balance sheet as inventory, rather than being expensed in full. Variable costing expenses all fixed manufacturing overhead immediately, so its cost of goods sold equivalent is higher for that period.
Q: A manager is considering producing 5,000 extra units at year-end despite no customer orders. Under which costing method would this decision increase reported operating income, and why?
A: Absorption costing. The extra production would absorb fixed overhead into inventory, reducing the amount expensed in the current period and increasing reported profit. Under variable costing, the additional production would have no effect on operating income because all fixed manufacturing overhead is expensed regardless of output.
Q: If a firm uses variable costing for internal reports and absorption costing for external reports, what adjustment is needed to reconcile the two income figures?
A: Add (or subtract) the change in the fixed overhead component of inventory. Specifically: Absorption OI = Variable OI + (Fixed OH rate × increase in inventory units) or – (Fixed OH rate × decrease in inventory units).
This material connects directly to CVP analysis (Ch. 3), because variable costing aligns with the contribution-margin income statement used in break-even calculations. It also ties into budgeting and variance analysis (Ch. 7–8), since the production volume variance is a by-product of the absorption costing framework. Understanding this chapter is essential before tackling performance evaluation topics later in the course, where the choice of costing method can distort divisional profit measures and affect managerial decision-making.
Variable costing, direct costing, absorption costing, full costing, inventoriable costs, product costs, period costs, fixed manufacturing overhead, production volume variance, denominator level, operating income reconciliation, GAAP costing, contribution margin income statement, overproduction incentive, inventory build-up effect, cost accounting chapter 9