Overview

• What you’ll learn: The difference between absorption (full) costing and variable (direct) costing, how each method affects reported income, the capacity-level issue, and why managers under absorption costing may overproduce.
• Prerequisites: Lesson 7 — Fixed Overhead Variances
• Estimated reading time: 17 minutes

Introduction

The Grand Historian records: Two scholars stand before the emperor, each presenting the kingdom’s profit report. One includes the full cost of the fortress in the value of every arrow produced — this is absorption costing. The other insists the fortress cost is a period expense regardless of how many arrows were made — this is variable costing. Both are correct in their logic, yet they report different profits. The emperor, understandably, is confused.

Horngren (Chapter 9) explains this ancient dispute and its modern consequences. The choice between absorption and variable costing affects not just financial statements but managerial behavior — and therein lies the danger.

Absorption Costing (Full Costing)

Under absorption costing, all manufacturing costs — variable and fixed — are inventoriable product costs. Fixed manufacturing overhead is allocated to units produced and remains in inventory until those units are sold.

Product cost = DM + DL + Variable MOH + Fixed MOH

This is the method required by GAAP (Generally Accepted Accounting Principles) and IFRS for external financial reporting.

Variable Costing (Direct Costing)

Under variable costing, only variable manufacturing costs are inventoriable. Fixed manufacturing overhead is treated as a period expense — charged entirely to the income statement in the period incurred.

Product cost = DM + DL + Variable MOH

Period expense = Fixed MOH (entire amount)

Variable costing is not permitted for external reporting under GAAP or IFRS, but it is widely used for internal management reporting because it eliminates the profit distortions caused by inventory changes.

The Profit Difference

The fundamental rule:

The difference = Fixed MOH rate × Change in inventory units

When production exceeds sales, absorption costing “hides” fixed overhead in ending inventory — costs that variable costing expenses immediately. The fortress cost is buried in unsold arrows sitting in the warehouse.

The Overproduction Incentive

太史公曰：Herein lies the great peril of absorption costing. A manager evaluated on absorption-costing income can increase reported profit simply by overproducing — manufacturing more units than needed, thereby spreading fixed overhead over more units and deferring costs into inventory. No additional sales are required. This is the accounting equivalent of borrowing from tomorrow to pay for today’s feast.

This perverse incentive has destroyed many a company’s cash flow. Warehouses fill with unsold inventory, carrying costs mount, and eventually the reckoning arrives when inventory must be written down.

Variable costing eliminates this incentive because fixed overhead hits the income statement regardless of production volume. A manager cannot improve variable-costing income by producing units nobody wants.

Throughput Costing

An even more extreme alternative is throughput costing (super-variable costing), which treats only direct materials as inventoriable. Direct labor and all overhead become period expenses. This method focuses management attention on throughput — the rate at which the system generates money through sales.

Capacity Level and External Reporting

For external reporting, the choice of denominator level (theoretical, practical, normal, or master-budget capacity) affects the fixed overhead rate and thus inventory valuations. The IRS requires practical capacity for tax reporting in the US, while IFRS allows normal capacity. These choices ripple through to cost of goods sold, inventory on the balance sheet, and reported income.

Key Takeaways

• Absorption costing includes all manufacturing costs in inventory; variable costing includes only variable manufacturing costs.
• When production ≠ sales, the two methods report different incomes. The difference equals fixed MOH rate × change in inventory.
• Absorption costing creates an incentive to overproduce — variable costing removes it.
• GAAP and IFRS require absorption costing for external reporting; variable costing is used internally.
• Throughput costing is the most conservative approach, treating only direct materials as inventoriable.

What’s Next

This completes Module 7 on Budgeting and Variance Analysis. In Module 8, we shift from measuring the past to shaping the future — decision analysis, strategic pricing, and the balanced scorecard.