Absorption, Marginal and Throughput Costing

Introduction and context

Costing systems do more than attach monetary values to products. They shape reported profit, inventory values, performance evaluation, pricing analysis and managerial behaviour. Three important approaches are absorption costing, marginal (or variable) costing and throughput costing. Each answers a somewhat different question.

Absorption costing asks: what full manufacturing cost should be attached to units produced and inventory held? Marginal costing asks: how does profit change when sales volume changes, given that fixed manufacturing costs are treated as period costs? Throughput costing asks: how can the organisation maximise the rate at which it generates contribution from its principal constraint?

The methods may use the same underlying production data yet report different inventory values and profits. These differences do not necessarily indicate errors. They arise because the methods classify and assign costs differently. A financially literate manager must therefore understand both the arithmetic and the behavioural consequences of each method.

Learning objectives

By the end of this chapter, you should be able to:

1. Explain the principles and purposes of absorption, marginal and throughput costing.
2. Classify costs as fixed, variable, direct, indirect, operating and throughput-related.
3. Calculate product costs, inventory values and operating profit under competing methods.
4. Reconcile profit differences between absorption and marginal costing.
5. Apply throughput accounting to decisions involving scarce capacity.
6. Evaluate the effects of costing methods on production, inventory and performance incentives.
7. Identify limitations, governance risks and appropriate uses of each method.

1. Cost classification and the costing problem

A cost object is anything for which cost is measured, such as a product, service, customer or department. Direct costs can be traced economically to a cost object; indirect costs require allocation. A cost driver is a factor that causes or influences the amount of a cost, such as machine hours, labour hours, setups or purchase orders.

For short-term analysis, costs are often classified by behaviour:

• Variable cost changes in total in proportion to an activity measure, within a relevant range. Variable cost per unit is assumed constant within that range.
• Fixed cost remains constant in total within a relevant range and time period. Fixed cost per unit changes as volume changes.
• Mixed cost contains both fixed and variable elements.

The relevant range is important. A factory may have a monthly fixed cost of £120,000 while operating between 10,000 and 20,000 units. Beyond that range, an additional shift, machine or supervisor may make fixed cost step upward.

Manufacturing costs commonly include direct materials, direct labour and manufacturing overhead. Selling, distribution and administration costs are normally treated as period costs in both absorption and marginal costing, although organisations may use different internal classifications.

2. Absorption costing

Principle

Absorption costing assigns all manufacturing costs—variable and fixed—to units produced. Each unit absorbs a share of manufacturing overhead. Units sold carry manufacturing cost into cost of sales; units remaining in inventory carry manufacturing cost forward as an asset, subject to applicable inventory valuation rules and impairment or net-realizable-value tests.

A predetermined overhead absorption rate is often used:

The activity level may be direct labour hours, machine hours, units, or another cost driver. The rate has units such as £ per machine hour or £ per labour hour.

If a product uses h machine hours and the overhead rate is r per machine hour, absorbed overhead per unit is:

The method assumes that the selected allocation base provides a reasonably useful representation of resource consumption. In automated factories, machine hours or activity-based drivers may be more informative than direct labour hours.

Under- and over-absorption

Because rates are usually based on budgets, actual overhead may differ from overhead absorbed. The variance is:

A positive amount indicates under-absorption when actual cost exceeds absorbed cost; a negative amount indicates over-absorption. The variance may be written off to profit or investigated and allocated, depending on policy and materiality.

Strengths and limitations

Absorption costing reflects the idea that production requires both variable and fixed manufacturing resources. It is widely used for external financial reporting because inventory should generally include production costs, including an appropriate share of fixed production overhead, when normal capacity is used.

However, absorption costing can encourage production beyond current demand. Producing additional units transfers part of fixed manufacturing cost from the income statement into inventory. Reported profit can therefore rise even when sales do not increase. Allocation may also be arbitrary where overhead is driven by complexity, product variety or support activities rather than volume.

3. Marginal or variable costing

Principle

Marginal costing charges variable manufacturing costs to products and treats fixed manufacturing overhead as a period cost. Inventory is therefore valued at variable production cost only. The central performance measure is contribution:

Operating profit is:

For a single product, if p is selling price per unit, v is variable cost per unit and Q is units sold, then:

where p − v is contribution per unit. The formula assumes a constant selling price and variable cost per unit over the relevant range.

The contribution margin ratio is:

For a single product, this is also (p − v) ÷ p. The break-even volume, when fixed costs are covered exactly, is:

These measures are particularly useful for short-term decisions, such as accepting a special order when spare capacity exists, selecting a product mix, or assessing the effect of a change in volume.

Strengths and limitations

Marginal costing makes the volume–profit relationship transparent and prevents fixed production cost from being carried in inventory. It is therefore useful for internal planning and control. It is not, by itself, a complete long-term product-costing system: in the long run, capacity, facilities, engineering support and many other costs must be recovered from products or services.

A further risk is overemphasis on contribution per unit. A product with high contribution per unit may consume much more of a scarce machine resource than a product with lower contribution per unit. In constrained environments, contribution per unit of bottleneck resource is more relevant.

4. Throughput costing and throughput accounting

Throughput accounting is associated with the Theory of Constraints. It focuses on improving the flow of value through the system by identifying and managing the constraint, or bottleneck. In its strictest form, throughput costing treats only truly variable direct material as a product cost. Direct labour and most other operating expenses are treated as fixed in the short term because they cannot be eliminated immediately when one unit is not produced.

The core measures are:

The “truly variable cost” is commonly the cost of direct materials purchased for the unit. The exact definition should be stated clearly because some organisations have additional genuinely variable costs, such as usage-based royalties or transaction charges.

At the organisation level:

A ratio above 1 indicates that the return generated per bottleneck hour exceeds the operating expense assigned to that hour, subject to the assumptions of the model. “Cost per factory hour” is generally operating expense divided by available bottleneck hours.

Throughput accounting emphasises five focusing steps:

1. Identify the system constraint.
2. Exploit the constraint by using it as effectively as possible.
3. Subordinate other resources and decisions to the constraint.
4. Elevate the constraint by adding capacity or removing restrictions.
5. Repeat the process when the constraint moves.

Throughput costing is powerful for short-term constrained-resource decisions, but it is not normally a substitute for statutory inventory valuation or comprehensive long-term product costing. It also requires disciplined identification of genuinely variable costs and the current constraint.

5. Comparing the three methods

The methods are not mutually exclusive in practice. A company may use absorption costing for external reporting, marginal costing for budgeting and special-order decisions, and throughput measures for a bottleneck production cell.

6. Inventory valuation and profit reconciliation

The central difference between absorption and marginal costing is the treatment of fixed manufacturing overhead in inventory. Suppose opening inventory is zero and production exceeds sales. Under absorption costing, some fixed manufacturing overhead is held in closing inventory rather than expensed immediately. Absorption profit will therefore exceed marginal profit by the fixed overhead included in closing inventory.

With opening and closing inventories measured in units, and a constant fixed manufacturing overhead rate per unit, the reconciliation is:

The formula assumes a constant fixed overhead rate, no unusual capacity adjustments and consistent treatment of all other costs. If inventory falls, absorption profit is lower because fixed overhead previously deferred in inventory is released to cost of sales.

A more general reconciliation uses fixed production overhead deferred in inventory:

This identity is conceptually important. It shows that the difference is a timing difference, not necessarily a difference in cash generation. If all production is eventually sold, the cumulative profit under the two methods will converge, assuming no write-downs or other adjustments.

Worked example 1: absorption and marginal costing

Illustrative data for one month:

• Production: 10,000 units
• Sales: 8,000 units
• Selling price: £50 per unit
• Variable manufacturing cost: £22 per unit
• Fixed manufacturing overhead: £120,000
• Variable selling cost: £3 per unit sold
• Fixed selling and administration cost: £60,000
• Opening inventory: zero

#### Absorption costing

Fixed manufacturing overhead rate:

Absorption production cost per unit:

Sales revenue:

Cost of goods sold:

Variable selling cost is 8,000 × £3 = £24,000. Therefore:

Closing inventory is 2,000 units at £34, or £68,000.

#### Marginal costing

Variable manufacturing cost of units sold is 8,000 × £22 = £176,000. Total variable cost, including variable selling cost, is £176,000 + £24,000 = £200,000. Contribution is:

All fixed costs are expensed:

Closing inventory is valued at 2,000 × £22 = £44,000.

#### Reconciliation

Fixed overhead included in closing inventory under absorption costing is:

Thus:

The £24,000 difference is exactly the fixed overhead deferred in closing inventory. No additional economic cash has been generated merely because absorption profit is higher.

7. Throughput analysis under a bottleneck

Worked example 2: choosing a product mix

A factory makes Products A and B. Machine M is the bottleneck. Monthly data are:

Throughput per unit is selling price less direct material:

• A: £100 − £40 = £60
• B: £150 − £70 = £80

Throughput per bottleneck minute is:

• A: £60 ÷ 10 = £6
• B: £80 ÷ 20 = £4

Although B provides greater throughput per unit, A provides the greater return per scarce minute. If only 10,000 bottleneck minutes are available, the recommended sequence is to satisfy demand for A first: 1,000 A units use all 10,000 minutes and generate £60,000 throughput. Producing 500 B units instead would generate only £40,000 throughput.

This conclusion assumes that demand exists, selling prices and material costs are reliable, quality and delivery requirements are met, and no other constraint becomes binding. If the factory can sell only 600 A units, the remaining 4,000 minutes should be allocated to B, subject to its demand limit.

8. Capacity, incentives and managerial behaviour

Costing systems influence decisions because managers are often evaluated using accounting profit, cost variances, return on investment or budget achievement.

Production and inventory behaviour

Under absorption costing, producing more units than are sold can increase reported profit. Fixed overhead is spread over more units and some is placed in inventory. This creates an incentive to build inventory, run long production batches and resist product rationalisation. Such behaviour may increase storage, insurance, obsolescence and handling costs, while concealing weak demand.

Marginal costing reduces this particular incentive because fixed manufacturing cost is expensed in the period. However, managers may then focus excessively on contribution and accept business that does not recover long-run capacity costs. A low price may be sensible for a genuinely temporary order using idle capacity, but damaging if it becomes a precedent or displaces profitable regular sales.

Throughput accounting directs attention to flow and the constraint. It discourages local optimisation, such as maximising utilisation at a non-bottleneck machine while finished goods accumulate before the bottleneck. Nevertheless, throughput measures can create pressure to prioritise immediate output over maintenance, training, quality or employee welfare unless balanced scorecards and governance controls are used.

Capacity measures

Capacity should be distinguished as theoretical, practical, normal and budgeted capacity. Using an unrealistic denominator can produce an inappropriate absorption rate. For example, dividing fixed overhead by theoretical maximum hours produces a lower rate than dividing by practical capacity, but may create large under-absorption when downtime is normal and unavoidable.

Managers should investigate idle capacity rather than automatically blaming production departments. Idle capacity can arise from demand shortages, maintenance, labour absence, supply disruption or deliberate strategic flexibility. A cost system should make these causes visible.

9. Case study: Northbridge Components

The following is a fictional illustrative case.

Background

Northbridge Components manufactures two industrial parts. Its external accounts use absorption costing. The production manager receives a bonus based on monthly absorption-costing profit and machine utilisation. Demand has weakened for Part X, but the plant continues producing it to keep machines busy.

Problem

At year-end, finished-goods inventory has increased by 35%. The finance director reports improved profit, while the operations director reports rising storage costs and more obsolete stock. A customer-service review also finds that production of low-demand X has delayed urgent orders for Part Y.

Analysis

The finance team recalculates results using marginal costing. The absorption profit is higher because fixed manufacturing overhead has been included in the increased inventory. The difference disappears when the inventory build is removed from the analysis. A throughput review identifies the heat-treatment oven as the bottleneck. Part Y generates £90 of throughput and uses 15 oven minutes, or £6 per bottleneck minute. Part X generates £55 and uses 20 minutes, or £2.75 per bottleneck minute.

Approach and solution

Northbridge adopts three complementary reports:

1. Absorption-costing statements for external reporting and long-term cost recovery.
2. Marginal-costing contribution reports for short-term pricing and volume decisions.
3. Throughput reports ranking products by throughput per oven minute and reporting queue time, quality and delivery performance.

The bonus plan is redesigned. It includes contribution, inventory turnover, on-time delivery, quality and cash conversion rather than absorption profit alone. Production is authorised by demand and bottleneck priorities, not merely by machine utilisation.

Outcome

The inventory build is reduced, obsolete stock falls and urgent Y orders are delivered more reliably. Reported accounting profit initially declines because previously deferred fixed overhead is released from inventory, but cash flow and operating performance improve. Managers also gain a clearer understanding of the difference between accounting profit and economic progress.

Learning points

The case illustrates that no single costing method is sufficient for every purpose. External reporting, short-term decisions and constraint management require different information. It also demonstrates the governance principle that performance measures should not reward actions that improve an accounting number while damaging customers, cash flow or long-term capacity.

10. Limitations, governance and contemporary practice

Allocation and measurement limitations

Absorption costing depends on cost pools and allocation bases. If overhead is driven by product complexity rather than volume, a single volume-based rate can distort product costs. Activity-based costing may improve cost assignment by tracing costs through activities and cost drivers, although it increases data requirements and should not be confused with marginal or throughput costing.

Marginal costing depends on a credible separation of fixed and variable costs. In the medium term, many “fixed” costs become avoidable or step-variable. A short-term decision based on an apparently attractive contribution may be poor if it creates overtime, maintenance, regulatory or service obligations.

Throughput accounting depends on correctly identifying the current constraint. Constraints can be physical, such as a machine, or policy-based, such as approval rules, scheduling practices or quality procedures. A constraint may move after management action, so analysis must be repeated.

Financial reporting and governance

External inventory reporting is governed by the applicable accounting framework and jurisdiction. In many frameworks, inventories include costs of purchase, conversion and other costs incurred to bring inventory to its present location and condition; fixed production overhead is generally allocated systematically using normal capacity. Abnormal waste, selling costs and unrelated administrative costs are normally excluded. Inventory is also subject to lower-of-cost-and-net-realizable-value or equivalent impairment requirements, depending on the applicable framework.

Internal reports should be clearly labelled. A throughput inventory figure based only on direct material must not be presented as if it were the statutory inventory valuation. Reconciliations between management reports and financial statements support auditability and prevent misleading performance claims.

Technology and data

Enterprise resource planning systems can maintain parallel views of product cost, contribution and bottleneck consumption. Machine data and process-mining tools can estimate actual cycle times and identify queues. Artificial intelligence may assist in forecasting demand, detecting unusual overhead absorption or simulating product mixes, but outputs require human review. Poor master data, unstable cost drivers or biased historical decisions can make automated recommendations unreliable. Governance should include version control, traceable assumptions, access controls and review of model performance.

Best-practice framework

A professional costing system should:

1. Define the decision purpose before selecting the costing method.
2. Separate product costing, inventory valuation and decision analysis.
3. Use cost drivers that reflect resource consumption.
4. State relevant-range, capacity and demand assumptions.
5. Reconcile internal reports to financial statements.
6. Monitor inventory, cash flow, quality and customer service alongside profit.
7. Review bottlenecks whenever process conditions change.
8. Test incentives for unintended production or pricing behaviour.
9. Document estimates, rates, exclusions and impairment judgments.
10. Combine financial measures with operational and ethical controls.

Chapter Summary

Absorption costing assigns variable and fixed manufacturing costs to production and normally provides the basis for external inventory valuation. Marginal costing assigns variable costs to units, treats fixed costs as period costs and highlights contribution for short-term planning. Throughput costing focuses on the rate of generating contribution through the system’s constraint and commonly treats only truly variable material as a product cost.

When production exceeds sales, absorption profit is usually higher than marginal profit because fixed manufacturing overhead is deferred in closing inventory. The difference equals fixed overhead in closing inventory less fixed overhead in opening inventory, subject to consistent assumptions. When inventory falls, the reverse occurs.

Each method has a legitimate purpose but also behavioural risks. Absorption costing can encourage overproduction; marginal costing can understate the importance of long-run fixed costs; throughput costing can overemphasise short-term flow if broader performance and governance measures are ignored. Effective management therefore uses the methods together, labels their purposes clearly and evaluates profit alongside capacity, cash, quality, inventory and customer outcomes.

Key Terms

Cost object: Anything for which costs are measured.

Direct cost: A cost economically traceable to a specific cost object.

Indirect cost: A cost that cannot be traced conveniently and is allocated.

Variable cost: A cost that changes in total with activity within a relevant range.

Fixed cost: A cost constant in total within a relevant range and period.

Absorption costing: A method assigning all manufacturing costs to units produced.

Marginal costing: A method assigning variable costs to units and expensing fixed costs in the period.

Contribution: Sales revenue less variable cost.

Throughput: Sales revenue less truly variable cost, commonly direct material.

Cost driver: A factor causing or influencing cost.

Overhead absorption rate: Predetermined manufacturing overhead divided by a budgeted activity base.

Under-absorption: Actual overhead exceeding absorbed overhead.

Over-absorption: Absorbed overhead exceeding actual overhead.

Relevant range: The activity interval within which cost assumptions are expected to hold.

Bottleneck: A resource or policy that limits system output.

Throughput per bottleneck unit: Throughput contribution divided by scarce-resource usage.

Practical capacity: Capacity allowing for normal unavoidable interruptions.

Inventory deferral: The carrying of costs in inventory until related units are sold.

Net realizable value: Estimated selling price less costs of completion and sale.

Theory of Constraints: A management approach focused on identifying and improving system constraints.

Review Questions

5 Short Answer Questions

1. What is the principal difference between absorption and marginal costing?
2. Define contribution and explain why it is useful for short-term decisions.
3. What is meant by a bottleneck?
4. Why can absorption costing encourage inventory building?
5. State the five focusing steps of the Theory of Constraints.

5 Long Answer Questions

1. Explain the principles, uses and limitations of absorption costing.
2. Compare marginal costing and throughput costing as tools for product-mix decisions.
3. Derive and explain the reconciliation between absorption profit and marginal profit.
4. Discuss how capacity assumptions affect overhead absorption rates and managerial performance evaluation.
5. Evaluate the claim that one costing method should be used for all management purposes.

3 Analytical / Application Questions

1. A business produces 12,000 units, sells 9,000 units and has fixed manufacturing overhead of £180,000. If the absorption rate is based on production, calculate the fixed overhead included in closing inventory and explain its effect on absorption profit relative to marginal profit.
2. Product P yields £48 throughput and uses 8 bottleneck minutes. Product Q yields £70 throughput and uses 15 bottleneck minutes. Which product should receive priority when the bottleneck is fully utilised, and what additional information should management check?
3. A manager increases production while sales remain unchanged and reports a higher absorption profit. Assess whether this represents genuine performance improvement, identifying financial, operational and governance measures that should be reviewed.

Multiple Choice Questions with Answers

Question 1

Which cost is included in inventory under absorption costing but normally treated as a period cost under marginal costing?

A. Direct material

B. Variable manufacturing overhead

C. Fixed manufacturing overhead

D. Variable selling cost

Correct Answer: C. Fixed manufacturing overhead

Question 2

A company produces 10,000 units and incurs fixed manufacturing overhead of £150,000. What is the fixed overhead absorption rate per unit?

A. £10

B. £15

C. £100

D. £150

Correct Answer: B. £15

Question 3

If closing inventory exceeds opening inventory, and all other assumptions remain constant, absorption profit will normally be:

A. Lower than marginal profit

B. Equal to marginal profit

C. Higher than marginal profit

D. Unrelated to marginal profit

Correct Answer: C. Higher than marginal profit

Question 4

A product sells for £80 and has variable cost of £50. What is its contribution per unit?

A. £30

B. £50

C. £80

D. £130

Correct Answer: A. £30

Question 5

Under throughput costing, the most commonly treated truly variable product cost is:

A. Factory rent

B. Direct material

C. Production supervision

D. Depreciation

Correct Answer: B. Direct material

Question 6

Product A generates £40 throughput using 5 bottleneck minutes. Product B generates £60 throughput using 10 bottleneck minutes. Which product has the higher throughput per bottleneck minute?

A. Product A, at £8 per minute

B. Product A, at £5 per minute

C. Product B, at £6 per minute

D. Product B, at £4 per minute

Correct Answer: A. Product A, at £8 per minute

Question 7

Which formula correctly expresses the reconciliation between absorption and marginal profit under constant fixed overhead per unit?

A. Absorption profit − marginal profit = sales units × variable cost

B. Absorption profit − marginal profit = change in inventory units × fixed overhead rate

C. Absorption profit − marginal profit = fixed cost ÷ sales price

D. Absorption profit − marginal profit = production units × selling price

Correct Answer: B. Absorption profit − marginal profit = change in inventory units × fixed overhead rate

Question 8

What is a major behavioural risk of absorption costing?

A. Managers may avoid all production

B. Managers may build inventory to defer fixed overhead

C. Managers cannot calculate contribution

D. Managers must ignore capacity constraints

Correct Answer: B. Managers may build inventory to defer fixed overhead

Question 9

If selling price is £100 and variable cost is £60, the contribution margin ratio is:

A. 20%

B. 40%

C. 60%

D. 160%

Correct Answer: B. 40%

Question 10

What is the first focusing step in the Theory of Constraints?

A. Elevate every resource simultaneously

B. Identify the system constraint

C. Eliminate all fixed costs

D. Maximise non-bottleneck utilisation

Correct Answer: B. Identify the system constraint