Overhead Allocation and Activity-Based Costing

Introduction

Cost and management accountants must assign indirect costs to products, services, customers, projects and responsibility centres in a way that is economically meaningful and useful for decisions. Direct materials and direct labour can often be traced to a cost object with reasonable accuracy. Overheads—such as production supervision, depreciation, maintenance, information technology, procurement and quality assurance—cannot usually be traced economically to individual units. They must therefore be allocated, apportioned and absorbed through a systematic costing process.

The choice of method matters. A single plant-wide overhead rate may be adequate where products consume resources in similar proportions. It may be seriously misleading where products differ in complexity, batch size, setup requirements, engineering support or distribution demands. Activity-based costing (ABC) was developed to address this problem by relating resource consumption to activities and cost drivers. Activity-based management (ABM) uses the resulting information to improve processes and decisions. Time-driven ABC (TDABC) simplifies ABC by estimating the cost of supplying capacity and the time required for transactions.

This chapter examines the logic, calculations, managerial uses and limitations of these approaches. It is particularly relevant to CFM professionals who must evaluate product profitability, improve operational efficiency and design credible internal cost information systems.

Learning objectives

After studying this chapter, you should be able to:

1. Distinguish direct costs, indirect costs, cost centres and cost objects.
2. Allocate and apportion overheads using appropriate bases.
3. Calculate overhead absorption rates and explain under- and over-absorption.
4. Apply repeated and reciprocal methods for allocating service-department costs.
5. Explain traditional volume-based costing and its limitations.
6. Design and calculate activity-based costing systems.
7. Compare traditional costing, ABC, ABM and TDABC.
8. Interpret costing information for pricing, product mix, process improvement and strategic decisions.
9. Evaluate behavioural, ethical, technological and governance issues in overhead costing.

1. The architecture of overhead costing

A cost object is anything for which a cost is measured—for example, a product, customer, service, contract or distribution channel. A cost centre is a location, function or activity for which costs are accumulated. Production departments are operating cost centres; maintenance, stores and information technology are commonly service cost centres.

A cost is direct when it can be traced economically to a cost object. A cost is indirect when tracing it would be impractical or more costly than the information benefit obtained. Directness is therefore partly a matter of economic feasibility, not merely physical observability. Electricity used by a single machine may be directly measurable, but factory rent is normally indirect.

The basic flow is:

Resources → cost centres or activities → cost objects → managerial decisions

Overhead accounting involves four related processes:

• Allocation: assigning an entire cost to one cost centre when the cost is specifically attributable to that centre.
• Apportionment: distributing a common cost among two or more cost centres using a rational basis.
• Re-apportionment: transferring service-department costs to operating departments.
• Absorption: charging operating-department overheads to products or services using an overhead absorption rate.

The distinction between allocation and apportionment is important. A production supervisor’s salary may be allocated wholly to one production department. Factory rent may be apportioned between departments according to floor area.

2. Allocation and apportionment of overheads

Selecting an apportionment basis

A basis should reflect the causal relationship between the shared cost and the receiving cost centres. Common bases include:

No basis is universally correct. Floor area may be suitable for rent but inappropriate for electricity if departments use different levels of machine power. A professionally designed system documents the rationale for each basis and reviews it when operating conditions change.

Primary distribution schedule

A primary distribution schedule assigns direct overheads to cost centres and apportions common overheads. Consider the following illustrative data:

The schedule is a control document. Its column totals should reconcile to the total overheads recorded in the general ledger. The service-centre amounts are then transferred to operating departments.

3. Service-department re-apportionment

Service departments support production but do not normally manufacture saleable units. Their costs must be assigned to operating departments. Three methods are common.

Direct method

The direct method transfers service-department costs only to operating departments. It ignores services exchanged between service departments. It is simple and often acceptable where inter-service support is immaterial.

If Maintenance costs £18,500 and provides 70% of its service to Machining and 30% to Assembly, the transfers are £12,950 and £5,550 respectively. Stores costs are then transferred using its own service proportions. This method is easy to explain but may distort costs when service departments significantly support one another.

Step-down method

The step-down method recognizes some inter-service relationships. Service departments are ranked, usually by the proportion or value of services provided to other service departments. The first service department is allocated to all other departments; the next is allocated only to the remaining departments. Once a department has been closed, it receives no further allocation.

The ranking rule should be documented because alternative rankings can produce different product costs. The method is more realistic than the direct method but remains dependent on an arbitrary sequence.

Reciprocal method

The reciprocal method recognizes all mutual services among service departments. It is theoretically the most complete of the three traditional methods. Simultaneous equations are used to calculate the fully loaded cost of each service department.

Suppose Maintenance has direct cost of £18,500 and receives 10% of Stores’ services. Stores has direct cost of £14,500 and receives 20% of Maintenance’s services. Let:

• M = total cost of Maintenance after receiving Stores’ service;
• S = total cost of Stores after receiving Maintenance’s service.

Then:

Substitute the second equation into the first:

Then:

Thus, £20,357.14 is the full cost of Maintenance and £18,571.43 is the full cost of Stores. These amounts are allocated to the production departments using the proportions of service supplied to them, excluding the reciprocal percentages already represented in the equations.

The equations assume that service percentages are stable and that the stated proportions are internally consistent. If service relationships are complex, matrix methods can be used. The reciprocal method is precise, but its informational value should justify its additional complexity.

4. Overhead absorption

After overheads have been assigned to operating departments, they are absorbed into products or services. An overhead absorption rate (OAR) is generally calculated as:

The activity level must be expressed in the same unit as the rate denominator. Typical bases are direct labour hours, machine hours, direct labour cost and units produced.

For example, if a department budgets overhead of £240,000 and 12,000 machine hours, then:

A product requiring 3 machine hours absorbs £60 of departmental overhead. The calculation assumes that machine hours are a reasonable predictor of overhead consumption and that the budgeted capacity level is appropriate.

A plant-wide rate uses one rate for the whole factory. Departmental rates use separate rates for different operating departments and are generally more accurate where technology and overhead structures differ.

Under- and over-absorption

Actual overhead and actual activity often differ from budget. The absorbed overhead is:

The difference between actual overhead incurred and overhead absorbed is the absorption variance:

A positive result indicates under-absorption; a negative result indicates over-absorption. Under-absorption may arise from higher-than-expected costs, lower activity, idle capacity or an inappropriate rate. It should not automatically be treated as evidence of poor operational performance.

At period end, the variance may be written to profit or loss when immaterial. If it is material and relates to work in progress, finished goods and cost of sales, a rational apportionment may be required, subject to the applicable financial reporting framework. For management purposes, the causes should be investigated rather than simply written off.

5. Traditional volume-based costing

Traditional costing normally assigns overheads using one or a small number of volume measures, such as labour hours or machine hours. The approach works reasonably well where:

• direct costs form a large proportion of total cost;
• products are homogeneous;
• overhead consumption is strongly related to production volume;
• production processes are stable; and
• customers and products require similar support activities.

Its weakness is the assumption that overhead is driven primarily by output volume. Many overheads are instead driven by the number of batches, purchase orders, engineering changes, inspections, product designs, customer accounts or delivery routes.

A high-volume, standard product may be overcosted when it consumes few support activities. A low-volume, complex product may be undercosted because it requires disproportionate setups, design support and quality inspections. Cross-subsidization can then lead to incorrect prices, unprofitable product retention and poor strategic decisions.

6. Activity-based costing

ABC assigns resource costs first to activities and then assigns activity costs to cost objects using cost drivers. An activity is a task or group of tasks that consumes resources. A cost driver is a factor that causes or strongly influences the cost of an activity.

A typical ABC hierarchy is:

The ABC procedure is:

1. Identify significant activities.
2. Assign resource costs to activity cost pools.
3. Select a cost driver for each pool.
4. Estimate practical or expected driver volume.
5. Calculate an activity rate.
6. Assign activity costs to products, services or customers.
7. Interpret the results in operational and strategic context.

The activity rate is:

For an activity pool of £90,000 and 300 setups:

A product requiring 4 setups is assigned £1,200 of setup cost. The unit of the rate must match the driver unit.

Worked ABC comparison

Assume a factory makes Product X and Product Y. Traditional overhead is £180,000 and is assigned using machine hours. X uses 8,000 machine hours and Y uses 2,000, so the traditional rate is:

Traditional assignments are £144,000 to X and £36,000 to Y.

An ABC analysis identifies two pools:

The processing rate is £10 per machine hour. The setup rate is £800 per setup. ABC assigns:

• X: (8,000 × £10) + (20 × £800) = £96,000;
• Y: (2,000 × £10) + (80 × £800) = £84,000.

Traditional costing assigns £144,000 to X and £36,000 to Y, whereas ABC assigns £96,000 and £84,000. Product Y is low volume but setup-intensive; traditional costing substantially undercosts it. Product X is overcosted under the traditional system. The result does not prove that ABC is always superior: the conclusion depends on the quality of the activity analysis and driver measurements.

7. Activity-based management

ABM uses cost and activity information to manage activities and improve organizational performance. It has two complementary dimensions:

• Operational ABM: improves efficiency, reduces waste, shortens cycle time and removes non-value-adding activities.
• Strategic ABM: supports product design, customer profitability, outsourcing, channel strategy, pricing and product portfolio decisions.

Value is context-dependent. An activity is often described as value-adding when it contributes directly to what the customer values or is necessary to meet a required specification. Inspection may be non-value-adding from the customer’s perspective, although it may be necessary under current processes. The correct response may be prevention and process redesign rather than simply eliminating inspection.

ABM should not be used mechanically to cut every activity. Reducing customer service may lower short-term cost while damaging retention and lifetime profitability. Managers should examine cost, quality, risk, capacity and strategic importance together.

8. Time-driven activity-based costing

Traditional ABC can become expensive because it requires many interviews, surveys and separate activity pools. TDABC estimates two central quantities:

1. the cost per unit of practical capacity supplied; and
2. the time required to perform each transaction or transaction variant.

The capacity cost rate is:

If a customer-service department costs £360,000 and supplies 12,000 practical labour hours, the rate is:

If a standard customer enquiry takes 6 minutes, its cost is:

For a complex enquiry requiring an additional 4 minutes, the time equation is:

The cost is £30 × 10/60 = £5. TDABC can therefore represent transaction complexity without creating a separate activity pool for every variant.

Unused capacity is visible. If supplied practical capacity is 12,000 hours but only 9,000 hours are used, 3,000 hours represent unused capacity. At £30 per hour, the capacity cost of unused time is £90,000. This information supports staffing, scheduling, process redesign and capacity reduction decisions. Practical capacity should allow for breaks, training, maintenance, meetings and normal interruptions; using theoretical capacity would overstate efficiency.

9. Comparing costing approaches

The choice should be based on decision relevance and cost-benefit analysis. A sophisticated system with unreliable data is not better than a simple system with credible data. Hybrid systems are often appropriate: standard rates for routine manufacturing, ABC for customer service and TDABC for logistics or transaction processing.

10. Illustrative case: Northstar Components

Background

Northstar Components, an illustrative manufacturer, produces 100,000 standard units and 10,000 customised units. Management uses one machine-hour rate. Customised units require many engineering changes, small batches and inspections, but their reported margin appears attractive.

Problem

The finance director suspects that standard products are subsidising customised orders. Production overhead is £1.2 million. Machine hours are 40,000 for standard units and 20,000 for customised units. The traditional rate is:

Thus, £800,000 is assigned to standard products and £400,000 to customised products.

An operational review identifies the following activity pools:

Analysis

The ABC rates are £10 per machine hour, £2,000 per setup, £3,000 per engineering change and £600 per inspection.

Standard products receive:

• machine processing: £400,000;
• setups: £60,000;
• engineering changes: £30,000;
• inspections: £60,000;
• total: £550,000.

Customised products receive:

• machine processing: £200,000;
• setups: £180,000;
• engineering changes: £150,000;
• inspections: £120,000;
• total: £650,000.

The traditional method understated customised overhead by £250,000 and overstated standard overhead by the same amount. Management should not immediately abandon customised products. It should first review prices, order minimums, engineering charges, batch sizes and design standardisation.

Approach and outcome

Northstar introduced a quotation surcharge for engineering changes, consolidated small orders, reduced unnecessary design variants and negotiated delivery schedules with customers. The revised system also reported unused engineering capacity separately, preventing managers from interpreting all allocated costs as avoidable.

Learning points

1. Product volume is not necessarily the main cause of overhead.
2. Cost visibility can change pricing and process priorities.
3. ABC information is most useful when linked to operational action.
4. Allocated costs should not automatically be treated as avoidable costs.
5. A costing redesign should include data governance, staff involvement and periodic validation.

11. Implementation, governance and professional judgement

A successful system requires more than formulas. Important practices include:

• define the decision purpose before designing the model;
• involve operations, sales, engineering and finance personnel;
• distinguish causal drivers from convenient proxies;
• use practical capacity rather than unrealistic maximum capacity;
• reconcile model totals to the general ledger;
• document allocation rules and changes;
• test sensitivity to alternative drivers;
• review rates when technology, prices, process design or product mix changes;
• separate costs relevant to a decision from merely allocated historical costs; and
• communicate uncertainty and limitations to users.

Ethical risks include manipulating drivers to make a department or product appear profitable, allocating corporate costs to penalize managers for costs they cannot control, and using customer profitability data in ways that unfairly discriminate against small or vulnerable customers. Confidential customer data used in ABC or TDABC models must be protected. Automated costing systems and AI-assisted driver selection should be auditable, explainable and subject to human review. AI may detect unusual cost-driver relationships or forecast activity volumes, but it cannot replace managerial judgement about causality, capacity, contractual obligations and strategic value.

12. Limitations and critical evaluation

Overhead allocation is inherently approximate. Even an elaborate ABC model may fail when:

• activities are poorly defined;
• drivers are correlated but not genuinely causal;
• employees provide biased time estimates;
• capacity fluctuates sharply;
• costs are lumpy or committed rather than variable;
• products share resources in ways the model does not capture; or
• managers treat allocated costs as avoidable short-run costs.

ABC also has implementation and maintenance costs. More cost pools do not automatically create more accuracy. The relevant test is whether the information improves decisions by more than it costs to produce. Cost systems should therefore be designed for different purposes: external reporting, inventory valuation, operational control, pricing and strategic analysis may require different levels of aggregation and different treatment of capacity costs.

Chapter Summary

Overhead costing assigns indirect costs to cost centres and cost objects through allocation, apportionment, service-department re-apportionment and absorption. Direct, step-down and reciprocal methods differ in how they treat services exchanged among service departments; the reciprocal method uses simultaneous equations and recognizes all mutual services.

Traditional absorption costing is efficient where volume measures explain overhead consumption. It can be misleading in diverse environments because batch-level, product-sustaining and customer-sustaining activities are not necessarily driven by output volume. ABC assigns resource costs to activity pools and then to cost objects using causal drivers. ABM uses this information to improve operations and strategic decisions. TDABC uses capacity cost rates and time equations, making transaction complexity and unused capacity more visible.

No method is universally best. Professional practice requires causal reasoning, reliable data, reconciliation, sensitivity analysis, ethical governance and a clear distinction between allocated cost, avoidable cost and strategic cost. The value of a costing system lies not in its complexity but in the quality of decisions it supports.

Key Terms

Allocation: Assignment of an entire cost to one cost centre or cost object.

Apportionment: Distribution of a common cost among two or more cost centres.

Absorption: Charging operating overheads to products or services.

Cost object: Item for which costs are measured.

Cost centre: Organizational or functional unit used to accumulate costs.

Direct cost: Cost economically traceable to a cost object.

Indirect cost: Cost not economically traceable to a cost object.

Overhead absorption rate: Budgeted overhead divided by budgeted activity.

Under-absorption: Actual overhead exceeds absorbed overhead.

Over-absorption: Absorbed overhead exceeds actual overhead.

Service department: Department supporting operating departments.

Reciprocal method: Service-cost method recognizing mutual services through simultaneous equations.

Activity-based costing: Costing system assigning costs through activities and cost drivers.

Cost driver: Factor causing or influencing activity cost.

Cost pool: Accumulated cost assigned to a defined activity.

Activity-based management: Use of activity information for operational and strategic improvement.

Time-driven ABC: ABC approach based on capacity cost rates and estimated transaction times.

Practical capacity: Usable capacity after allowing for normal interruptions.

Unused capacity: Supplied capacity not consumed by actual transactions.

Review Questions

Short Answer Questions

1. Distinguish allocation from apportionment.
2. What is an overhead absorption rate?
3. Why can a plant-wide rate be misleading?
4. Define a cost driver and give two examples.
5. What is the principal purpose of ABM?

Long Answer Questions

1. Explain the direct, step-down and reciprocal methods of service-department cost allocation.
2. Discuss the assumptions and limitations of traditional volume-based costing.
3. Describe the stages involved in designing an ABC system.
4. Compare ABC and TDABC in terms of data requirements, complexity and capacity information.
5. Explain how overhead costing information can support strategic pricing and product portfolio decisions.

Analytical / Application Questions

1. A department budgets overhead of £360,000 and 18,000 machine hours. Product A uses 5 machine hours. Calculate the rate and overhead absorbed by Product A, and explain one limitation of the chosen base.
2. A service department has direct cost of £50,000 and receives 15% of a second service department’s cost. The second department has direct cost of £30,000 and receives 10% of the first department’s cost. Set up and solve the reciprocal equations.
3. A low-volume product consumes 15% of machine hours but 60% of setups and engineering changes. Explain why traditional costing may misstate its cost and recommend information that management should collect before changing prices.

Multiple Choice Questions with Answers

Question 1

Which statement best distinguishes allocation from apportionment?

A. Allocation distributes a common cost, whereas apportionment traces a direct cost.

B. Allocation assigns a cost wholly to one cost centre, whereas apportionment distributes a common cost among cost centres.

C. Allocation is used only for variable costs, whereas apportionment is used only for fixed costs.

D. Allocation calculates product profit, whereas apportionment calculates cash flow.

Correct Answer: B. Allocation assigns a cost wholly to one cost centre, whereas apportionment distributes a common cost among cost centres.

Question 2

A department budgets overhead of £240,000 and 12,000 machine hours. What is its overhead absorption rate?

A. £12 per machine hour

B. £18 per machine hour

C. £20 per machine hour

D. £28 per machine hour

Correct Answer: C. £20 per machine hour

Question 3

Which service-department method recognizes mutual services between service departments?

A. Direct method

B. Step-down method

C. Reciprocal method

D. Single-rate method

Correct Answer: C. Reciprocal method

Question 4

If actual overhead is £100,000 and absorbed overhead is £94,000, the result is:

A. £6,000 over-absorption

B. £6,000 under-absorption

C. £194,000 under-absorption

D. No variance

Correct Answer: B. £6,000 under-absorption

Question 5

Which cost is most likely to be batch-level rather than unit-level?

A. Direct material per unit

B. Machine processing per unit

C. Machine setup for a production run

D. Packaging material per unit

Correct Answer: C. Machine setup for a production run

Question 6

An ABC activity pool costs £90,000 and has 300 setups. What is the activity rate?

A. £30 per setup

B. £270 per setup

C. £300 per setup

D. £900 per setup

Correct Answer: C. £300 per setup

Question 7

A product uses 8,000 machine hours and 20 setups. ABC rates are £10 per machine hour and £800 per setup. What overhead is assigned?

A. £80,000

B. £96,000

C. £144,000

D. £160,000

Correct Answer: B. £96,000

Question 8

What is a distinctive feature of time-driven ABC?

A. It uses only direct labour cost as a driver.

B. It ignores practical capacity.

C. It uses capacity cost rates and estimated transaction times.

D. It assigns all facility costs directly to products.

Correct Answer: C. It uses capacity cost rates and estimated transaction times.

Question 9

Why might a low-volume customised product be undercosted by traditional costing?

A. It uses no direct materials.

B. It consumes disproportionate setup, engineering and inspection activities not captured by volume.

C. Traditional costing always assigns all overhead to high-volume products.

D. Its direct labour rate is necessarily lower.

Correct Answer: B. It consumes disproportionate setup, engineering and inspection activities not captured by volume.

Question 10

Which is an appropriate governance practice for an overhead-costing model?

A. Change drivers whenever a product appears unprofitable.

B. Avoid reconciling the model to the general ledger.

C. Document driver logic and periodically validate the model.

D. Treat every allocated cost as avoidable in the short run.

Correct Answer: C. Document driver logic and periodically validate the model.