Study guide
3.4 Overhead Absorption and Activity-Based Costing
AAT Level 3 Diploma in Accounting - Management Accounting Techniques MATS
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1 Chapter 1: Purpose and Use of Management Accounting
1.1 Role and Scope of Management Accounting 1.2 Cost Classification and Responsibility Accounting 1.3 Information Needs and Management Reporting
2 Chapter 2: Techniques for Dealing with Costs
2.1 Cost Behaviour Patterns 2.2 High-Low Method and Cost Estimation 2.3 Costing Methods: Job, Batch, Unit, and Service Costing 2.4 Cost Accounting Records and Journals
3 Chapter 3: Attributing Costs to Organisational Requirements
3.1 Material Costs and Inventory Valuation 3.2 Labour Costing and Remuneration Systems 3.3 Overhead Allocation, Apportionment, and Reapportionment 3.4 Overhead Absorption and Activity-Based Costing
4 Chapter 4: Investigating Deviations from Budgets - Variance Analysis
4.1 Standard Costing and Flexible Budgets 4.2 Direct Material and Direct Labour Variances 4.3 Overhead and Sales Variances 4.4 Reconciliation Operating Statements and Variance Interpretation
5 Chapter 5: Spreadsheet Techniques
5.1 Spreadsheet Design Principles and Data Integrity 5.2 Essential Spreadsheet Functions for Accounting 5.3 Data Analysis, Formatting, and Security
6 Chapter 6: Short-Term Decision Making
6.1 Marginal vs Absorption Costing 6.2 Cost-Volume-Profit (CVP) and Break-Even Analysis 6.3 Relevant Costing and Non-Routine Decisions 6.4 Limiting Factor Analysis and Make-or-Buy Decisions
7 Chapter 7: Principles of Cash Management
7.1 Cash vs Profit and Cash Flow Forecasting 7.2 Working Capital Cycle and Cash Control 7.3 Liquidity, Treasury, and Financing Options
3.4 Overhead Absorption and Activity-Based Costing
Key Takeaways Explain This Topic AI
Overhead Absorption Rate (OAR) absorbs production overheads into cost units by dividing budgeted overheads by budgeted activity volume.
Under-absorption occurs when actual overhead incurred exceeds absorbed overhead; over-absorption occurs when absorbed overhead exceeds actual overhead.
The absorption base reflects department operations: machine hours for automated cost centres, direct labour hours for manual cost centres.
Activity-Based Costing (ABC) pools overheads by operational activities and attributes them to products using activity cost drivers.
ABC eliminates cost distortions inherent in traditional volume-based absorption, providing accurate costs for low-volume complex products.
Last updated: August 2026
3.4 Overhead Absorption and Activity-Based Costing
Once production department overheads are fully aggregated (following allocation, primary apportionment, and service department reapportionment), they must be incorporated into the total unit cost of products. In AAT Level 3 Management Accounting Techniques (MATS) , candidates learn how to calculate traditional Overhead Absorption Rates (OAR) , analyze under- and over-absorption , and contrast traditional absorption costing with modern Activity-Based Costing (ABC) .
Traditional Overhead Absorption Rate (OAR)
Under traditional absorption costing, overheads are absorbed into units of product using a predetermined rate calculated prior to the financial period: Predetermined OAR = Budgeted Production Overheads Budgeted Activity Level \text{Predetermined OAR} = \frac{\text{Budgeted Production Overheads}}{\text{Budgeted Activity Level}} Predetermined OAR = Budgeted Activity Level Budgeted Production Overheads
Common Activity Bases for OAR
Absorption Base Formula for OAR Most Suitable Operational Context
Direct Labour Hours Budgeted Overheads Budgeted Direct Labour Hours \frac{\text{Budgeted Overheads}}{\text{Budgeted Direct Labour Hours}} Budgeted Direct Labour Hours Budgeted Overheads Manual, labour-intensive production cost centres
Machine Hours Budgeted Overheads Budgeted Machine Hours \frac{\text{Budgeted Overheads}}{\text{Budgeted Machine Hours}} Budgeted Machine Hours Budgeted Overheads Highly automated, machinery-intensive cost centres
Units of Output Budgeted Overheads Budgeted Units of Output \frac{\text{Budgeted Overheads}}{\text{Budgeted Units of Output}} Budgeted Units of Output Budgeted Overheads Single-product environments with identical output units
% of Direct Labour Cost ( Budgeted Overheads Budgeted Direct Labour Cost ) × 100 \left(\frac{\text{Budgeted Overheads}}{\text{Budgeted Direct Labour Cost}}\right) \times 100 ( Budgeted Direct Labour Cost Budgeted Overheads ) × 100 Stable hourly wage environments
% of Prime Cost ( Budgeted Overheads Budgeted Prime Cost ) × 100 \left(\frac{\text{Budgeted Overheads}}{\text{Budgeted Prime Cost}}\right) \times 100 ( Budgeted Prime Cost Budgeted Overheads ) × 100 Simple, uniform material and labour job structures
Under- and Over-Absorption of Overheads
Because predetermined OARs are calculated using budgeted figures, the total overhead absorbed into production during a period will rarely equal the actual overhead expenditure incurred. Overhead Absorbed = Actual Activity Level Achieved × Predetermined OAR \text{Overhead Absorbed} = \text{Actual Activity Level Achieved} \times \text{Predetermined OAR} Overhead Absorbed = Actual Activity Level Achieved × Predetermined OAR Absorbed vs Actual Variance = Overhead Absorbed − Actual Overhead Incurred \text{Absorbed vs Actual Variance} = \text{Overhead Absorbed} - \text{Actual Overhead Incurred} Absorbed vs Actual Variance = Overhead Absorbed − Actual Overhead Incurred
Over-Absorption : Occurs when Absorbed Overhead > Actual Overhead Incurred . (Favourable impact: excess overhead charged to products is credited back to profit).
Under-Absorption : Occurs when Absorbed Overhead < Actual Overhead Incurred . (Adverse impact: insufficient overhead charged to products is debited as an additional expense to profit).
Causes of Under/Over-Absorption
Actual overhead expenditure differed from budgeted overhead expenditure.
Actual activity hours worked differed from budgeted activity hours.
Worked Example: Under/Over Absorption Calculation
Scenario : For August, the Machining Department budgeted overheads of £60,000 and 15,000 machine hours. Actual results were £63,500 actual overheads incurred and 15,500 actual machine hours worked.
Predetermined OAR : OAR = £ 60 , 000 15 , 000 machine hrs = £ 4.00 / machine hour \text{OAR} = \frac{\pounds 60,000}{15,000 \text{ machine hrs}} = \mathbf{\pounds 4.00 / \text{machine hour}} OAR = 15 , 000 machine hrs £60 , 000 = £4.00/ machine hour
Overhead Absorbed : Absorbed Overhead = 15 , 500 actual hrs × £ 4.00 = £ 62 , 000 \text{Absorbed Overhead} = 15,500 \text{ actual hrs} \times \pounds 4.00 = \mathbf{\pounds 62,000} Absorbed Overhead = 15 , 500 actual hrs × £4.00 = £62 , 000
Absorption Variance : Variance = Absorbed (£62,000) − Actual (£63,500) = − £ 1 , 500 ( U n d e r − a b s o r b e d ) \text{Variance} = \text{Absorbed (\pounds 62,000)} - \text{Actual (\pounds 63,500)} = -\mathbf{\pounds 1,500 \quad (Under-absorbed)} Variance = Absorbed (£62,000) − Actual (£63,500) = − £1 , 500 ( Under − absorbed )
Interpretation : The department under-absorbed overheads by £1,500 because actual expenditure exceeded absorbed amounts. This £1,500 under-absorption is charged as an expense in the monthly profit statement.
Activity-Based Costing (ABC)
Traditional volume-based absorption costing (using direct labour or machine hours) was developed when direct labour dominated production costs. In modern manufacturing, automation and complex product ranges mean that indirect overheads form a huge proportion of cost, driven by activities rather than simple volume.
Core ABC Terminology
Cost Pool : An aggregation of indirect costs associated with a specific business activity (e.g., Machine Setup Cost Pool, Quality Inspection Cost Pool, Order Processing Cost Pool).
Cost Driver : The factor or event that causes the cost of a specific activity to increase (e.g., Number of Setups, Number of Inspections, Number of Purchase Orders).
Activity Driver Rate : Activity Driver Rate = Total Cost Pool Expenditure Total Volume of Cost Driver \text{Activity Driver Rate} = \frac{\text{Total Cost Pool Expenditure}}{\text{Total Volume of Cost Driver}} Activity Driver Rate = Total Volume of Cost Driver Total Cost Pool Expenditure
Product Cost Distortion Under Traditional Costing vs ABC
Traditional Costing Distortion : High-volume, simple products consume many direct hours and are assigned an unfairly high proportion of overheads. Low-volume, complex bespoke products require frequent setups and inspections, but consume few direct hours and are severely under-costed .
ABC Solution : ABC attributes overheads based on actual activity consumption, providing accurate product costing for pricing and profitability decisions.
Worked Numerical Example: Traditional Absorption vs ABC
Scenario : Enterprise Ltd manufactures Product X (High Volume Standard) and Product Y (Low Volume Complex).
Total Overheads = £120,000 , consisting of:
- Machine Setups Pool: £60,000 (Cost Driver: Number of Setups; Total = 100 setups)
- Quality Inspection Pool: £60,000 (Cost Driver: Number of Inspections; Total = 200 inspections)
Operational Data :
- Data Item Product X (Standard) Product Y (Complex) Total
- Production Volume 10,000 units 1,000 units 11,000 units
- Direct Labour Hours per Unit 1.0 hr 2.0 hrs
- Total Direct Labour Hours 10,000 hrs 2,000 hrs 12,000 hrs
- Number of Setups 20 setups 80 setups 100 setups
- Number of Inspections 50 inspections 150 inspections 200 inspections
- 1. Traditional OAR Method (based on Labour Hours)
OAR = £ 120 , 000 12 , 000 labour hrs = £ 10.00 / labour hour \text{OAR} = \frac{\pounds 120,000}{12,000 \text{ labour hrs}} = \mathbf{\pounds 10.00 / \text{labour hour}} OAR = 12 , 000 labour hrs £120 , 000 = £10.00/ labour hour .
Overhead per Unit of Product X : 1.0 hr × £ 10.00 = £ 10.00 / unit 1.0 \text{ hr} \times \pounds 10.00 = \mathbf{\pounds 10.00 / \text{unit}} 1.0 hr × £10.00 = £10.00/ unit .
Overhead per Unit of Product Y : 2.0 hrs × £ 10.00 = £ 20.00 / unit 2.0 \text{ hrs} \times \pounds 10.00 = \mathbf{\pounds 20.00 / \text{unit}} 2.0 hrs × £10.00 = £20.00/ unit .
2. Activity-Based Costing (ABC) Method
Setup Driver Rate : £ 60 , 000 100 setups = £ 600 / setup \frac{\pounds 60,000}{100 \text{ setups}} = \mathbf{\pounds 600 / \text{setup}} 100 setups £60 , 000 = £600/ setup .
Inspection Driver Rate : £ 60 , 000 200 inspections = £ 300 / inspection \frac{\pounds 60,000}{200 \text{ inspections}} = \mathbf{\pounds 300 / \text{inspection}} 200 inspections £60 , 000 = £300/ inspection .
Overhead Allocation via ABC:
Product X (10,000 units) :
- Setups: 20 × £ 600 = £ 12 , 000 20 \times \pounds 600 = \pounds 12,000 20 × £600 = £12 , 000
- Inspections: 50 × £ 300 = £ 15 , 000 50 \times \pounds 300 = \pounds 15,000 50 × £300 = £15 , 000
Total ABC Overhead = £27,000 → £ 2.70 / unit \rightarrow \mathbf{\pounds 2.70 / \text{unit}} → £2.70/ unit .
Product Y (1,000 units) :
- Setups: 80 × £ 600 = £ 48 , 000 80 \times \pounds 600 = \pounds 48,000 80 × £600 = £48 , 000
- Inspections: 150 × £ 300 = £ 45 , 000 150 \times \pounds 300 = \pounds 45,000 150 × £300 = £45 , 000
Total ABC Overhead = £93,000 → £ 93.00 / unit \rightarrow \mathbf{\pounds 93.00 / \text{unit}} → £93.00/ unit .
Conclusion & Comparison
Under traditional absorption, Product Y was dramatically under-costed (£20 vs £93) , while Product X was over-costed (£10 vs £2.70) . ABC reveals that Product Y consumes 80% of setups and 75% of inspections despite low unit volume.