6.3 Overhead Absorption Rates & Under/Over Absorption

Overhead Absorption Rates & Under/Over Absorption

Core Principle: Overheads can only be absorbed into units of production using a predetermined absorption rate established before the accounting period begins. Because actual expenditure and actual production activity inevitably diverge from budget, the overhead absorbed into work-in-progress will rarely equal the actual overhead incurred, generating under- or over-absorbed overhead that must be reconciled in the Statement of Profit or Loss.

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1. Rationale for Predetermined Overhead Absorption Rates

Why Actual Rates Cannot Be Used

An enterprise cannot wait until the end of an annual accounting period to calculate its actual overhead costs and divide them by actual production volume. In business operations:

1. Immediate Decision-Making: Managers must quote selling prices to prospective customers daily, invoice completed jobs, and evaluate order profitability throughout the year.
2. Interim Inventory Valuation: Monthly and quarterly financial statements require valuations for work-in-progress and finished goods under IAS 2.
3. Seasonal Cost Fluctuations: Actual overheads vary seasonally (e.g., factory heating and electricity bills peak in winter, while annual machinery insurance is paid in January). If actual monthly rates were used, identical products manufactured in winter would bear substantially higher unit costs than those made in summer.
4. Volume Distortions: Production volumes often fluctuate due to holiday shutdowns or cyclical customer demand. Dividing a constant fixed overhead by a low monthly production volume creates an artificial surge in unit cost during quiet months.

The Predetermined OAR Formula

To ensure stable, timely, and consistent unit costing, management accountants calculate a Predetermined Overhead Absorption Rate (OAR) at the beginning of the annual budget cycle:

$\text{Predetermined OAR} = \frac{\text{Budgeted Production Overheads}}{\text{Budgeted Activity Level}}$

Where:

• Budgeted Production Overheads: The total budgeted factory overhead allocated, apportioned, and reapportioned to the cost centre for the upcoming year.
• Budgeted Activity Level: The anticipated annual volume of the chosen operational activity base (e.g., direct labour hours, machine hours, or units of output).

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2. Selecting an Appropriate Overhead Absorption Base

Selecting an inappropriate activity base distorts product costs, leading to poor pricing decisions and misstated product profitability. The ACCA syllabus requires understanding the five principal absorption bases:

1. Direct Labour Hour Rate

$\text{OAR per Direct Labour Hour} = \frac{\text{Budgeted Overheads}}{\text{Budgeted Direct Labour Hours}}$

• Suitability: Most appropriate for labour-intensive production environments where manual craftsmanship, assembly, or hand finishing dominates cycle times and overhead incurrence.

2. Machine Hour Rate

$\text{OAR per Machine Hour} = \frac{\text{Budgeted Overheads}}{\text{Budgeted Machine Hours}}$

• Suitability: Most appropriate for capital-intensive, highly automated, or machine-dominated environments (such as CNC machining, automated pressing, or robotic injection moulding), where machinery depreciation, power, and maintenance form the primary overhead drivers.

3. Unit of Output Rate

$\text{OAR per Unit} = \frac{\text{Budgeted Overheads}}{\text{Budgeted Production Units}}$

• Suitability: Appropriate ONLY when the cost centre produces a single homogeneous product, or where all units consume identical proportions of time and resources. If a department produces varied products (e.g., standard chairs and custom executive desks), a unit rate severely distorts costs.

4. Percentage of Direct Labour Cost

$\text{OAR \% of Direct Labour Cost} = \left(\frac{\text{Budgeted Overheads}}{\text{Budgeted Direct Labour Cost}}\right) \times 100\%$

• Evaluation: Easy to calculate, but can introduce significant distortion. Highly paid skilled artisans working on a delicate piece will absorb far more overhead than an unskilled worker operating a massive, expensive machine, even though the machine consumes vastly more electricity, maintenance, and space.

5. Percentage of Prime Cost

$\text{OAR \% of Prime Cost} = \left(\frac{\text{Budgeted Overheads}}{\text{Budgeted Prime Cost}}\right) \times 100\%$

• Evaluation: Heavily flawed because prime cost is dominated by direct materials. Products made from expensive raw materials (such as gold or titanium) would absorb immense overhead, while products made from cheap materials (such as plastics) would absorb negligible overhead, despite consuming identical factory resources.

Departmental vs. Blanket (Plant-Wide) Rates

• Blanket (Plant-Wide) OAR: A single overhead absorption rate calculated for the entire factory by dividing total factory overheads by total factory hours. This is acceptable only if all products spend roughly identical time in every department.
• Departmental OARs: Separate absorption rates established for each individual production department (e.g., a machine hour rate for Machining, and a direct labour hour rate for Assembly). This is far superior because products that spend 90% of their time in automated machining absorb mostly machine overheads, avoiding cross-subsidisation.

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3. Calculating Overhead Absorbed in Production

Once the predetermined OAR is set, it is used throughout the year to absorb overhead into jobs and products as work is performed.

$\text{Overhead Absorbed} = \text{Actual Activity Achieved} \times \text{Predetermined OAR}$

[!IMPORTANT] The Golden Rule of Overhead Absorption: Overhead is ALWAYS absorbed based on ACTUAL activity achieved (actual direct labour hours worked, actual machine hours run, or actual units produced) multiplied by the PREDETERMINED rate. You must NEVER multiply budgeted activity by the predetermined rate (which simply reproduces the budget), nor actual activity by an actual rate.

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4. Under-Absorption and Over-Absorption of Overheads

At the end of the accounting period, the actual overhead costs incurred are tallied and compared against the total overhead absorbed into production.

Definitions and Formulas

$\text{Net Absorption Difference} = \text{Overhead Absorbed} - \text{Actual Overhead Incurred}$

- If Overhead Absorbed > Actual Overhead Incurred ──> OVER-ABSORPTION (Credit adjustment / Profit Increase)
- If Overhead Absorbed < Actual Overhead Incurred ──> UNDER-ABSORPTION (Debit adjustment / Profit Decrease)

• Under-Absorption: The overhead absorbed into work-in-progress was insufficient to cover the actual indirect expenses incurred. Product costs during the period were effectively understated. The shortfall is an unrecovered cost that must be debited to the Statement of Profit or Loss as an additional expense, reducing profit.
• Over-Absorption: The overhead absorbed into work-in-progress exceeded the actual indirect expenses incurred. Product costs were overstated during the period. The excess is credited to the Statement of Profit or Loss, increasing reported profit.

Comprehensive Worked Example: Calculating Under/Over Absorption

Scenario: Vanguard Manufacturing established the following annual budget for two production departments:

At the end of the year, actual operational records revealed:

• Machining: Incurred actual overheads of $374,000; operated 46,200 actual machine hours; recorded 11,800 direct labour hours.
• Assembly: Incurred actual overheads of $194,000; worked 38,500 actual direct labour hours; ran 5,200 machine hours.

Step 1: Determine the Appropriate OAR for Each Department

• Machining: Dominated by machinery (45,000 machine hours vs 12,000 labour hours) $\implies$ Use Machine Hours: $\text{OAR}_{\text{Machining}} = \frac{360,000}{45,000 \text{ machine hours}} = \mathbf{8.00 \text{ per machine hour}}$
• Assembly: Dominated by manual labour (40,000 labour hours vs 5,000 machine hours) $\implies$ Use Direct Labour Hours: $\text{OAR}_{\text{Assembly}} = \frac{200,000}{40,000 \text{ labour hours}} = \mathbf{5.00 \text{ per direct labour hour}}$

Step 2: Calculate Overhead Absorbed in Each Department

• Machining: $\text{Overhead Absorbed} = 46,200 \text{ actual machine hours} \times 8.00 = \mathbf{369,600}$
• Assembly: $\text{Overhead Absorbed} = 38,500 \text{ actual labour hours} \times 5.00 = \mathbf{192,500}$

Step 3: Calculate Under- or Over-Absorption

• Machining Department:

  • Overhead Absorbed = $369,600
  • Actual Overhead Incurred = $374,000
  • Net Difference = $369,600 - $374,000 = -$4,400
  • Result: UNDER-ABSORPTION of $4,400 (Absorbed < Actual by $4,400).

• Assembly Department:

  • Overhead Absorbed = $192,500
  • Actual Overhead Incurred = $194,000
  • Net Difference = $192,500 - $194,000 = -$1,500
  • Result: UNDER-ABSORPTION of $1,500 (Absorbed < Actual by $1,500).

• Combined Enterprise Position:

  • Total Absorbed = $369,600 + $192,500 = $562,100
  • Total Incurred = $374,000 + $194,000 = $568,000
  • Total Under-Absorption = $5,900 (Transferred as a debit adjustment to Profit or Loss).

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5. Decomposing Under/Over Absorption: Root Causes

Under- or over-absorption is not a single random variance; it arises from two distinct operational root causes:

                    Total Under- or Over-Absorption
                                   │
         ┌─────────────────────────┴─────────────────────────┐
         ▼                                                   ▼
Expenditure (Spending) Variance                      Volume Variance
Did we spend more or less than budget?              Did we work more or fewer hours than budget?
(Budgeted Overhead - Actual Overhead)               (Actual Activity - Budgeted Activity) × OAR

1. The Expenditure (Spending) Variance

The expenditure variance measures whether the organisation spent more or less money on overhead items than originally budgeted:

$\text{Overhead Expenditure Variance} = \text{Budgeted Overheads} - \text{Actual Overheads}$

• Favourable ($F$): Actual overheads incurred were less than budgeted overheads (under-spending).
• Adverse ($A$): Actual overheads incurred exceeded budgeted overheads (over-spending).

2. The Volume Variance

The volume variance measures the impact of operating at an activity level different from planned budget capacity. Because fixed overheads are absorbed on an hourly rate, working more hours than budgeted absorbs extra fixed overhead:

$\text{Overhead Volume Variance} = (\text{Actual Activity} - \text{Budgeted Activity}) \times \text{Predetermined OAR}$

• Favourable ($F$): Actual hours worked exceeded budgeted hours (surplus capacity utilisation absorbs extra overhead).
• Adverse ($A$): Actual hours worked were lower than budgeted hours (idle capacity results in unabsorbed overhead).

Mathematical Proof: Synthesising Total Under/Over Absorption

Adding the Expenditure Variance and Volume Variance together yields the exact Total Under- or Over-Absorption:

$\text{Total Absorption Variance} = \text{Expenditure Variance} + \text{Volume Variance}$

Let us prove this mathematically using the Machining Department from our earlier example:

• Budgeted Overhead = $360,000; Budgeted Hours = 45,000 hrs; OAR = $8.00/hr
• Actual Overhead = $374,000; Actual Hours = 46,200 hrs

1. Expenditure Variance: $\text{Budgeted Overhead } (360,000) - \text{Actual Overhead } (374,000) = \mathbf{14,000 \text{ Adverse}}$
2. Volume Variance: $(\text{Actual Hours } 46,200 - \text{Budgeted Hours } 45,000) \times 8.00 = +1,200 \text{ hrs} \times 8.00 = \mathbf{9,600 \text{ Favourable}}$
3. Net Total Absorption Variance: $14,000 \text{ Adverse} + 9,600 \text{ Favourable} = \mathbf{4,400 \text{ Adverse (Under-Absorbed)}}$

Analysis: The department suffered a $14,000 cost overrun in actual spending, but partially mitigated this by running 1,200 extra machine hours (generating $9,600 in extra absorbed overhead), resulting in a net under-absorption of exactly $4,400.

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6. Accounting Treatment & Ledger Entries

The Production Overhead Control Account

All transactions relating to factory overhead flow through the Production Overhead Control Account:

• Debit Side (Actual Incurred): Records actual overhead expenses as they arise (credit Bank, Accounts Payable, Accumulated Depreciation, or Wages Control).
• Credit Side (Overhead Absorbed): Records overhead absorbed into production jobs as actual hours are worked (debit Work-in-Progress Control Account).

                     Production Overhead Control Account
---------------------------------------------------------------------------------
Dr (Actual Overhead Incurred)                      Cr (Overhead Absorbed into WIP)
Payables / Bank (Rent, Power)          $240,000 | WIP Control (Absorbed Overhead) $369,600
Wages Control (Indirect Labour)        $80,000  | 
Accumulated Depreciation (Plant)       $54,000  | Statement of Profit or Loss      $4,400
                                                | (Under-absorbed balance cleared)
------------------------------------------------|--------------------------------
Total                                  $374,000 | Total                          $374,000

Year-End Closing Journal Entries

At the end of the accounting period, the balance on the Production Overhead Control Account represents either under-absorbed (debit balance) or over-absorbed (credit balance) overhead. This balance is closed out to the Statement of Profit or Loss:

1. Closing Under-Absorbed Overhead (Debit Balance):

   • Debit: Statement of Profit or Loss (Additional overhead cost / Profit reduction)
   • Credit: Production Overhead Control Account (Clears debit balance to zero)

2. Closing Over-Absorbed Overhead (Credit Balance):

   • Debit: Production Overhead Control Account (Clears credit balance to zero)
   • Credit: Statement of Profit or Loss (Overhead recovery credit / Profit addition)

Presentation in Financial Statements

In the published or internal Statement of Profit or Loss, the adjustment is presented directly beneath Gross Profit or as an adjustment to Cost of Goods Sold:

Gross Profit (at standard absorption cost)                  $450,000
Adjustment for Overhead Absorption:
  Less: Under-absorbed production overhead                  ($4,400)
  [Or Add: Over-absorbed production overhead]               ───────
Adjusted Gross Profit                                      $445,600

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7. ACCA Exam Traps & Common Pitfalls

[!WARNING] Exam Trap 1: Multiplying Budgeted Hours by Predetermined OAR: The most frequent student mistake is calculating overhead absorbed as Budgeted Hours × Predetermined OAR. This merely reproduces the budgeted overhead dollar figure. Overhead absorbed must ALWAYS be Actual Activity Achieved × Predetermined OAR.

[!WARNING] Exam Trap 2: Inverting Under/Over Absorption: Remember:

• Absorbed > Actual indicates OVER-absorbed (Good news for cost recovery; increases profit).
• Absorbed < Actual indicates UNDER-absorbed (Shortfall in cost recovery; reduces profit).

[!WARNING] Exam Trap 3: Confusing Expenditure and Volume Variances: When an exam question asks for the volume variance, do NOT look at actual dollar spending. The volume variance depends strictly on the difference between actual hours and budgeted hours, valued at the predetermined OAR.