ACC2200 · Introduction to Management Accounting
A Closer Look at Overhead Cost
ACC2200 Introduction to Management Accounting at Monash University refines how manufacturing overhead reaches a product once a factory is split into departments. A single plantwide rate averages a machine-heavy department together with a labour-heavy one, so it quietly under-costs some products and over-costs others; departmental rates fix this by giving each production department its own rate and driver. Overhead travels in two stages — Stage 1 reassigns the cost of support departments (maintenance, stores, IT) to the production departments, then Stage 2 applies each production department's pooled overhead to products. The whole topic turns on one choice: how much service that support departments provide to each other you recognise — nothing (direct), one way (step-down), or in full (reciprocal).
What this chapter covers
- 011. Plantwide rate vs departmental rates — one blended rate for the whole plant versus a separate rate per production department
- 022. Why a single rate distorts — machine-heavy jobs get under-costed and labour-heavy jobs over-costed
- 033. Two-stage allocation — Stage 1 pushes support-department cost into production; Stage 2 applies each department's pool to products
- 044. Support vs production departments — who serves the factory (maintenance, stores) versus who works on product
- 055. The direct method — allocate support cost to production departments only, re-basing the denominator
- 066. The step-down method — sequenced, one-way allocation; the ordering rules; never flow cost back to a closed department
- 077. The reciprocal method — simultaneous equations that recognise mutual support-to-support service in full
- 088. Budgeted rates and departmental over/under-applied overhead — why budgeted data is preferred and how each department disposes of its own balance
Worked example: step-down allocation and departmental rates
- +1Order. Each support department serves one other support department, so the 'serves most other support depts' rule ties. Break the tie by the larger overhead budget: Maintenance $120,000 > Stores $60,000, so allocate Maintenance first, then Stores.
- +1Allocate Maintenance $120,000 over all remaining departments on maintenance hours 1,000 : 6,000 : 3,000 (total 10,000) → Stores 120,000 × 1,000 ÷ 10,000 = $12,000; Machining 120,000 × 6,000 ÷ 10,000 = $72,000; Assembly 120,000 × 3,000 ÷ 10,000 = $36,000.
- +1Allocate Stores. Stores now holds 60,000 + 12,000 = $72,000. Maintenance is closed, so allocate to production only on requisitions 500 : 300 (total 800 — the 200 to Maintenance is dropped) → Machining 72,000 × 500 ÷ 800 = $45,000; Assembly 72,000 × 300 ÷ 800 = $27,000.
- +1Total production overhead. Machining = 400,000 + 72,000 + 45,000 = $517,000; Assembly = 260,000 + 36,000 + 27,000 = $323,000.
- +1Reconcile (always do this): support cost allocated out = 120,000 + 60,000 = $180,000 = received by production (72,000 + 45,000) + (36,000 + 27,000) = 117,000 + 63,000 = $180,000. ✓
- +1Machining rate = 517,000 ÷ 10,000 machine hours = $51.70 per machine hour.
- +1Assembly rate = 323,000 ÷ 8,000 labour hours = $40.38 per labour hour (2 dp).
Key terms
- Plantwide overhead rate
- A single overhead rate applied across the whole factory on one base (e.g. total machine hours or total labour hours). Simple and cheap, but it assumes every product consumes overhead in the same proportion as that one base — so it distorts product cost when departments differ.
- Departmental overhead rate
- A separate predetermined rate for each production department, each on the driver that best explains that department's overhead. Computed in Stage 2 as the department's total overhead (after support-department cost has been reassigned) divided by its budgeted driver level.
- Two-stage allocation
- The route overhead takes to a product. Stage 1 reassigns each support department's cost to the production departments that use it; Stage 2 applies each production department's pooled overhead to products via its own predetermined rate.
- Support (service) department
- A department that serves the factory rather than making product — maintenance, stores, quality control, IT, a staff cafeteria. It incurs overhead but has no product to absorb it, so its cost must be reassigned to production departments in Stage 1.
- Direct method
- The simplest reassignment method: each support department's cost is allocated to production departments only, ignoring all service between support departments. The allocation base is re-based over the production departments alone (the other support department is dropped from the denominator).
- Step-down method
- Support departments are allocated one at a time to all remaining departments (support and production). Once a department has been allocated, it is closed and no cost flows back to it. Ordering rule: allocate first the department serving the most other support departments; break ties by the larger overhead budget.
- Reciprocal method
- The only fully accurate method: it recognises mutual service between support departments in both directions by solving simultaneous equations before any cost reaches production. Worth its extra effort only when support-to-support service is large — a cost-versus-benefit judgement.
- Over/under-applied overhead
- The gap between actual overhead and overhead applied at the predetermined rate. With departmental rates each production department runs its own applied account and its own balance — under-applied (applied < actual) is debited to Cost of Goods Sold; over-applied is credited.
A Closer Look at Overhead Cost FAQ
Can AI help me with overhead cost allocation?
Yes — ask Sia to walk through any overhead cost allocation problem or concept step by step, the way Monash University tests it. Sia is an AI tutor that explains the reasoning: why you allocate one support department before another, why no cost flows back to a closed department, and how each production department's rate is built — so you can reproduce the method yourself under closed-book exam conditions.
What is the difference between a plantwide rate and departmental rates?
A plantwide rate is one overhead rate for the whole factory on a single base; departmental rates give each production department its own rate on its own driver. Departmental rates are more accurate when departments consume overhead differently — for example a machine-heavy department billed on machine hours next to a labour-heavy department billed on labour hours — because each product is charged for the departments it actually passes through.
Which support-department method should I use — direct, step-down or reciprocal?
It is a cost-versus-benefit judgement. Direct ignores all service between support departments; step-down recognises it one way, in sequence; reciprocal recognises it fully via simultaneous equations and is the only completely accurate method. All three hand the same total support cost to production, so the answers are close — reciprocal is worth the extra work only when the service support departments provide each other is large.
In the step-down method, which department do I allocate first?
Allocate first the support department that serves the greatest number of other support departments. If that ties — as it often does with only two support departments — break the tie by allocating the department with the larger overhead budget first. The exam explicitly asks you to state the order and the reason, so write the rule as well as applying it.
Why do I re-base the denominator in the direct method?
Because the direct method ignores service between support departments entirely, you drop the other support department from the allocation base and spread the cost across the production departments only. For example, if a support department serves Machining, Assembly and another support department in the ratio 6,000 : 3,000 : 1,000, the direct method allocates on 6,000 : 3,000 (total 9,000), not on the full 10,000. Forgetting to re-base is a common mark-loser.
Do departmental rates always beat a plantwide rate?
No. Departmental rates are more accurate only if departments genuinely consume overhead in different proportions. If every product flows through every department the same way, a plantwide rate gives the same answer for far less bookkeeping, so the extra departmental detail is not worth its cost. Saying this — that it is a cost-versus-benefit judgement, not an automatic win — earns the discussion mark.
Studying with AI? Sia — free AI accounting tutor works through ACC2200 step by step.
Exam move
Drill the two-stage picture until it is automatic: Stage 1 empties support departments into production departments, Stage 2 divides each production department's pool by its budgeted driver to get a rate. For step-down, state the ordering rule (serves most other support departments; tie-break on the larger budget) and apply it before you allocate, never flow cost back to a closed department, drop the other support department from the direct-method denominator, and always finish with the reconciliation that allocated support equals the original support totals. Keep the discussion reflex sharp too — every choice in this chapter (plantwide vs departmental, direct vs step-down vs reciprocal) is an accuracy-versus-cost trade-off, and the exam pays for naming it. Because Week 6 is a primary-emphasis topic, expect it in both the group case-study report and the closed-book final e-exam, where roughly 60% of marks are calculation and 40% discussion.
