Activity Based Costing (PM): A Complete Guide

Activity Based Costing (PM): A Complete Guide

Activity Based Costing (ABC) is a method of assigning overhead costs to products and services. It does this by identifying specific activities and allocating costs based on the actual consumption of each activity, using cost drivers, rather than simply using a single, broad overhead absorption rate.

Why this actually matters

When you're short on time, you need to know what to focus on. Traditional costing can lead you to mis-price products and make poor decisions, which is exactly what the examiner will test. Mastering ABC gives you certainty that you are preparing for a likely exam question and not wasting time on outdated, less examinable methods.

Structured breakdown

To apply ABC correctly, follow this exact five-step process. This is the pro forma examiners expect to see in your answer.

• Step 1: Group overheads into cost pools. Identify the major activities that cause overheads in the organisation (e.g., machine setups, quality inspections, purchase ordering).
• Step 2: Identify the cost driver for each activity. A cost driver is a factor that influences the cost of an activity. For example, the number of setups drives machine setup costs.
• Step 3: Calculate the cost driver rate. For each activity, divide the total cost in the cost pool by the total volume of the cost driver. This gives you a rate (e.g., $50 per machine setup).
• Step 4: Absorb the activity costs into products. Multiply the cost driver rate by the number of cost driver units consumed by each product. This allocates the overheads from the cost pool to the specific products.
• Step 5: Calculate the full production cost. Sum the allocated overheads from all activities for each product and add this to the prime cost (direct materials + direct labour) to get the final unit cost.

Worked example

Scenario: Company Z makes two products, Product A and Product B. The direct costs are:

• Product A: Direct materials $20, Direct labour $10.
• Product B: Direct materials $30, Direct labour $15.

Total production overheads are $150,000. These have been analysed into two cost pools:

• Machine-related costs: $100,000 (Cost driver: machine hours)
• Production setup costs: $50,000 (Cost driver: number of setups)

Production details are as follows:

Requirement: Calculate the full cost per unit for Product A and Product B using Activity Based Costing.

Workings:

Step 1 & 2: Identify Cost Pools and Drivers (Done in the scenario)

• Cost Pool 1: Machine-related costs ($100,000), Driver: Machine hours
• Cost Pool 2: Production setup costs ($50,000), Driver: Number of setups

Step 3: Calculate Cost Driver Rates

1. Machine-related cost driver rate: $100,000 / 40,000 total machine hours = $2.50 per machine hour
2. Production setup cost driver rate: $50,000 / 250 total setups = $200 per setup

Step 4: Absorb Overheads into Products

For Product A:

• Machine-related costs: 10,000 machine hours * $2.50/hour = $25,000
• Production setup costs: 100 setups * $200/setup = $20,000
• Total overheads for Product A: $25,000 + $20,000 = $45,000
• Overhead per unit for Product A: $45,000 / 5,000 units = $9.00

For Product B:

• Machine-related costs: 30,000 machine hours * $2.50/hour = $75,000
• Production setup costs: 150 setups * $200/setup = $30,000
• Total overheads for Product B: $75,000 + $30,000 = $105,000
• Overhead per unit for Product B: $105,000 / 10,000 units = $10.50

Step 5: Calculate Full Production Cost per Unit

Answer and Marking Logic:

The full cost for Product A is $39.00 and for Product B is $55.50. Marks are awarded for each correct calculation: calculating the cost driver rates (2 marks), correctly allocating the overheads to each product based on consumption (2 marks), and calculating the final correct unit costs (2 marks). Showing your workings clearly in the stepped pro forma is essential for securing all available marks, even if you make a calculation error.

If you're working full-time

Studying after a long day at work is draining. Don't try to master all of ABC in one go. Break it down. One evening, focus only on calculating cost driver rates. The next, focus on allocating the overheads. This micro-learning approach is more manageable and effective than a single, exhausting weekend session. Spend 25-30 minutes on a small component, then stop. This builds confidence without leading to burnout.

Common mistakes

Avoid these common errors to protect your marks:

• Using the wrong denominator: Calculating the cost driver rate using production units instead of the actual cost driver (e.g., using 15,000 units instead of 40,000 machine hours).
• Forgetting to add prime costs: Calculating the overhead per unit correctly but then forgetting to add the direct material and direct labour costs to get the full unit cost.
• Mixing up total vs per-unit drivers: Using a 'per unit' driver number (e.g. 2 machine hours) instead of the total (e.g. 10,000 machine hours) when calculating the total overhead for a product line.
• Calculation errors under pressure: Simple mistakes with multiplication or division. Laying out your workings clearly helps mitigate this and can earn follow-through marks.
• Choosing an illogical cost driver: In a scenario question, you must justify your choice of driver. Don't just pick one at random; link it logically to the activity cost pool.
• Ignoring the question's focus: The question might ask for the cost per unit, the total profit, or a comparison with traditional costing. Answer the specific question asked.
• Poorly structured answer: Presenting your answer as a single block of text and numbers is hard for the examiner to mark. Use the step-by-step pro forma.

How examiners think

Examiners want to see that you understand why ABC provides a "fairer" and more accurate cost, not just that you can perform the calculations. They reward students who can explain the link between activities, resource consumption, and cost. Marks are not just for the final number. They are for the method. A clear, structured answer following the 5 steps, even with a final error, will score highly. Conversely, a correct final number with no visible workings will score very few, if any, marks. The examiner is testing your ability to apply a systematic costing technique, not just your ability to use a calculator.

FAQ

Calm summary

This week, take two concrete steps. First, review the 5-step ABC process and write it down from memory. Make sure you can define "cost pool" and "cost driver" accurately. Second, find one past exam question on ABC and attempt only the calculation of the cost driver rates. Do not try to answer the whole question. Just complete that one small part. This builds technical precision and confidence on a core skill that unlocks the rest of the question.