Activity-based vs traditional costing: which one tells the truth.

The same two products, two costing systems

The argument is easier to settle with numbers than with adjectives. Take a plant with an overhead pool of 3,600,000 euro a year and two products. Product A is simple and high volume: 100,000 units, and because it runs continuously it needs only 10 machine setups in the year. Product B is complex and low volume: 10,000 units, but it is made in short runs with constant changeovers, so it needs 150 setups.

Traditional absorption spreads the pool by volume. Divide 3,600,000 by the 110,000 units made and every unit absorbs about 32.73 euro of overhead, whichever product it belongs to. Activity-based costing spreads it by what causes the cost, which here is the setup. Divide 3,600,000 by the 160 setups and each one carries 22,500 euro.

Illustrative figures. Both systems allocate the same pool in full: 3,272,727 + 327,273 = 3,600,000 and 225,000 + 3,375,000 = 3,600,000. The accounts balance either way, which is why nobody notices.

What changes is the per-unit answer, and it changes by a factor of about ten. Under the volume rate Product B carries 32.73 euro of overhead a unit. Under the setup rate it carries 337.50. If the selling price of B was set on the first figure, the company has been discounting a product that consumes ten times the overhead it was charged for, and funding it out of the margin of Product A, which was carrying overhead it never caused. Neither number is an accounting error. They are two defensible answers to two different questions, and only one of them is a basis for a price.

Common mistakes when comparing the two methods

These come up in almost every conversation a company has before it changes method, and most of them are reasonable positions taken one step too far.

• Treating absorption costing as an error to be corrected. It answers a financial reporting question about inventory valuation, and it answers it well. It becomes a problem only when someone prices on it in a business whose complexity has outgrown a single rate.
• Confusing the accounting question with the decision question. A number can reconcile to the ledger to the cent and still be useless for deciding which product to push. The example above reconciles perfectly in both columns.
• Building classic ABC and then abandoning it. ABC based on staff surveys of percentage time is expensive to collect and ages badly. A good number of the ABC projects of the 1990s died of maintenance rather than of method, which is the gap Time-Driven ABC was written to close.
• Buying precision that does not pay. In a stable operation with a narrow range, a detailed model costs more to run than the decisions it improves are worth. Choose the method against the complexity you actually carry, not against the complexity you might have one day.
• Ignoring the capacity you paid for and did not use. A single absorption rate distributes the whole pool across whatever was produced, so idle capacity is invisible by construction. TDABC reports it as its own line, and in most first models it is the largest single surprise.

How to move from one to the other

If your range, order sizes or customer types have grown since the costing was set up, the single rate is probably distorting decisions somewhere. That does not mean a costing project. It usually means testing one department first, because a department is small enough to finish and large enough to prove the point.

The practical route is TDABC rather than classic ABC: two parameters per resource pool, a cost per minute of practical capacity and a short time equation, built from data you already hold in the ERP and in your financial and operational records. Orders, deliveries, invoices, setups and activity times are usually enough for a first pass, and a first pass on one department is a few weeks of work, not a programme. CostCtrl is where we put that model when it has to survive monthly, but the arithmetic on this page works perfectly well in a spreadsheet for as long as one spreadsheet can carry it. If the question is wider than one department, our cost management consulting page sets out how the diagnostic runs, what it costs, and when a cut is the right answer after all.

If you would rather see where you sit before deciding anything, the Profit Check takes 10 minutes and will tell you whether your overhead and your range are in the territory where a single rate still holds. And if you already know the answer and want to talk about the department to start with, 20 minutes on a call is usually enough to pick one.

Common questions

What is the difference between activity-based and traditional costing?

Traditional, or absorption, costing spreads overhead across products using a single volume-based rate such as labour hours or units made. Activity-based costing first identifies the activities that consume resources, then traces overhead to products and customers by how much of each activity they use. Traditional costing is simpler; activity-based costing is more accurate where complexity is high.

Is traditional absorption costing wrong?

Not wrong, just blunt. Where a business makes a few similar products in similar volumes, a single overhead rate is close enough and cheap to run. It distorts the truth when product range, order sizes and customer behaviour vary a lot, because a volume-based rate cannot see that variety. Then it over-costs the simple, high-volume lines and under-costs the complex, low-volume ones.

When should I switch from traditional to activity-based costing?

Switch when complexity has outgrown the single rate: a wide product range, very different order profiles, significant overhead relative to direct cost, or pricing decisions that keep being contradicted by reality. If overhead is small and the mix is uniform, the simpler method is fine. The test is whether the numbers are driving good decisions.

How does TDABC relate to activity-based costing?

Time-Driven Activity-Based Costing is a faster, more maintainable form of ABC. Instead of surveying staff for the percentage of time spent on each activity, it estimates the time each activity takes and the cost per minute of capacity. It keeps the accuracy of ABC while removing much of the cost and maintenance that made classic ABC hard to sustain.

Related: TDABC vs ABC, compared·What cost-to-serve analysis reveals·The margin cascade, layer by layer