The Cost of Unused Capacity
Idle capacity is real money that has already been spent. When it is measured on its own line rather than smeared into unit cost, the numbers stay honest, prices stay defensible, and the demand-death-spiral of costing on volume you no longer have is avoided.
The cost of unused capacity is the portion of a resource's committed cost that supports capability you paid for but did not use in the period. It is calculated by costing resources at their practical capacity - the realistic output a person, machine or facility can deliver allowing for breaks, maintenance and normal downtime - rather than at whatever volume actually occurred. The capacity cost rate is the total cost of a resource group divided by its practical capacity in time units; multiplying that rate by the time actually used charges work with the cost of the capacity it consumed, and the balance - rate times idle time - is reported separately as the cost of unused capacity. This one discipline stops idle cost from inflating the unit cost of the work that did happen, keeps prices from chasing falling volumes upward, and turns spare capacity into a visible number a manager can act on: sell it, redeploy it, or remove it.
Do not smear idle cost into unit cost
Traditional costing divides a period's total cost by the period's actual volume. In a slow month, the same fixed cost is spread over fewer units, so the reported cost per unit rises - even though nothing about the product or the process changed. If prices or decisions follow that number, the business raises prices into weak demand, loses more volume, and recomputes an even higher unit cost next period. Kaplan and Cooper named this trap the death spiral, and its root cause is a single accounting choice: letting the cost of idle capacity ride along inside the cost of good output.
The fix is to cost resources at the capacity they can realistically supply, not the capacity a particular period happened to consume. The cost of the work done is then stable regardless of volume, and the cost of what was not done becomes its own visible figure. That figure is not a rounding error to be buried; it is management information. Unused capacity has an owner, a cause and a set of choices attached to it, and it belongs on its own line where those choices can be debated.
Practical capacity, not theoretical
Theoretical capacity is the output if a resource ran flat out with no interruptions - every minute of every shift, no breaks, no maintenance, no setup. It is a ceiling nobody reaches, and costing against it understates every rate and overstates idle cost.
Practical capacity is the sensible working level: theoretical capacity less the time any real resource loses to breaks, training, maintenance, changeovers and unavoidable slack. A common convention is to take roughly 80% to 85% of theoretical time for people, and a similar allowance for machines depending on their duty cycle. Practical capacity is the honest denominator - high enough that genuine idleness shows up as idleness, not as a padded standard, yet realistic enough that a fully-loaded resource is not reported as impossibly over-utilised.
Getting this denominator right matters more than any decimal in the rate above it. Set it too high and normal working looks wasteful; set it too low and real slack disappears from view. Practical capacity is a judgement, made once and reviewed, and it is the quiet foundation the whole calculation rests on.
The capacity cost rate
The engine is a single rate. Take the total cost of a resource group for the period - salaries, space, equipment, supervision, everything committed to keep that capability available - and divide it by the practical capacity of that group expressed in time units, usually minutes or hours. The result is the capacity cost rate: the cost of one minute of that resource being ready to work.
From there the arithmetic is simple and self-balancing. Multiply the rate by the time actually consumed by the work and you get the cost charged to that work. Multiply the same rate by the time left idle and you get the cost of unused capacity. The two always add back to the total committed cost, so nothing is lost and nothing is invented - the period's spend is simply split into the part that did work and the part that stood ready. This is the signature move of Time-Driven Activity-Based Costing (TDABC), and it is what lets cost-to-serve figures stay stable while quietly surfacing spare capacity as a managed quantity rather than a hidden subsidy inside every rate.
Capacity supplied versus capacity used
A processing team of 10 people costs €600,000 a year, all in (illustrative figures, not client data). Each person supplies about 1,700 productive hours a year after holidays, training and breaks, so practical capacity is 17,000 hours, or 1,020,000 minutes. The capacity cost rate is €600,000 / 1,020,000 = €0.588 per minute, roughly €35.3 per hour.
This year the team actually handled work that consumed 13,600 hours - 816,000 minutes. Cost of work done is 816,000 × €0.588 = €480,000. The remaining 3,400 hours (204,000 minutes) were unused: 204,000 × €0.588 = €120,000, the cost of unused capacity. The two figures add back to the full €600,000.
Note what the traditional method would have done. Dividing €600,000 by only the 13,600 hours actually worked gives €44.1 per hour - a rate 25% higher, built entirely on idleness. Every job costed that way would carry a quarter of someone else's idle time, and a manager reading it would conclude the work had become more expensive when in truth demand had simply fallen. The capacity view instead reports the same €35.3 per hour on the work, plus a clean €120,000 line that reads: this is the cost of the empty chairs, and here is roughly two people's worth of capacity to either fill or release.
What the idle line is telling you
| Cause of the unused capacity | The decision it points to |
|---|---|
| Seasonal or cyclical trough in demand | Hold the capacity if the peak needs it; do not price the trough into the product |
| Structural loss of volume that will not return | Resize the resource - the idle cost is the size of the cut on the table |
| Capacity deliberately held for growth or resilience | A conscious investment; report it as such, not as waste |
| Bottleneck elsewhere starving this resource of work | Fix the constraint, not this resource; the idle here is a symptom |
| Product or customer mix that under-loads the resource | Sell the spare time, or route more suitable work through it |
The point of the separate line is that idle capacity is rarely nobody's fault and rarely a single thing. Splitting the committed cost into used and unused does not by itself say whether the idleness is prudent or wasteful - it puts the number on the table so that question can finally be asked of the right person.
When capacity costing earns its keep
Strengths. Costing on practical capacity keeps unit costs stable through the demand cycle, kills the death spiral, and converts vague talk of "spare capacity" into a euro figure a manager can defend, sell or cut. It is the natural foundation for TDABC and for any honest cost-to-serve or customer-profitability model, because it stops idle cost from silently taxing whichever product or customer happened to be measured.
Limits. The whole calculation rests on the practical-capacity estimate, which is a judgement and can be gamed - set it generously and idle cost inflates, set it tightly and it vanishes. It works cleanly for committed, time-based resources and less so for genuinely variable costs that scale with volume anyway. And the idle line is a diagnosis, not a decision: it tells you capacity is unused, not whether keeping it is wise. That fuller picture - which customers and products actually consume the capacity, and which ones a business would keep - is what cost-to-serve, the whale curve of customer profitability, and product-level analysis across this encyclopedia are built to reveal.
Common questions about the cost of unused capacity
- Why report the cost of unused capacity separately?
- Because folding it into unit cost makes products look more expensive whenever volume falls, which pushes prices up into weak demand and drives the death spiral. A separate line keeps the cost of work done stable and turns idle capacity into a visible, actionable number that has an owner and a set of choices attached.
- What is practical capacity, and why not use theoretical capacity?
- Practical capacity is the realistic output a resource can sustain after allowing for breaks, maintenance, training and normal downtime - typically around 80% to 85% of theoretical time for people. Theoretical capacity assumes uninterrupted running that never happens, so costing against it understates rates and overstates idle cost. Practical capacity is the honest denominator.
- How is the capacity cost rate calculated?
- Divide the total committed cost of a resource group by its practical capacity in time units. The result is the cost per minute (or hour) of that resource being available. Multiplying by time used charges the work; multiplying by time idle gives the cost of unused capacity, and the two always sum back to the total cost.
- How does this connect to TDABC?
- The capacity cost rate is the heart of Time-Driven Activity-Based Costing. TDABC costs each activity as rate times the time it consumes, which automatically leaves the unused time - and its cost - as a residual. That is what lets TDABC produce stable cost-to-serve figures while surfacing spare capacity, instead of hiding idle cost inside inflated activity rates.
- Does unused capacity always mean waste?
- No. Idle capacity can be a seasonal trough worth holding, deliberate slack kept for growth or resilience, or the symptom of a bottleneck elsewhere. The separate cost line does not judge the idleness - it quantifies it so the right manager can decide whether to fill it, redeploy it, hold it as an investment, or remove it.
References
Kaplan, R. S. & Cooper, R. Cost & Effect: Using Integrated Cost Systems to Drive Profitability and Performance (unused capacity and the death spiral). · Kaplan, R. S. & Anderson, S. R. Time-Driven Activity-Based Costing (practical capacity and the capacity cost rate). · Horngren, C. T., Datar, S. M. & Rajan, M. V. Cost Accounting: A Managerial Emphasis (denominator-level choices and capacity analysis). · CIMA, Official Terminology (definitions of practical capacity and idle capacity). · Institute of Management Accountants (IMA), Statements on Management Accounting (measuring the cost of capacity).