You are paying for capacity you never use. Here is how to cost it.
Every resource you supply has a cost, whether you use it or not. A machine bought for 10,000 hours a year, a team paid for a full week, a ward built for a certain number of beds. Capacity costing is the discipline of valuing those resources at what they can practically deliver, not at what they happened to deliver last month. It is the quiet engine inside time-driven costing, and it is the reason TDABC can show you, in money, the cost of the capacity you are paying for and not using.
Cost and Profitability Consulting · 150+ models since 2010 · TDABC
Capacity costing values the cost of resources at their practical capacity, not their actual use. The capacity cost rate equals the total cost of supplied resources divided by practical capacity. The gap between supplied and used capacity is the cost of unused capacity, which TDABC surfaces explicitly rather than burying it in the rates charged to the products and customers that did use the resource. This keeps unit costs stable and tells managers what idle capacity actually costs.
What a unit of a resource's time actually costs.
Capacity costing answers one question: what does a unit of a resource's time actually cost. Not on average, not after a busy month, but at the rate the resource is built to run.
The instrument is the capacity cost rate. Take the total cost of supplying a resource over a period, including people, space, equipment, technology and supervision, and divide it by the practical capacity of that resource over the same period. The result is a cost per unit of capacity, usually a cost per minute or per hour. The choice of denominator is the whole argument, and there are three candidates.
Theoretical capacity
The resource running flat out, no breaks, no maintenance, no holidays. A person available every paid minute, a machine that never stops. Nobody operates here.
Practical capacity
Theoretical minus the unavoidable: breaks, training, set-up, maintenance, the slack a real operation needs. Roughly 80 to 85 percent of theoretical, for people and machines alike.
Actual usage
Whatever the resource happened to do last period. Tempting because the data is right there, and wrong because it changes every month and punishes products for slack they did not create.
Capacity costing uses practical capacity. That single decision is what separates a stable, defensible costing system from one that swings with every dip in demand.
SUPPLIED VS PRACTICAL VS USED CAPACITY
Illustrative. Theoretical capacity is the full bar; practical capacity is the cut line at 80 to 85 percent; used capacity is the solid fill. The band between practical and used is the cost of unused capacity.
Cost at actual usage and your unit costs rise when volumes fall.
Suppose a resource costs 100,000 a year and could practically deliver 100,000 minutes of work. The capacity cost rate is 1.00 per minute. Now demand softens and the resource only does 70,000 minutes of real work. If you divide the same 100,000 by 70,000, the rate jumps to 1.43 per minute. The products and customers that used the resource suddenly look 43 percent more expensive, not because they consumed any more, but because other work did not show up.
That is the trap. Costing at actual usage spreads the cost of idleness across whoever happened to use the resource, and it makes your unit costs rise exactly when volumes fall, which is the worst possible moment to be telling managers their products got pricier. Capacity costing refuses to do this. It holds the rate at 1.00 per minute, charges the 70,000 minutes of real work at that stable rate, and leaves 30,000 minutes, or 30,000 of cost, sitting in plain view as the cost of unused capacity.
This is the point of the whole method. The cost of unused capacity is real money, and it belongs to someone, but it does not belong to the products and customers that used the resource. It belongs to the decisions that sized the resource: the demand forecast that did not materialise, the seasonal pattern, the deliberate buffer held for growth, the contract that ended. TDABC puts that figure on the table so it can be managed, defended or removed, rather than smearing it across unit costs where no manager can see it or act on it. Idle capacity costed honestly becomes a management lever. Idle capacity hidden in inflated rates becomes a mystery that makes everything look unprofitable.
WHERE THE COST OF IDLE CAPACITY GOES
Illustrative. Capacity costing holds the rate steady and parks idle cost in the open; costing at actual usage crams the same cost into the work that did show up, inflating the rate from 1.00 to 1.43 per minute.
Two lines of arithmetic, and the idle cost falls out automatically.
Capacity costing rests on two lines of arithmetic: a rate built on practical capacity, then a cost per activity built on that rate.
Capacity cost rate = total cost of resources supplied
/ practical capacity (in minutes)
Cost of an activity = capacity cost rate x time the activity takes
A worked illustration. A support team costs 480,000 a year. It has 8 people, each paid for about 1,920 hours a year, which is 921,600 minutes of theoretical capacity. At 80 percent practical, that is roughly 737,000 minutes of usable capacity.
Capacity cost rate = 480,000 / 737,000 minutes
= 0.65 per minute (approx)
A 20-minute support case = 0.65 x 20 = 13.00
A 90-minute onboarding = 0.65 x 90 = 58.50
Multiply each activity's time by the rate, sum across everything the team actually did, and the difference between that total and the 480,000 supplied is the cost of unused capacity. Two parameters, no surveys, and the idle cost falls out automatically.
THE CAPACITY COST RATE
Illustrative. The rate is total supplied cost over practical capacity; the cost of any activity is that rate times the time it takes.
Roughly right and held steady beats exactly wrong and stale.
Practical capacity is an estimate, and there are two clean ways to make it, depending on whether the resource is people or equipment.
People. Start from paid time and subtract the time that is genuinely not available for productive work: breaks, meetings, training, administration, holidays already netted out. The 80 to 85 percent rule of theoretical capacity is a reasonable default for knowledge and service work, and it has the virtue of being defensible without a stopwatch study. The aim is not false precision. An estimate that is roughly right and applied consistently beats a survey that is exactly wrong and stale within a quarter.
Equipment. Drive practical capacity from expected uptime. Take the calendar hours the asset is available, subtract planned maintenance, changeovers and the realistic share of unplanned downtime, and you have the hours the machine can practically run. For physical assets this connects directly to operational measures the plant already tracks, which is why capacity costing sits so naturally next to OEE, bed occupancy, network utilisation and seat or room occupancy in the industries that live by those numbers.
The discipline in both cases is the same: pick a practical capacity, hold it steady across periods, and let actual usage vary against it. The variance is the signal.
AI does not abolish capacity costing. It changes what fills the denominator.
When an AI agent absorbs work that used to take a person twenty minutes, the human capacity that work consumed is freed, and unless something else fills it, it becomes unused capacity with a cost attached. Automation tends to convert variable labour into fixed capacity: the model or the platform is paid for whether it processes a thousand cases or a million, which makes the practical-capacity denominator larger and the cost of idleness more sensitive to volume. The organisations that handle this well are the ones that already cost at practical capacity, because they can see, in money, exactly how much freed-up capacity they have created and decide whether to redeploy it, retrain into it or remove it. The ones that cost at actual usage simply watch their unit costs lurch around and cannot tell automation's saving from a demand dip.
The shared foundation under every place idle capacity destroys margin.
Capacity costing is a method pillar, so industry pages link up to it rather than the other way round. Two method pillars sit closest: TDABC versus ABC explains why time-driven costing handles capacity cleanly while traditional activity-based costing tends to hide idle cost in its rates, and time-driven activity-based costing is the implementation, where this capacity cost rate is the first parameter and time equations are the second.
Frequently asked questions
- What is practical capacity?
- Practical capacity is the realistic amount of work a resource can deliver once unavoidable downtime is removed: breaks, maintenance, set-up, training. A common rule of thumb sets it at about 80 to 85 percent of theoretical capacity for both people and equipment. It is the denominator capacity costing uses to keep cost rates stable.
- How do you calculate the capacity cost rate?
- Divide the total cost of supplying a resource over a period by its practical capacity in minutes or hours over the same period. The result is a cost per unit of time. Multiply that rate by the time an activity takes and you have the cost of the activity.
- What is the cost of unused capacity?
- It is the cost of the practical capacity you supplied but did not use. If a resource can deliver 100,000 minutes and only 70,000 are used, the 30,000 idle minutes, costed at the capacity cost rate, are the cost of unused capacity. TDABC reports this figure separately rather than hiding it in unit costs.
- What is the difference between practical and theoretical capacity?
- Theoretical capacity is the resource running with no breaks, no maintenance and no downtime, a level no real operation reaches. Practical capacity subtracts the unavoidable interruptions and reflects what the resource can actually deliver, usually around 80 to 85 percent of theoretical.
- Why does TDABC use practical capacity instead of actual usage?
- Because dividing cost by actual usage makes unit costs rise whenever volumes fall, punishing products and customers for idle time they did not cause. Practical capacity holds the cost rate steady and isolates the cost of unused capacity as a separate, manageable number.
Put a number on the capacity you are paying for and not using.
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