Capacity Costing and the True Cost of Idle Time

A machine that runs three shifts has a low cost per hour. The same machine running one shift — same depreciation, same supervisor, same rent — has a high cost per hour. Capacity costing is how you measure and manage that gap. Get it right and pricing, sales targets and investment decisions all sharpen up. Here is the working method.

Why this matters

Fixed overhead — rent, depreciation, supervisor salaries, insurance, minimum power — does not pause when production pauses. Whether you run 8,000 hours a month or 4,000, those costs are largely the same.

When you absorb that fixed overhead onto units produced, the fewer units you produce, the more overhead each one carries. Under-utilised capacity makes every unit look more expensive than it should — and the prices you quote based on that cost can end up the highest in your market.

Practical and theoretical capacity

Two reference points:

• Theoretical capacity — what the plant could produce running 24/7 with no downtime. A useful ceiling but never achievable.
• Practical capacity — what the plant can realistically sustain given planned downtime (maintenance, shift breaks, holidays). The right benchmark for costing.

Most management accountants use practical capacity as the denominator when computing an overhead rate, not "what we actually produced." Why? Because using actual output to set the rate punishes you twice when output drops — you spread the same fixed cost over fewer units and you build that high cost into next period's prices.

The overhead absorption gap

If your monthly fixed overhead is ₹2,40,000 and practical capacity is 8,000 labour hours, your fixed overhead rate is ₹30/hour.

• If you actually run 8,000 hours → ₹2,40,000 absorbed. Fully recovered.
• If you actually run 6,000 hours → ₹1,80,000 absorbed, ₹60,000 under-absorbed. That ₹60,000 is the cost of idle capacity — hit it on the P&L as a separate line.

The under-absorbed amount is not a cost of producing what you produced. It is the cost of capacity you paid for but did not use. It deserves its own visible line because it is a different kind of problem — a sales or planning problem, not a production problem.

Over-absorption is also a signal

Run more than practical capacity (extra shifts, weekend running) and you over-absorb — costs absorbed into units exceed actual overhead. Usually that means:

• You should re-set your practical capacity assumption (you have been understating it)
• Or you are running unsustainable conditions (overtime, deferred maintenance) that will catch up later

Either way, sustained over-absorption is worth investigating.

Why pricing on "actual cost" is dangerous

If you cost on actual hours run rather than practical capacity, slow months make products look expensive — and you raise prices defensively. The higher prices reduce demand. Volume drops further. Cost looks even higher. You raise prices again.

It is a death spiral and it happens to manufacturers all the time.

Costing on practical capacity stops the spiral: prices reflect the cost at normal utilisation, idle capacity is visible separately, and the conversation moves to how to fill the capacity — not how to price as though we are smaller than we are.

What idle capacity is really telling you

A high cost of idle capacity is a signal worth interpreting. Common causes:

• Demand shortfall — sales gap. The fix is sales / marketing.
• Imbalanced product mix — some lines are over-capacity, others under. The fix is mix or scheduling.
• Bottleneck downstream — upstream sits idle waiting. The fix is the bottleneck.
• Over-investment — capacity was built for volume that never came. The fix is hard — divest, repurpose, or accept it.

The idle capacity number does not tell you which one — but it tells you the size of the problem, and that should drive an honest conversation.

How to track it monthly

A simple capacity report:

• Practical capacity (hours): the constant benchmark
• Actual hours run: from production logs
• Utilisation %: actual ÷ practical
• Fixed overhead absorbed: at the standard rate × actual hours
• Fixed overhead actually incurred: from the books
• Under- (or over-) absorbed: the difference, P&L impact

Track utilisation by week, by line, by shift. Patterns emerge quickly.

How Booksmor helps

Booksmor lets you set practical capacity and an absorption rate per work centre. Production runs absorb at the standard rate; the actuals from the books are reconciled monthly with the under-/over-absorbed variance posted to its own P&L line — making the cost of idle (or stretched) capacity visible. Start a 30-day free trial and put capacity in the conversation.