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Strategic Cost Management · Throughput Accounting

Theory of Constraints and Bottlenecks Explained

Updated 11 October 2026 · Fact-checked

The Theory of Constraints says a system's output is limited by its slowest resource, the bottleneck. To solve a question, find the resource with the highest load against its capacity, then apply the five focusing steps: identify, exploit, subordinate, elevate, and repeat. Rank products by throughput per bottleneck hour.

Understand Theory of Constraints and Bottlenecks

Every process has a resource that limits how much the whole system can produce. This resource is the constraint or bottleneck. Think of a road with one narrow bridge. It does not matter how wide the rest of the road is. Traffic moves at the speed of the bridge.

The Theory of Constraints (TOC) was developed by Eliyahu Goldratt. Its core idea is simple. Time lost at the bottleneck is time lost for the whole system. An hour saved at a non-bottleneck resource saves nothing, because that resource already has spare capacity. Extra output there only builds up stock in front of the bottleneck.

TOC gives five focusing steps: identify the constraint, exploit it, subordinate everything else to it, elevate it, and then go back to step one because a new constraint may appear. Exploit means get the most out of the constraint without spending money, for example by avoiding idle time, setting up machines during breaks, and not wasting its time on defective or low-value work. Subordinate means all other resources work only at the pace the constraint can absorb. Elevate means invest, such as buying another machine, adding shifts or outsourcing, to increase the constraint's capacity.

TOC links to throughput accounting. Throughput is sales revenue less totally variable cost, usually direct material. When a bottleneck exists, you rank products by throughput per bottleneck hour, not by contribution per unit or per unit of sales. The product with the highest return per bottleneck hour gets priority.

Drum-Buffer-Rope (DBR) is the scheduling method for TOC. The drum is the bottleneck, which sets the pace of production. The buffer is a protective stock or time cushion placed before the bottleneck so it never starves. The rope is the signal that ties material release at the start of the process to the drum's rate, so no more material enters than the bottleneck can process.

Key rules to remember

Throughput
Throughput = Sales revenue − Totally variable costs (usually direct material)
Labour and overheads are treated as operating expenses unless they are truly variable with output.
Throughput per unit of bottleneck resource
Throughput per bottleneck hour = Throughput per unit ÷ Bottleneck hours per unit
Use this to rank products and allocate scarce bottleneck time.
Bottleneck identification
Load on resource = Units demanded × Hours per unit; Bottleneck = resource where Load > Available capacity by the largest margin (or highest Load ÷ Capacity)
Compare load with available hours for each resource at the planned output.
Factory cost per bottleneck hour
Factory cost per hour = Total factory cost ÷ Bottleneck hours available
Total factory cost means labour and overheads (operating expenses) plus other conversion costs, as the question defines.
Throughput accounting ratio (TPAR)
TPAR = Throughput per bottleneck hour ÷ Factory cost per bottleneck hour
TPAR above 1 means the product earns more than the cost of the bottleneck hour. Below 1 means it does not cover it.
Five focusing steps
Identify → Exploit → Subordinate → Elevate → Repeat
Write them in this order and add one practical action for each.

How to solve Theory of Constraints and Bottlenecks questions

Use this method for any numerical or theory question on bottlenecks and TOC.

  1. 1List each resource or process with its available hours or capacity for the period.
  2. 2Compute the hours needed on each resource to meet the planned or demanded output (units × hours per unit).
  3. 3Compare load with capacity. The resource with the greatest shortfall, or highest load as a share of capacity, is the bottleneck. Say so explicitly.
  4. 4Compute throughput per unit for each product: selling price less direct material and other totally variable cost.
  5. 5Divide throughput per unit by bottleneck hours per unit to get throughput per bottleneck hour, and rank products.
  6. 6Allocate bottleneck hours to the highest-ranked product up to its demand, then to the next, until hours run out. Check the allocation against other resources' capacity.
  7. 7Compute total throughput, deduct operating expenses (labour and overheads) to get profit. Compute TPAR if asked.
  8. 8Add a recommendation, mapped to the focusing steps: exploit, subordinate, elevate. State whether elevating pays off by comparing extra throughput with extra cost.

Quickest way: Rank by throughput per bottleneck hour

When to use it: Use when the question gives hours per unit on several machines and asks for the best product mix or profit.

  1. Multiply demand by hours per unit for each machine and compare with available hours. The machine that is short is the bottleneck.
  2. Calculate throughput per unit as price less material only.
  3. Divide by bottleneck hours per unit. Rank.
  4. Fill bottleneck hours in rank order up to demand limits.
  5. Total throughput minus fixed operating costs gives profit. Write one line of recommendation.

Common mistakes in Theory of Constraints and Bottlenecks

  • Ranking products by contribution per unit or by selling price instead of throughput per bottleneck hour.

    Students are used to simple marginal costing, where the highest margin per unit looks best.

    Fix: Once a bottleneck exists, always divide by the bottleneck hours per unit before ranking.

  • Deducting direct labour and variable overhead when calculating throughput.

    Students treat throughput as the same as contribution.

    Fix: Deduct only totally variable costs, usually direct material, unless the question says other costs vary fully with output. Treat labour and overheads as operating expenses.

  • Naming the bottleneck by the longest processing time per unit rather than by load against capacity.

    Students compare hours per unit across machines and forget that available capacity differs.

    Fix: Compute total hours required and compare with hours available on each resource.

  • Writing the five steps in the wrong order or giving definitions without actions.

    Students memorise the names without understanding the logic.

    Fix: Remember: find it, squeeze it, align to it, expand it, repeat. Add one practical example for each step.

  • Recommending that non-bottleneck resources run at full speed to avoid idle time.

    Traditional efficiency measures reward high utilisation everywhere.

    Fix: Explain that extra output at non-bottlenecks only creates inventory. Subordinate them to the drum's pace.

  • Forgetting to re-check the bottleneck after elevating capacity.

    Students stop at step four.

    Fix: After adding capacity, recompute loads. A different resource may now be the constraint, so say 'go back to step one'.

Worked examples

Example 1

Meera Foods Ltd makes two products, X and Y, using machines M1 and M2. Weekly availability is 40 hours on M1 and 30 hours on M2. Per unit: X needs 0.4 hour on M1 and 0.5 hour on M2; Y needs 0.5 hour on M1 and 0.3 hour on M2. Weekly demand is 100 units of X and 60 units of Y. Identify the bottleneck.

Show the solution
  1. Load on M1 = (100 × 0.4) + (60 × 0.5) = 40 + 30 = 70 hours. Available = 40 hours. Shortfall = 30 hours.
  2. Load on M2 = (100 × 0.5) + (60 × 0.3) = 50 + 18 = 68 hours. Available = 30 hours. Shortfall = 38 hours.
  3. Load as a share of capacity: M1 = 70 ÷ 40 = 1.75. M2 = 68 ÷ 30 = 2.27 (rounded).
  4. M2 is more heavily overloaded on both measures.

Answer: M2 is the bottleneck: it needs 68 hours against 30 available. Both machines are short of capacity, but M2 is the binding constraint. Schedule production around M2's 30 hours, and check M1's 40 hours when the plan is made.

Example 2

Rohan Engineering Ltd has a bottleneck machine with 600 hours available per month. Product A sells at ₹900, with direct material ₹300, and needs 2 hours on the bottleneck. Product B sells at ₹1,000, with direct material ₹400, and needs 1.5 hours. Monthly demand is 200 units of A and 300 units of B. Operating expenses (labour and overheads) are ₹1,20,000 per month. Find the best mix, profit, and TPAR for each product.

Show the solution
  1. Throughput per unit: A = 900 − 300 = ₹600. B = 1,000 − 400 = ₹600.
  2. Throughput per bottleneck hour: A = 600 ÷ 2 = ₹300. B = 600 ÷ 1.5 = ₹400.
  3. Rank: B first, A second.
  4. Hours needed for full demand of B = 300 × 1.5 = 450 hours. Remaining = 600 − 450 = 150 hours.
  5. Units of A possible = 150 ÷ 2 = 75 units (demand is 200, so limited by hours).
  6. Total throughput = (300 × 600) + (75 × 600) = 1,80,000 + 45,000 = ₹2,25,000.
  7. Profit = 2,25,000 − 1,20,000 = ₹1,05,000.
  8. Factory cost per bottleneck hour = 1,20,000 ÷ 600 = ₹200.
  9. TPAR: A = 300 ÷ 200 = 1.5. B = 400 ÷ 200 = 2.0.

Answer: Make 300 units of B and 75 units of A. Total throughput is ₹2,25,000 and profit is ₹1,05,000. TPAR is 2.0 for B and 1.5 for A, so both products cover the cost of a bottleneck hour, and B is the better use of it. Consider elevating the bottleneck if extra hours cost less than ₹300 per hour, since each extra hour used on A earns ₹300 throughput.

Exam tips

  • For theory questions, write the five steps in order and give one practical action under each. This scores more than definitions alone.
  • In numerical questions, state the bottleneck explicitly in one line with the load and capacity figures. Examiners look for this.
  • Write the formula for throughput, then show each product's throughput per bottleneck hour in a small working table so partial marks are safe.
  • End decision questions with a recommendation: what to produce, whether to elevate the constraint, and why non-bottleneck efficiency should not be pushed.
  • For Drum-Buffer-Rope, define all three terms in one line each and say what problem each solves.

Practice questions from Throughput Accounting

Theory of Constraints and Bottlenecks in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Theory of Constraints and Bottlenecks: frequently asked questions

What are the five steps of the Theory of Constraints?

They are: identify the constraint, exploit it, subordinate everything else to it, elevate it, and repeat the process. Exploit means getting the most from existing capacity. Elevate means spending money to increase capacity. After elevating, check whether a new constraint has appeared.

How do I identify the bottleneck resource in a numerical question?

Calculate the hours each resource needs to meet demand and compare with hours available. The resource with the largest shortfall, or the highest load relative to capacity, is the bottleneck. Do not simply pick the machine with the longest time per unit.

What is Drum-Buffer-Rope in simple words?

It is a scheduling method built around the bottleneck. The drum is the bottleneck that sets the pace. The buffer is protective stock or time before it, so it never runs out of work. The rope links the release of material to the drum's rate.

Is throughput the same as contribution?

Not always. Throughput is sales less totally variable cost, usually only direct material. Contribution normally deducts all variable costs, including labour and variable overheads. Follow the definition the question gives.