Throughput Accounting: How to Decide by the Bottleneck

What throughput accounting is

Throughput accounting is the decision method that grew out of the Theory of Constraints. It measures plant performance with three numbers: throughput (T), inventory (I) and operating expense (OE). Instead of allocating overhead to each product, it asks how much money each product generates per minute of the resource that limits output.

That changes everyday decisions: which product to prioritize, which order to accept and what to charge. A product with a great margin under traditional costing can be the worst use of the bottleneck. The method builds on the theory of constraints (TOC), which shows where the limit is; throughput accounting shows what that limit is worth in money.

Visual map of throughput accounting

The map summarizes the three measures, the derived ratios and the example of two products competing for the same bottleneck.

The three measures and the ratios

• Throughput (T): sales revenue minus totally variable costs (TVC), meaning costs that exist only if the unit is sold: raw material, purchased components, sales commission, outbound freight.
• Inventory (I): money invested in items the company intends to sell. It is valued at the purchase cost of materials, without adding labor or overhead allocation.
• Operating expense (OE): all the money spent to turn I into T: salaries, rent, fixed energy, depreciation, maintenance.

T = Revenue − TVC  |  Net profit = T − OE  |  ROI = (T − OE) ÷ I

Two ratios complete the dashboard: productivity = T ÷ OE (how much throughput each dollar of expense produces) and inventory turns = T ÷ I. To choose between products, the key figure is throughput per bottleneck minute:

T per bottleneck minute = (Price − TVC) ÷ minutes on the constraint resource

In the classic version, direct labor goes into OE because it does not vary with each unit sold. If the company pays by the piece, that pay is variable and belongs in TVC. Set the rule before calculating and apply it equally to every product.

Example: two products, one bottleneck

Fictional example. A machining cell has its machining center as the bottleneck, with 2,400 minutes per week available. Weekly demand is 200 units of product A and 300 of product B. Operating expense for the plant is $12,000 per week and inventory is $150,000.

By traditional margin, product A looks better (30% against 21.4%). By throughput per bottleneck minute, B is worth $10 against $7. Total demand (200 × 10 + 300 × 5 = 3,500 min) exceeds the 2,400 min available, so a choice is needed:

Prioritizing by the bottleneck adds $3,300 per week of throughput without changing operating expense. With the same $150,000 of inventory, the weekly return on investment rises from 4.0% (6,000 ÷ 150,000) to 6.2% (9,300 ÷ 150,000), and productivity T ÷ OE goes from 1.50 to 1.78.

Applying it to order and pricing decisions

The constraint resource is the one that carries a price. Every minute taken by an order is a minute not used by the best product still unserved; in the example, A at $7.00 per minute.

• Extra order of product B: minimum price = material + minutes × $7.00 = 20 + 5 × 7 = $55. Below that, the bottleneck earns more producing A.
• Bottleneck with slack: if capacity is left over, the opportunity cost is zero and any price above TVC increases throughput.
• Outsourcing: outsourcing operations on non-bottleneck resources rarely pays; freeing bottleneck time for a high T-per-minute product often does.
• Investing: the gain from an improvement is the T it releases at the bottleneck, not the cost saved on resources with spare capacity.

To schedule the plant with this logic, see drum-buffer-rope; for the cost of the resource itself, the machine-hour rate.

Throughput accounting versus traditional costing

The methods do not compete; they answer different questions. Absorption costing is required for statutory and tax reporting. The contribution margin is the closest bridge: it also separates variable costs, but divides by the unit and not by the scarce resource. To see how lean companies apply this by value stream, read about lean accounting.

How to implement it in 5 steps

1. Identify the bottleneck from load and queue data, not from opinion.
2. Define TVC for each product and document what is in (material, commission, freight) and what is out.
3. Calculate T and T per minute for every product that crosses the bottleneck and rank them.
4. Build the mix until the available minutes run out, respecting demand and contracts.
5. Track T, I and OE weekly or monthly and re-check the bottleneck when mix or capacity changes.

Common mistakes and limitations

• Calculating T per minute without a defined bottleneck: with no capacity constraint, the criterion becomes throughput per unit or per order.
• Ignoring the moving bottleneck: when you add capacity, the constraint moves and so does the product ranking.
• Treating everything as fixed expense: in the long run OE changes too; the approach suits short and medium-term decisions.
• Dropping low T-per-minute products: they may be needed by contract, for a strategic customer, or because slack periods exist.
• Using it as the only accounting: statutory and tax reports follow accounting standards; throughput is a management model.
• Mixing TVC rules: comparing products with different criteria distorts the ranking.

Frequently asked questions

What is throughput accounting?

A Theory of Constraints management method that evaluates decisions by throughput (T), inventory (I) and operating expense (OE), focused on the constraint resource.

What is the throughput formula?

T = sales revenue − totally variable costs. Net profit is T − OE and ROI is (T − OE) ÷ I.

How do you calculate throughput per bottleneck minute?

Divide throughput per unit (price − TVC) by the minutes the unit takes on the constraint resource. In the example: 70 ÷ 10 = $7.00 for A and 50 ÷ 5 = $10.00 for B.

Is direct labor a variable cost here?

In the classic version, no: it is operating expense. It enters TVC only when pay varies directly with each unit sold, such as piece-rate pay.

How does it differ from contribution margin?

Both separate variable cost, but contribution margin is measured per unit while throughput is measured per scarce resource minute. With a bottleneck, that difference can reverse the priority order.

Do I have to abandon absorption costing?

No. It is still needed for the balance sheet and taxes; throughput accounting serves mix, pricing and investment decisions.

About the author

Vagner Soares

Lean Manufacturing & Behavioral Management Specialist

Over 20 years in the automotive and metalworking industries (GM and Dana), Lean Manufacturing practitioner since 2006. SENAI instructor and mentor in Brazil’s Brasil Mais Produtivo program, delivering consulting, training and audits for 50+ companies, combining quality, productivity and people development.

LinkedInInstagramFull profile →