Make-or-Buy Decision: Relevant Costs, Calculator and Example

A make-or-buy decision compares the future costs and benefits that differ between doing work internally and purchasing the same acceptable deliverable from a supplier. It is a relevant-cost decision: a cost that continues unchanged under both alternatives cannot make one option cheaper than the other.

Use the calculator above to compare a single period and a consistent quantity. Then assess quality, lead time, supply continuity, cash timing and realistic internal capacity. A mathematically cheaper quote is not automatically the better operating decision, and the tool does not certify that either option is feasible.

Make-or-buy decision formula

Relevant make cost = required quantity × avoidable make cost per unit + avoidable fixed make costs + contribution forgone by making. Relevant buy cost = required quantity × supplier unit price + additional buying-only period costs. The modeled cost advantage is the difference between those two amounts.

Include material purchases that change with the decision, labor that is genuinely avoidable, variable processing and the fixed commitments that actually disappear if production is outsourced. For buying, include freight, inspection, onboarding, handling, expected rework and other incremental costs where supported by evidence. Keep a cost either in the unit quote or the additional period line; do not count it twice.

An allocated share of rent is not avoidable merely because an accounting report charges it to the component. If the lease and payment remain unchanged, the amount is common to both alternatives. Likewise, the historical purchase price of existing equipment is a sunk cost. Future sale proceeds or actual cancellation costs can be relevant, but require their own evidenced cash flows and may need a more detailed model.

A worked make-or-buy example with capacity

A fictional small business needs 1,000 acceptable product components this month. Making costs $14 per component in truly avoidable materials, paid labor and running costs. It can avoid $3,000 of period setup/support expense if it buys. A supplier quotes $18 per acceptable component, with an additional $500 for delivery and inspection. A separate $4,000 commitment continues under either alternative.

If the internal team makes the components, it cannot perform another feasible job that would earn $2,000 net contribution after that alternative job's own variable costs. That is an opportunity cost of making, not another supplier invoice. These USD figures are invented examples, not market quotations or verified supplier terms.

Buying has a $500 lower modeled economic cost. Including the unchanged $4,000 in both columns leaves exactly the same difference. Removing that common cost from only one alternative would manufacture a false saving.

Economic comparison and cash comparison are different here. Relevant make cash outflows are $17,000, whereas buy cash outflows are $18,500. Buying uses $1,500 more direct cash under this boundary, but releases an opportunity worth $2,000 net contribution if it is genuinely feasible. Its modeled economic advantage is therefore $500. Actual collections, deposits and payment dates still require a cash forecast.

Idle capacity can reverse the result

If there is no feasible alternative use of the released resources, enter zero opportunity contribution. Relevant make cost becomes $14,000 + $3,000 = $17,000; buy remains $18,500. Making now has a $1,500 advantage. Do not claim a released employee hour earns revenue just because the hour becomes available.

Conversely, if making requires overtime, additional equipment or delayed higher-contribution work, those consequences may change the comparison. A salaried employee who stays employed can be an unavoidable cash commitment even when their scarce time has an economic opportunity cost. Model the relevant distinction without charging the same salary twice.

For service work, the equivalent decision can be internal delivery versus subcontracting. Use comparable scope, acceptance standards, customer obligations and rework responsibilities. See the employee-versus-freelancer guide and subcontractor job-cost guide for related operating contexts.

Calculate the crossover quantity carefully

With constant unit rates and period amounts, equate the relevant cost lines. Crossover quantity = (avoidable make fixed costs + contribution forgone − buying-only period costs) ÷ (buy unit cost − make variable cost per unit).

Using the main example: ($3,000 + $2,000 − $500) ÷ ($18 − $14) = 1,125 components. Both alternatives have a relevant economic cost of $20,750 at that quantity. At 1,000, buying is cheaper; at 1,200, making costs $21,800 versus $22,100 buying, so making is cheaper by $300 under the same assumptions.

The opportunity amount is held constant in that mathematical illustration. In reality it may depend on released hours, demand or the quantity made. Supplier price tiers, minimum orders, overtime and capacity steps can also invalidate the crossover. A negative threshold is not a useful positive production target; equal unit rates may produce no unique threshold or a tie at every quantity.

Records to collect before deciding

1. A comparable specification: quantity, usable yield, quality tolerance, packaging, warranty and required date.
2. A verified internal cost list: mark each line as avoidable, unavoidable, sunk or opportunity-related. Confirm cancellation and reassignment possibilities.
3. A complete supplier quote: price basis, freight, inspection, minimum order, payment terms, rework responsibility and validity period.
4. A capacity record: usable hours, bottlenecks, required setup time and the best feasible alternative work.
5. An opportunity calculation: expected retained revenue less the alternative job's incremental costs, supported by demand and delivery capacity.
6. A cash schedule: deposit dates and outflows, expected collections, available funding and a realistic buffer.

Keep the original comparison and record what happens after implementation: accepted quantities, delivery time, defects, rework, actual supplier charges and retained internal commitments. A decision that looked cheaper can deteriorate when the quoted scope differs from the work actually supplied.

Check allocation before treating it as a saving

A cost-driver allocation can assign a support pool to a job without making that amount avoidable. Keep the documented driver and cost pool for job reporting, then independently identify which commitments disappear under each make-or-buy alternative.

Common mistakes and practical controls

Comparing a full allocated make cost with a partial buy quote. Reconcile both columns to the same cost boundary. Treating all direct labor as avoidable. Review actual employment and scheduling commitments. Using alternative revenue as opportunity benefit. Use net contribution from a feasible alternative after its relevant costs.

Ignoring the cost of failures. A low quote is not comparable if it shifts inspection, waste or warranty obligations back to you. Confusing lower economic cost with cash affordability. An early supplier deposit can create a funding gap despite an attractive comparison. Use the cash forecast separately.

Confusing a make-or-buy crossover with business break-even. The crossover compares two cost alternatives. Business break-even compares revenue with all included operating costs. Review the break-even formula guide for that separate question.

What is the maximum acceptable supplier price?

The crossover quantity answers how volume changes the comparison. A different question is the supplier unit-price ceiling at a fixed quantity. With the original 1,000 components, $14 avoidable internal unit cost, $3,000 avoidable fixed cost, $2,000 feasible opportunity contribution and $500 buying-only cost, the economic ceiling is ($19,000 − $500) ÷ 1,000 = $18.50 per acceptable component. At that quote both options have the same modeled economic cost. Below it buying is lower; above it making is lower, within the stated assumptions.

Without an alternative job, relevant make cost is $17,000 and the ceiling falls to ($17,000 − $500) ÷ 1,000 = $16.50. The $2 difference is the $2,000 opportunity contribution spread over 1,000 components. A buyer should therefore not negotiate against an $18.50 ceiling unless that alternative contribution is evidenced. This is an economic ceiling, not a statement that a deposit or invoice is affordable.

How much alternative contribution makes outsourcing worthwhile?

At the original $18 supplier quote, buying costs $18,500 and avoidable make cash costs are $17,000. Buying needs at least $1,500 of feasible released-capacity contribution to reach an economic tie. At $1,000 contribution, making remains lower by $500. At $2,000, buying becomes lower by $500. Use net contribution after the alternative job’s incremental costs, not its invoice value or gross revenue.

Confirm that outsourcing releases the actual bottleneck. If inspection still requires the same scarce technician, purchased components may free machine time without enabling the alternative job. Check setup, transfer time, delivery dates and customer demand together. A scenario table makes the assumption visible; it does not establish the probability of earning the contribution.

Convert a quote into cost per acceptable component

A quoted purchased unit and an acceptable delivered unit are not necessarily the same denominator. In a separate fictional case, the supplier charges $10 per purchased unit, expected usable yield is 95%, and the business needs 1,000 acceptable units. Before considering minimum order sizes, it must purchase at least ceiling(1,000 ÷ 0.95) = 1,053 units. Purchase cost is $10,530; with $300 incremental inspection and delivery, the modeled cost is $10,830, or $10.83 per required acceptable unit.

The extra 53 purchased units are a planning allowance, not a guarantee that exactly 1,000 will pass. A yield estimate needs evidence and a contingency for variability. If the contract bills only accepted units or provides replacements, use those terms instead. Do not apply both a yield adjustment and the same defect allowance again as rework cost. Minimum orders, freight tiers and surplus inventory can invalidate a smooth unit-cost crossover, so compare actual order-sized alternatives.

Check when an internal cost can actually disappear

Attach a date and evidence to each claimed saving. Materials not yet ordered may be avoidable immediately. A cancellable machine rental may disappear only after its notice period. A supervisor salary that continues for the next three months is not an immediate saving merely because the production line stops. A reassigned employee can remain a cash commitment while released hours have a separate opportunity value.

For a one-month decision, include only the costs and benefits relevant to that month. For a longer decision, show the transition months, cancellation payments, severance where applicable, continuing commitments, working capital and eventual equipment proceeds separately. Do not divide a first-year saving by monthly volume and present it as a permanent unit advantage. The embedded single-period calculator cannot discount a multi-year project or recognize staged cash flows; build a dated schedule where the decision requires one.

Write a decision note someone else can verify

A defensible note states the required quantity and acceptance specification; the quote validity date; make and buy cost boundaries; avoidable commitments and supporting cancellation terms; evidence for alternative contribution; and the owner responsible for delivery and quality. Include the base comparison plus a scenario with no opportunity contribution. In the original case, the note must state both the $500 economic buying advantage and the $1,500 higher direct cash outflow before the alternative work is collected.

Set operational conditions before committing: agreed sample approval, inspection responsibility, acceptable delivery timing, escalation contacts and treatment of rejected units. Record which costs remain after outsourcing and compare actual accepted output with the assumptions after implementation. If the $2,000 alternative job is cancelled, revisit the decision rather than preserving a saving that no longer exists.

Download the make-or-buy decision worksheet

Download the formula-based make-or-buy review CSV. The original defaults calculate $19,000 relevant make cost, $18,500 relevant buy cost, a $500 buying advantage, an $18.50 supplier-price ceiling and a $1,500 opportunity-contribution tie threshold. The positive crossover is 1,125 components when period amounts stay constant. Retain the input and formula rows, enable formulas on import and check those results before editing. A blank crossover means no positive unique crossing under the entered assumptions; it does not mean zero units.

Use contribution margin to define the net contribution released capacity can earn. Keep business break-even separate: it asks how sales cover operating costs, rather than which supplier or production alternative is lower. The evidence principles follow OpenStax’s relevant-information framework; the thresholds and sensitivity figures above are original worked examples.

Method source

OpenStax's make-or-buy lesson discusses relevant costs and quantitative and qualitative considerations. This guide's figures and the capacity sensitivity are original fictional examples; they do not establish actual supplier savings.