Make-or-Buy Decision: Compare Cost and Capacity

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Estimated reading time: 10 minutes

A make-or-buy decision determines whether to manufacture a component or perform an operation in-house, or purchase it from an external supplier. The right choice combines the costs that change with the capacity, quality and delivery performance each option can provide.

The difficult part is rarely finding two unit prices. It is knowing what disappears if you stop making the part, what other work could use the released hours, and whether the purchased component will be ready when assembly needs it.

That is why the cost comparison belongs alongside the production schedule. SkyPlanner APS makes your internal load and connected operations visible, so a cheaper component does not quietly become a late customer order. Once an external operation is approved, it can stay in the same plan as the work before and after it.

Start with the same component, quantity and delivery requirement

Before comparing costs, agree on exactly what you are buying or making. Use the same drawing revision, material specification, accepted quantity, quality requirements and date needed for the next operation. A quote for unfinished blanks is not comparable to the cost of inspected, finished parts.

Choose the decision period too. Buying one urgent batch while a machine is overloaded is a different decision from closing an internal process and outsourcing its annual volume. In the first case, a machine lease may remain unchanged. In the second, future equipment replacement, staffing and retained expertise may materially change the answer.

Ask engineering and quality to identify requirements that neither option may compromise. If a supplier cannot achieve the required tolerance or approval, a low quote does not make it a feasible option. Apply the same test internally: owning a machine is not proof that your process can make the component consistently.

Compare relevant costs, not the allocated unit cost

Relevant costs are future costs and benefits that differ between the alternatives. For a short-term make-or-buy comparison, ask: which cash flows change because we choose this option?

An allocated share of rent or management salaries can make a component look expensive without being a cost you can remove. If those expenses remain after outsourcing, buying the part adds a supplier bill without eliminating that overhead.

The reverse matters as well. A dedicated fixture, additional shift or avoidable service contract is relevant when the decision causes or removes the expenditure. Fixed does not automatically mean irrelevant, and variable does not excuse you from checking the assumption.

Cost or benefit	What to establish before including it
Materials and conversion costs	The materials, consumables, energy and other expenditure that actually change with internal production.
Labor	Whether you need extra paid hours, temporary staff or overtime, or would use employees already being paid. Released hours are not automatically wage savings.
Batch-specific and fixed costs	Which setup expenditure, tooling, contracts or other costs arise or can genuinely be avoided over the chosen period.
Purchased components	The supplier price for the required accepted quantity, plus incremental freight, duties, inspection, handling and other procurement costs.
Existing allocated overhead	Whether the total expense changes. Leave unchanged allocations out of the differential comparison.
Alternative use of capacity	The additional net benefit from feasible work you would otherwise give up, not its sales revenue alone.

Past expenditure is not recovered by choosing to make another batch. Use the future consequences of the decision rather than the book value of an existing machine. If an asset could instead be sold or used profitably elsewhere, that forgone benefit does matter.

Finance should validate the comparison, especially the treatment of retained wages and overhead. Production supplies the hours and resource requirements; purchasing supplies comparable quotes and delivery terms. Together, they turn a tempting price difference into a usable business case.

Make-or-buy example: the cheaper part can use the more valuable hours

Imagine you need 2,000 finished components for the next production period. The following figures are illustrative, not customer results or market rates. Both options meet the same specification and delivery requirement.

Making the batch requires $6 of material, $4 of additional labor and $2 of variable conversion expenditure per component. Here, the labor means temporary or overtime hours paid only if you make this batch. There is also $3,000 of batch-specific expenditure that you avoid by buying.

A supplier quotes $15 per component. Freight adds $1,000 and incoming inspection adds another $1,000 for the batch.

Relevant cost for 2,000 components	Make	Buy
Material	$12,000	Included in supplier price
Additional labor	$8,000	Included in supplier price
Variable conversion expenditure	$4,000	Included in supplier price
Avoidable batch-specific expenditure	$3,000	$0
Supplier price	$0	$30,000
Freight and incoming inspection	$0	$2,000
Total relevant cost	$27,000	$32,000

With usable spare capacity and no better alternative use, making saves $5,000. Suppose the accounts also allocate $6,000 of unchanged factory overhead to this batch. Including that common expense would bring the totals to $33,000 for making and $38,000 for buying. It does not change the $5,000 difference. Comparing the fully allocated $33,000 make cost against the $32,000 supplier-side cost would wrongly favor buying.

Now put the batch on the bottleneck

Suppose making those parts requires 100 hours on a constrained machining center. Confirmed alternative orders could use those same hours and generate $7,000 of additional net contribution, after all additional costs. The other resources and materials are available, and those orders can actually be completed and sold within the period.

Buying now costs $5,000 more for the components but enables $7,000 of additional contribution. The business is $2,000 better off buying under these assumptions.

You can show the same result by adding the $7,000 opportunity cost to making: $27,000 becomes $34,000, compared with $32,000 for buying. Use one presentation or the other; do not count the alternative benefit twice.

The decisive question is not whether the factory is busy. It is whether those particular hours, on that particular resource, can create a better result elsewhere. If the alternative orders lack material, miss their dispatch dates or merely move to another bottleneck, the assumed benefit needs revisiting.

SkyPlanner helps establish that operational side of the comparison. Its finite schedule connects the component’s operations to the resource availability and other jobs competing for time. You can see the production consequences before committing to a decision based only on the component’s cost.

When a break-even calculation helps

For a stable volume comparison, write the relevant cost of each option as:

Make cost = Fm + vm × Q

Buy cost = Fb + vb × Q

Here, Q is quantity, F is the relevant fixed expenditure over the same decision period, and v is the relevant cost per unit. The subscripts m and b identify making and buying.

When buying has the higher per-unit cost, the cost crossover is:

Q = (Fm − Fb) ÷ (vb − vm)

In the example, the crossover is ($3,000 − $2,000) ÷ ($15 − $12), or approximately 333 components. At 334 whole components, making becomes cheaper on this cost-only basis. That calculation assumes both options remain available and the quoted rates and batch costs stay valid.

This is a crossover between alternatives, not a sales break-even or a capacity guarantee. Minimum purchase quantities, price tiers, extra shifts and opportunity costs can change the comparison. For a multi-year investment, finance also needs to evaluate the timing of cash flows and the investment’s wider consequences.

Check quality, supply risk and the capability you want to keep

A financial advantage is useful only if the chosen option can deliver acceptable parts. Make the risks specific enough to investigate rather than giving each option a vague score for “reliability.”

Decision factor	Questions that can change the choice
Quality and acceptance	Can the process meet the specification consistently? Who approves the first batch, and who pays for defects or rework?
Delivery and flexibility	Does lead time include production, transport and inspection? What happens if quantity or drawing revision changes?
Supply continuity	Is there a qualified backup source? How would either a supplier failure or an internal machine failure affect assembly?
Know-how and control	Is this process part of your differentiation? What expertise, tooling access or design knowledge must you retain?
Demand and reversibility	Are volumes firm enough to justify capacity or purchase commitments? How difficult would it be to bring the work back?

Purchasing should verify the supplier’s commitment, quality should approve the acceptance plan, and engineering should assess the capability implications. A weighted decision matrix can summarize their findings, but it should not allow several small benefits to outweigh a non-negotiable quality or delivery requirement.

The answer can also be a mix: retain a critical operation internally, buy a specialist treatment, or use an approved supplier for peak demand. Define which quantities and operations go where. Splitting the work still creates tooling, coordination and quality obligations; it is not automatically the lowest-risk option.

Use SkyPlanner to connect the decision to the production plan

A make-or-buy comparison needs a credible answer to “can we fit this work?” SkyPlanner brings workstations, shifts, operators, materials and priorities into one scheduling environment. Arcturus, its scheduling AI, calculates the sequence against the capacity and dependencies you define. The Gantt view shows the work assigned to resources and how it connects to later operations.

For internal production, review the required process steps, setup and processing durations on the resources that can perform them. Check the completion dates of the component and the other orders affected by its load. Spare hours after assembly needs the part do not solve today’s shortage.

For an approved subcontracted operation, SkyPlanner can represent the subcontractor as a workstation with its own scheduled queue. The external step can therefore remain connected to the internal work before and after it, rather than disappearing into a separate follow-up list. Record realistic timing and update progress so the plan reflects what is happening.

If you buy the complete component instead, material-aware scheduling can connect its availability to the internal operations that need it. Buying a component and subcontracting one operation are different routes; model the route you actually approve.

You set the priorities and protected decisions; Arcturus handles the scheduling calculation. That reduces the manual work of reconciling queues and makes delivery consequences easier to see. Keep the finance-approved cost comparison alongside this operational view: the sourcing decision needs both.

SkyPlanner also integrates with any ERP or MES through its open REST API. You can keep your existing business systems while giving production a connected finite schedule.

What capacity visibility changed at Fredman Operations

Before SkyPlanner, food packaging manufacturer Fredman Operations prioritized jobs manually without precise visibility into machine release times or shift-by-shift capacity. Urgent rescheduling could take an hour or more.

With SkyPlanner integrated with L7 and LTR, the team gained a shared Gantt view of free and booked capacity. Fredman reports faster responses to schedule changes and a reduced need for overtime. When a customer asks to bring forward an entire order backlog, the team can answer in seconds rather than spending hours on manual calculations.

That experience illustrates why the schedule belongs in the sourcing discussion: management needs to know what the shop can actually promise, not just what a component costs on paper.

Approve the choice, then revisit the assumptions

Record the selected component or operation, quantity, period, relevant cost difference, capacity consequence and quality approval. Name the person responsible for the supplier commitment or internal completion date. Agree on review triggers such as a volume change, a revised quote, persistent defects or a changed bottleneck.

A good make-or-buy decision explains why the chosen option serves the business now, and what would make the answer change. SkyPlanner keeps the production side of that decision visible as the order book moves on.

Book a SkyPlanner demo with one typical component, its internal route and an approved external option. Use the discussion to examine the capacity each route needs and the other deliveries it affects. If you prefer to explore first, try SkyPlanner with demo data.