A printer manufacturer has already spent $2 million building its factory. It costs $80 in materials and labor to produce one more printer. Whether to produce that printer has nothing to do with the $2 million factory — that cost exists regardless. The only cost that matters for the decision is $80. If the printer sells for more than $80, produce it. If it sells for less, don't. That decision-level cost — the cost of the next unit — is marginal cost.
The formula
Marginal Cost (MC) = ΔTotal Cost ÷ ΔQuantity
Since fixed costs don't change with output, MC reflects only changes in variable costs:
MC = ΔVariable Cost ÷ ΔQuantity
A firm producing 100 units at $10,000 total variable cost and 101 units at $10,095 total variable cost: MC = ($10,095 – $10,000) ÷ 1 = $95
Reading the result
MC has a characteristic shape: it typically falls initially (as variable input per unit of output falls due to specialization) then rises (as diminishing returns set in and more expensive inputs are required for each additional unit).
The critical decision rule: produce any unit where the selling price (or marginal revenue) exceeds marginal cost; stop when MC exceeds price.
For competitive firms, price = marginal revenue. The profit-maximizing output is where P = MC.
For any firm, the universal profit-maximizing rule is MR = MC: produce until the additional revenue from the next unit exactly equals the additional cost of producing it.
Worked example
A coffee roaster produces specialty coffee in batches. Marginal cost data:
| Batch | MC per bag |
|---|---|
| 1–5 | $8 |
| 6–10 | $10 |
| 11–15 | $14 |
| 16–20 | $19 |
Market price: $13 per bag. The roaster should produce batches 1–10 (where MC ≤ $13) and stop at batch 11 (where MC = $14 > $13). Producing batch 11 would cost more than it earns.
The EPA's environmental cost-benefit methodology applies the same marginal cost logic to pollution abatement: the optimal emission reduction is where the marginal cost of additional abatement equals the marginal social benefit (the social cost of carbon) — the environmental policy equivalent of MR = MC.
Where it's used
MC is the cost variable relevant to every production decision. Past costs — fixed costs, sunk costs — don't affect the marginal cost of the next unit and should not influence whether to produce it. This is one of the most common and costly errors in business decision-making: continuing or abandoning production based on average or total costs rather than marginal cost. The relevant question is always: does the next unit pay for itself?
