Marginal cost is the extra cost of producing one more unit of a good or service. Economists, managers and regulators use it to decide how much to produce, where to set prices and whether an extra order, flight or transaction is worth taking. It is one of the central ideas of microeconomics, because most choices about output are made at the margin rather than all at once.
Marginal cost is built from a firm's variable costs: fixed costs such as rent on a factory do not change when one more unit is made, so they do not enter the marginal figure in the short run. It is often compared with marginal revenue, the extra income from selling one more unit, and with marginal utility, the matching idea on the buyer's side. Marginal cost is also different from average cost, which spreads every cost over all units produced.
Marginal cost in plain terms
Picture a bakery that has already paid for its ovens and its shop. When a customer asks for one extra loaf, the baker does not need a new oven. The only new spending is a little flour, yeast, power and the baker's time. That small top-up is the marginal cost of the loaf.
Now suppose the bakery is already running flat out. The next loaf might mean paying staff overtime or renting a second oven for the night. The extra loaf suddenly costs much more. Marginal cost captures exactly this: what the next unit adds to the bill, given everything that is already in place.
Formula and example
Marginal cost is the change in total cost divided by the change in output:
- is marginal cost per unit.
- is the change in total cost.
- is the change in the quantity produced.
Suppose a print shop spends $5,000 to produce 1,000 brochures in a week and $5,300 to produce 1,100. The change in total cost is $300 and the change in output is 100, so the marginal cost is $300 ÷ 100 = $3 per brochure. If the shop is offered an extra order at $4 a brochure, it covers the marginal cost and adds to profit, even though the shop's average cost may be higher once rent and equipment are included.
What makes marginal cost rise or fall
In the short run, marginal cost usually falls at first and then rises, giving the familiar U-shaped curve in textbooks. The rise comes from the law of diminishing returns: when more workers or materials are added to a fixed plant, each addition contributes less extra output, so each extra unit costs more.
Over the long run, when every input can change, firms may enjoy economies of scale that keep average and marginal costs low as they grow. Digital goods are an extreme case: once software or a music file exists, serving another customer costs almost nothing. That near-zero marginal cost, combined with the network effect, helps explain why a platform company can scale quickly and why many digital markets drift toward a few large firms.
Marginal cost, pricing and market structure
A profit-seeking firm keeps producing as long as an extra unit brings in more than it costs, so output settles where marginal revenue equals marginal cost. Under perfect competition, where no seller can influence the price, marginal revenue equals the market price, so firms produce until price equals marginal cost. The supply curve of a competitive industry is therefore built from its firms' marginal cost curves, which links the concept directly to supply and demand.
A monopoly faces a downward-sloping demand curve, so its marginal revenue is below price. It restricts output and charges a price above marginal cost, and the size of that gap is a standard measure of market power. Economists treat price equal to marginal cost as a condition for Pareto efficiency, because at that point no further trade could make a buyer better off without making the seller worse off. For businesses, the same thinking appears in unit economics: a product only scales profitably if the revenue per customer exceeds the marginal cost of serving that customer.
Private and social marginal cost
The cost a firm pays is not always the full cost to society. When production causes pollution, noise or congestion, there is an externality, and the social marginal cost is the private marginal cost plus the damage imposed on others. Markets left alone then produce too much of the good. A Pigouvian tax or a carbon tax tries to close the gap by charging producers for the external damage, so that the price they face reflects the true marginal cost.
Regulators of utilities face a related problem. Networks such as electricity grids and water systems have high fixed costs and low marginal costs. If prices were set equal to marginal cost, the utility could not recover its fixed investment, so regulators often use two-part tariffs: a fixed standing charge plus a usage charge close to marginal cost.
Limitations
- Hard to measure. Accounting systems record total and average costs. Isolating what one extra unit adds requires judgment and estimates.
- Lumpy costs. Many costs jump in steps. The marginal cost of the 101st passenger on a 100-seat aircraft includes a second aircraft, so a single per-unit figure can mislead.
- Time horizon matters. A cost that is fixed this month may be variable next year, so the same decision can look different over different periods.
- Ignoring alternatives. Marginal cost counts money spent, not the opportunity cost of capacity that could serve a better-paying customer. Money already spent and unrecoverable, a sunk cost, should be left out entirely.