Marginal Cost: The Cost of One More
Imagine you run a small bakery. Every morning you bake 50 loaves; your total cost β rent, flour, yeast, electricity, your own time β comes to βΉ2,000, an average of βΉ40 per loaf.
Now a customer asks for one more loaf. What does baking that 51st loaf actually cost? Not βΉ40 β the rent doesn't change and your time is already paid; the oven is already hot. You need only a little more flour, yeast, and electricity: maybe βΉ15.
That βΉ15 is the marginal cost.
The Core Intuition
Marginal cost answers one question: "If I produce one more unit, how much does my total cost increase?"
It is not the average cost. It is not the total cost. It is the change in total cost when output changes by exactly one unit.
Think of it as the "extra cost" β the cost of the next step, not the cost of all steps so far.
The Precise Definition
Let TC(Q) be the total cost of producing Q units. The marginal cost of the Q-th unit β of increasing output from Qβ1 to Q β is:
MC(Q)=TC(Q)βTC(Qβ1)
When output can change continuously, we use the derivative:
MC(Q)=dQdTCβ
MC=ΞQΞTCβorMC=dQdTCβ
Why It Matters
Marginal cost is the decision-maker's cost. When a firm asks "Should I produce one more unit?", the answer depends on whether the marginal revenue exceeds the marginal cost. If yes, produce it; if no, stop.
This is why marginal cost typically:
- Falls initially β fixed costs are spread out and workers specialise.
- Rises eventually β because of diminishing returns (more workers in a fixed kitchen get in each other's way).
A common mistake: confusing marginal cost with average cost. If average cost is βΉ40 and marginal cost is βΉ15, producing one more lowers the average β but the decision is still based on marginal cost, not average.
A Quick Example
| Loaves | Total Cost (βΉ) | Marginal Cost (βΉ) |
|---|
| 0 | 500 | β |
| 1 | 520 | 20 |