The Profit Maximisation Condition: Why Firms Produce What They Do

Think about the last time you decided how many samosas to eat. You stop when the next samosa gives you less pleasure than the effort of eating it costs you. A firm does exactly the same thing — but with money.

The Core Intuition

Every extra unit a firm produces brings in some additional revenue (called Marginal Revenue, MR) and costs some additional money to make (called Marginal Cost, MC). As long as the revenue from one more unit exceeds its cost — that is, as long as $MR > MC$ — the firm should keep producing. Each such unit adds to total profit.

But the moment the cost of the next unit exceeds the revenue it brings — $MC > MR$ — producing that unit would reduce total profit. So the firm stops.

The sweet spot, the point of maximum profit, is where:

This is the Profit Maximisation Condition. It is not a guess — it is a logical necessity.

What Each Symbol Means

• MR (Marginal Revenue): The change in total revenue when one more unit is sold. In perfect competition, MR equals the market price ($P$), because the firm can sell any quantity at that price. In imperfect competition, MR is less than price and falls as output rises.
• MC (Marginal Cost): The change in total cost when one more unit is produced. It typically falls initially (due to specialisation) and then rises (due to diminishing returns).

Why the Condition Holds — A Simple Proof

Suppose a firm produces $Q$ units. Consider producing one more unit.

• If $MR > MC$, the extra unit adds $(MR - MC)$ to profit. So profit increases. The firm should produce it.
• If $MR < MC$, the extra unit subtracts $(MC - MR)$ from profit. So profit falls. The firm should not produce it.

The only point where no further increase or decrease in output can raise profit is when $MR = MC$. At that point, the last unit produced adds exactly zero to profit — any change would reduce it.

A Diagram in Words

Draw a standard cost-revenue diagram:

• The MC curve is U-shaped (falls, then rises).
• The MR curve is a horizontal line at price $P$ (under perfect competition) or a downward-sloping line (under monopoly).

The two curves intersect at two points — once where MC is falling (the first intersection) and once where MC is rising (the second intersection). The profit-maximising output is at the second intersection, where MC cuts MR from below. Why? Because to the left of this point, $MR > MC$ (so profit rises as output increases), and to the right, $MR < MC$ (so profit falls). The first intersection is a profit minimum — the firm would be better off producing nothing than stopping there.

Why This Matters for Exams …