8.2 How Perfectly Competitive Firms Make Output Decisions

Profit Maximization in Perfect Competition

Profit-Maximizing Quantity

Perfectly competitive firms are price takers. They face a perfectly elastic demand curve and must accept the market equilibrium price ($P$). Because every unit sells at the same price, the revenue from selling one more unit (marginal revenue) is just the price itself: $MR = P$.

To maximize profits, the firm produces where $MR = MC$. Since $MR = P$ for a price taker, the profit-maximizing rule becomes: produce where $P = MC$.

Why does this work? Think about it from both sides:

• If the firm produces less than the $P = MC$ quantity, there are still units it could sell where price exceeds marginal cost. Each of those units would add to profit, so the firm is leaving money on the table.
• If the firm produces more than the $P = MC$ quantity, it's making units that cost more to produce than they bring in. Each of those units shrinks profit.

The $P = MC$ quantity is the point where the firm squeezes out every profitable unit without tipping into unprofitable ones.

Evaluating Profits and Losses

Profitability

Once a firm finds its profit-maximizing quantity, it checks profitability by comparing $P$ to average total cost ($ATC$) at that quantity.

• $P > ATC$: The firm earns economic profits. Revenue per unit exceeds cost per unit, so the firm has a positive profit margin.
• $P < ATC$: The firm incurs economic losses. Revenue per unit falls short of cost per unit, giving a negative profit margin.
• $P = ATC$: The firm earns zero economic profit (also called normal profit). Revenue per unit exactly equals cost per unit. The firm is breaking even. This doesn't mean the owners get nothing; it means they're earning exactly their opportunity cost.

You can also calculate total profit (or loss) as a rectangle on the graph: $(P - ATC) \times Q$. If that value is positive, the firm profits. If negative, it's losing money.

Short-Run Shutdown Decision

Even when a firm is losing money, shutting down isn't always the best move. The key question is whether producing covers enough costs to make it worth staying open.

The decision hinges on average variable cost ($AVC$), which represents per-unit costs the firm can avoid by shutting down (things like electricity, materials, and hourly labor).

Here's the shutdown rule, step by step:

1. Find the profit-maximizing (loss-minimizing) quantity where $P = MC$.
2. Compare $P$ to $AVC$ at that quantity.
3. If $P \geq AVC$: Keep producing. Revenue covers all variable costs and chips away at fixed costs (rent, equipment leases). The loss from producing is smaller than the loss from shutting down and still owing all those fixed costs.
4. If $P < AVC$: Shut down temporarily. Revenue doesn't even cover variable costs, so every unit produced actually makes the loss worse. The firm is better off producing nothing, paying only its fixed costs, and waiting for conditions to improve.