Short-Run Profit and Loss in Perfect Competition

A perfectly competitive firm takes price as given and picks quantity to maximize profit. It produces where marginal cost equals price, because that is the last unit where the revenue it brings in still covers the cost of making it. Once MC rises above price, the next unit would cost more than it earns, so the firm stops there. That output rule never changes. What changes is whether the firm is happy with the result.

To see profit or loss, compare price to average total cost at that quantity. If price is above ATC, the firm earns a profit on every unit, shown as a rectangle above the ATC curve on the graph: height is price minus ATC, width is quantity. If price sits below ATC, that same rectangle sits below the curve and marks a loss. If price equals ATC exactly, the firm breaks even, earning zero economic profit while still covering all its costs, including the opportunity cost of the owner's time and money.

This is a snapshot, not the end of the story. Short-run profit draws new firms into the industry, and short-run loss pushes firms out, since entry and exit are free in the long run. As firms enter an industry that was profitable, market supply grows, price falls, and each firm's demand curve slides down until price equals minimum ATC and profit disappears. That long-run adjustment is the next step after this one.