To the left is a simple numerical example illustrating why profit maximization for a perfectly competitive firm means that it should produce a level of output such that P = MC.

   Increasing output where MC < P, increases profits (the Profit or "Profit Change" column) because the contribution to revenue exceeds the added costs. Output should not be increased when MC > P; if it is, profits are reduced. For this particular firm, 13 is the profit maximizing level of output, the level at which P = MC.