The general rule is that firm maximizes profit by producing that quantity of output where marginal revenue equals marginal costs. The profit maximization issue can also be approached from the input side. That is, what is the profit maximizing usage of the variable input?  To maximize profit the firm should increase usage of the input "up to the point where the input's marginal revenue product equals its marginal costs".  So mathematically the profit maximizing rule is MRP L = MC L , where the subscript L refers to the commonly assumed variable input, labor. The marginal revenue product is the change in total revenue per unit change in the variable input. That is MRP L = ∆TR/∆L. MRP L is the product of marginal revenue and the marginal product of labor or MRP L = MR x MP L .