What is the Rule of Maximizing Utility?

When making purchases you should choose the item with the greatest marginal utility per dollar spent (i.e., the most bang for the buck).

For a finite budget (limited funds to expend), and making a choice of the quantity of two goods to purchase, the utility maximizing choice should occur where the marginal utility per dollar spent is the same for both goods.

In summary, when you are deciding how to spend your limited budget between two items, you should (if possible) purchase a quantity of both goods where purchasing the next unit of either good would bring you the exact same value or utility.

This point would be the most valuable or utility-maximizing way to spend your limited budget.