Glossary

Marginal Utility

Marginal utility is the extra satisfaction you get from consuming one more unit of something. Think about eating pizza when you are hungry: the first slice is the best thing you have eaten all day, the fourth is fine, and by the sixth you can barely finish it. Each additional slice adds less satisfaction than the one before — that pattern is the rule, not the exception.

More formally, marginal utility ( $MU$ ) is the change in total utility ( $TU$ ) — the overall satisfaction from all units consumed — divided by the change in quantity consumed. The consistent tendency for $MU$ to fall as consumption rises is called the law of diminishing marginal utility. This law is why demand curves slope downward: if each extra unit is worth less to you, you will only buy more of it at a lower price. It also leads directly to the equimarginal principle: when you have a fixed budget, you get the most out of every dollar by allocating spending until the last dollar on each good delivers the same extra utility. If one good gives you more utility per dollar than another, you should shift spending toward it — and keep doing so until the gap closes.

MU = \frac{\Delta TU}{\Delta Q}, \qquad \frac{MU_x}{P_x} = \frac{MU_y}{P_y}

Marginal utility and consumer optimum

Marginal utility is the change in total utility per extra unit consumed. The consumer's optimum requires that utility per dollar be equal across goods: if

MU_x/P_x > MU_y/P_y

, shifting a dollar from

y

x

raises total utility, so you keep reallocating until the two ratios are equal.

Total utility: The cumulative satisfaction from consuming all units of a good up to a given quantity, $TU$ . Marginal utility is the slope of the total utility curve.
Law of diminishing marginal utility: The principle that $MU$ falls as consumption of a good increases, holding everything else constant. It is why the second slice of pizza is never as satisfying as the first.
Equimarginal principle: The rule that a consumer maximizes total utility when $MU_x/P_x = MU_y/P_y$ for every pair of goods $x$ and $y$ in the budget. Any inequality signals a reallocation that would raise overall satisfaction.
Consumer equilibrium: The combination of goods at which a consumer, facing given prices and a fixed budget, maximizes total utility — reached when the equimarginal principle holds across all goods purchased.