Marginal Utility

Marginal utility is the additional satisfaction or benefit a person gets from consuming one more unit of a good or service. It is the foundation of modern consumer theory in microeconomics: economists use it to explain why demand curves slope downward, how people split a budget across many goods and why the market price of a thing reflects its usefulness at the margin rather than in total.

The idea is usually paired with the principle of diminishing marginal utility, which says that each extra unit tends to add less satisfaction than the one before. Marginal utility is the buyer-side mirror of marginal cost on the seller's side; where the two meet, through prices, markets reach equilibrium. It should not be confused with total utility, the overall satisfaction from all units consumed, or with intrinsic utility as used in crypto discussions.

Imagine coming home thirsty after a long walk. The first glass of water is wonderful. The second is pleasant. By the fourth, you would hardly notice if someone took it away. Each glass is the same water, but the extra benefit of the next one keeps shrinking. That shrinking extra benefit is marginal utility.

This explains everyday choices. People rarely spend all their money on one favorite thing, because the tenth slice of cake gives far less pleasure than a first coffee or a bus ticket. They spread spending around, moving money to whatever gives the biggest boost for the next dollar.

Marginal utility is the change in total utility divided by the change in quantity consumed:

Here $MU$ is marginal utility, $\Delta TU$ is the change in total utility and $\Delta Q$ is the change in units consumed. A consumer gets the most from a fixed budget when the last dollar spent on each good brings the same extra utility:

$MU_x$ and $MU_y$ are the marginal utilities of goods x and y, and $P_x$ and $P_y$ are their prices. Suppose a student's next cinema ticket at $10 would add 40 units of utility (4 per dollar) and the next meal at $5 would add 30 units (6 per dollar). The meal gives more per dollar, so the student should move spending toward meals until the ratios are equal. As more meals are bought, their marginal utility falls, and the ratios converge. Because people buy more of a good only when its price falls enough to match its lower marginal utility, the rule produces a downward-sloping demand curve, the basis of supply and demand analysis.

Adam Smith noted in The Wealth of Nations (1776) that water is essential but cheap while diamonds are useless for survival but costly. Writers in classical economics largely explained value through costs of production, and Marx built on the labor theory of value. In the 1870s three economists working separately, William Stanley Jevons in England (1871), Carl Menger in Austria (1871) and Léon Walras in Switzerland (1874), argued that value depends on marginal utility. Water is abundant, so the marginal glass is worth little; diamonds are scarce, so the marginal stone is worth a lot. This "marginal revolution" laid the foundations of neoclassical economics, while Menger's line of thought became the Austrian school. Hermann Heinrich Gossen had stated diminishing marginal utility in 1854, and Daniel Bernoulli had used a similar idea in 1738 to explain why people avoid fair gambles.

• Risk and insurance. If the marginal utility of money falls as wealth rises, losing $1,000 hurts more than gaining $1,000 helps. This underpins risk aversion and the demand for insurance.
• Taxation and redistribution. One argument for a progressive tax is that an extra dollar is worth less to a rich household than to a poor one. Critics reply that utility cannot be compared between people.
• Price changes. The response of demand to a price change splits into a substitution effect and an income effect. Rare exceptions, such as Giffen goods and Veblen goods, show where the simple story breaks down.

Utility cannot be measured directly. From the 1930s, following work by John Hicks and Roy Allen, mainstream theory moved to ordinal utility, which only ranks choices, and expressed the same logic through indifference curves and marginal rates of substitution. Behavioral economics adds further doubts: findings such as loss aversion suggest that people value gains and losses from a reference point rather than from total wealth, and that habits, framing and social comparison shape satisfaction. Diminishing marginal utility is also not universal; collectors and fans may enjoy the tenth item more than the first.

• OpenStax, Principles of Economics 3e: Consumption Choices