Marginal utility

The first glass of water on a hot day is worth a great deal. The fifth is worth much less. Marginal utility is the economics of that difference.

Marginal utility is the additional satisfaction a person gets from consuming one more unit of a good. It is the value of the next unit, not the average value of all units, and it is usually the figure that actually drives decisions.

Why the next unit is what matters

People do not decide whether to consume a good in the abstract; they decide whether to have a bit more or a bit less. So what matters for the choice is the value of the marginal unit, the next one, not the total satisfaction the good provides. This focus on the margin, rather than the total, was one of the great clarifying moves in economics, and it dissolved puzzles that had confused earlier thinkers.

Diminishing returns to consumption

Marginal utility typically falls as consumption rises: each additional unit adds less satisfaction than the one before. The first slice of cake delights; the fourth may be unwelcome. This diminishing marginal utility explains why people spread their spending across many goods rather than pouring everything into one, and why demand curves slope downward, since a buyer will pay less for a unit that adds less satisfaction.

Resolving the diamond-water paradox

Marginal utility resolves an old puzzle: why water, essential to life, is cheap, while diamonds, useless by comparison, are dear. The answer is that price reflects marginal, not total, utility. Water is so abundant that the marginal litre is worth little, even though water in total is priceless. Diamonds are scarce, so the marginal diamond carries high value. Value at the margin, not in total, sets the price.

The concept is a reminder that good decisions are made at the edges. The relevant question is rarely how much something is worth in total, but how much the next unit adds, and that figure is almost always smaller than our feelings about the thing as a whole suggest.