When a traveler is stranded in a desert, the first glass of water is worth almost everything. The second is still precious. By the time the crisis is over and water is freely available, the hundredth glass of water is worth almost nothing — the person is fully hydrated and can barely be induced to drink more. The marginal utility of water — the satisfaction from one more glass — has fallen from enormous to near-zero as consumption increased. That pattern is one of the most important regularities in consumer economics.
The formula
Marginal Utility (MU) = ΔTotal Utility ÷ ΔQuantity
For a consumer whose total utility from eating 1 slice of pizza is 20 utils, 2 slices is 35 utils, and 3 slices is 45 utils:
- MU of 2nd slice = (35 – 20) ÷ 1 = 15 utils
- MU of 3rd slice = (45 – 35) ÷ 1 = 10 utils
Marginal utility is falling — each additional slice adds less satisfaction than the one before. This is the law of diminishing marginal utility in action.
Reading the result
Marginal utility is the marginal benefit side of every consumer decision. A rational consumer should consume one more unit of a good if and only if the marginal utility from that unit exceeds the marginal cost (typically the price). The consumer should stop consuming when MU = Price — at that point, the additional satisfaction just equals the additional cost.
For a consumer choosing between two goods: Utility maximization requires: MU_A / P_A = MU_B / P_B
If the marginal utility per dollar of Good A exceeds that of Good B, the consumer should shift spending from B to A. They keep shifting until the ratios equalize — the point of maximum total utility given the budget. This is the consumer equilibrium condition, and it underlies every demand curve in economics.
Worked example
A consumer allocates her lunch budget between sandwiches ($8 each) and coffee ($4 each). Currently she buys one sandwich and one coffee.
- MU of the sandwich: 40 utils. MU per dollar: 40 ÷ 8 = 5
- MU of the coffee: 16 utils. MU per dollar: 16 ÷ 4 = 4
The sandwich provides more satisfaction per dollar. She should buy an extra sandwich and cut the coffee. After adding the second sandwich, its MU falls to 24 utils (MU per dollar = 3). Now coffee is the better deal. She buys a second coffee — its MU per dollar rises in her marginal calculation as she consumes less coffee. The process continues until MU per dollar is equalized across the two goods within her budget.
The Bureau of Labor Statistics Consumer Expenditure Survey captures aggregate revealed-preference data — spending allocation across categories — that reflects millions of individual marginal utility calculations made by households balancing food, housing, transportation, and entertainment spending simultaneously.
Where it's used
Marginal utility is the analytical workhorse of demand theory. It explains why demand curves slope downward (because MU falls as quantity increases, consumers pay less for each additional unit), why consumers diversify purchases (equalizing MU per dollar across goods), and why water — despite being essential to life — is cheap while diamonds — despite being luxury items — are expensive. Water's MU at typical consumption levels is very low (we have so much); diamond's MU at typical consumption levels is very high (we have very few). Price reflects MU at the margin, not total utility or necessity.
