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Business Economics · Consumer demand and behaviour

Utility Theory and Marginal Utility Explained

Updated 11 October 2026 · Fact-checked

Utility is the satisfaction a consumer gets from goods. Total utility (TU) is the satisfaction from all units. Marginal utility (MU) is the extra TU from one more unit. MU falls as you consume more. A consumer is in equilibrium when MU per rupee is equal across all goods and the budget is spent.

Understand Utility Theory and Marginal Utility

Utility is a measure of the satisfaction or benefit a consumer gets from consuming a good or service. Economists use it to explain why people buy what they buy. In cardinal utility theory, satisfaction can be measured in numbers, often called utils. You can then say that one good gives 20 utils and another gives 10.

Total utility (TU) is the satisfaction from all units consumed. Marginal utility (MU) is the extra satisfaction from consuming one more unit. So MU = change in TU ÷ change in quantity. When MU is positive, TU rises. When MU is zero, TU is at its maximum. When MU is negative, TU falls.

The law of diminishing marginal utility says that, as a consumer takes more units of a good in a given period, the MU of each extra unit eventually falls. The first glass of water when you are thirsty is worth a lot. The fifth is worth much less. This holds when tastes, income and the time period are unchanged and units are of the same size and quality. It is the reason a demand curve slopes downward: you will pay less for extra units because they add less satisfaction.

A consumer has limited income, so must choose. The equimarginal principle says that you get the most total utility when the last rupee spent on each good gives the same extra utility. That is MUx ÷ Px = MUy ÷ Py = ... and the whole budget is spent. If one good gives more MU per rupee, shift spending towards it. Its MU falls as you buy more, and the ratios move together until they are equal.

Ordinal utility is the alternative view. It says you cannot measure utility in numbers, only rank bundles as better, worse or equal. Indifference curves use this approach. Cardinal utility needs a measurable unit; ordinal utility needs only a preference ranking.

Key rules to remember

Marginal utility
MU = ΔTU ÷ ΔQ
For one extra unit, MU = TU(n) − TU(n−1). In calculus form, MU = dTU/dQ.
Total utility from marginal utilities
TU(n) = MU(1) + MU(2) + ... + MU(n)
TU is the running sum of MU. TU is maximum where MU = 0.
Law of diminishing marginal utility
MU falls as Q rises (other things constant)
Holds for a given period, with unchanged tastes and income and identical units.
Equimarginal principle (consumer equilibrium)
MUx ÷ Px = MUy ÷ Py = ... = MU of money
Must also satisfy the budget: Px·x + Py·y + ... = income.
Single good equilibrium
MU of good = Price × MU of money
If MU of a rupee is 1 util, buy until MU = P.

How to solve Utility Theory and Marginal Utility questions

Use this method for any question on utility, MU and consumer choice.

  1. 1Write down the data: TU or MU schedule, prices and the budget.
  2. 2If given TU, compute MU as the change in TU for each extra unit. If given MU, build TU as the running total.
  3. 3Check the pattern. MU falling shows diminishing marginal utility. Note where MU hits zero or turns negative.
  4. 4For each good, compute MU ÷ P at every quantity level.
  5. 5Allocate the budget one rupee or unit at a time to the option with the highest MU per rupee, until the money is spent.
  6. 6Check the equilibrium: MU per rupee is equal across goods (or as close as whole units allow) and spending equals income.
  7. 7State the answer with units: quantities bought, total utility, and the reason.

Quickest way: Highest MU per rupee, step by step

When to use it: Use for numerical choice questions with a table of MU and a fixed budget.

  1. Divide every MU by its price to get MU per rupee in a table.
  2. Rank all the cells from highest to lowest.
  3. Buy in that order, subtracting each price from the budget, until the budget is used.
  4. Count how many units of each good you bought. Add the MU values for total utility.
  5. Check that the last unit bought of each good has similar MU per rupee.

Common mistakes in Utility Theory and Marginal Utility

  • Saying diminishing marginal utility means total utility falls.

    The words 'diminishing' and 'utility' get mixed up.

    Fix: TU keeps rising while MU is positive, even if MU is falling. TU falls only when MU is negative.

  • Equating MU of goods instead of MU per rupee.

    Students forget that prices differ.

    Fix: Always compare MU ÷ P. Equal MU is correct only when prices are equal.

  • Stopping at equal ratios without checking the budget.

    The equal ratio looks like the full answer.

    Fix: Check that total spending equals income. Equal ratios at the wrong spending level are not the answer.

  • Computing MU from the wrong TU difference.

    Students subtract in the wrong order or mislabel the first unit.

    Fix: MU of the first unit equals TU of one unit. Then MU(n) = TU(n) − TU(n−1).

  • Mixing up cardinal and ordinal utility.

    Both describe preferences, so the labels blur.

    Fix: Cardinal means measurable in utils; ordinal means only ranked. Marginal utility analysis is cardinal; indifference curves are ordinal.

  • Stating diminishing MU without its conditions.

    It is memorised as a bare slogan.

    Fix: Add: same period, same tastes and income, identical units, consumption of one good with others held constant.

Worked examples

Example 1

A consumer's total utility from a good is 0, 10, 18, 24, 28, 28 and 25 utils for 0 to 6 units. (a) Find the marginal utility of each unit. (b) At which quantity is TU maximum? (c) Does the data show diminishing marginal utility?

Show the solution
  1. MU(1) = 10 − 0 = 10.
  2. MU(2) = 18 − 10 = 8.
  3. MU(3) = 24 − 18 = 6.
  4. MU(4) = 28 − 24 = 4.
  5. MU(5) = 28 − 28 = 0.
  6. MU(6) = 25 − 28 = −3.
  7. TU is highest at 28, reached at 4 units and unchanged at 5. MU = 0 at the fifth unit.
  8. MU falls steadily: 10, 8, 6, 4, 0, −3.

Answer: MU = 10, 8, 6, 4, 0, −3. TU is maximum (28 utils) at 5 units, where MU = 0. Yes, MU diminishes with every extra unit.

Example 2

A consumer has ₹12 to spend on tea (price ₹2 per cup) and samosas (price ₹3 each). MU of tea for cups 1 to 4 is 20, 16, 12, 8 utils. MU of samosas for units 1 to 3 is 24, 15, 9 utils. Find the utility-maximising bundle.

Show the solution
  1. MU per rupee for tea: 20÷2 = 10, 16÷2 = 8, 12÷2 = 6, 8÷2 = 4.
  2. MU per rupee for samosas: 24÷3 = 8, 15÷3 = 5, 9÷3 = 3.
  3. Rank from highest: tea 1 (10), tea 2 (8), samosa 1 (8), tea 3 (6), samosa 2 (5), tea 4 (4).
  4. Buy tea 1: cost ₹2, left ₹10.
  5. Buy tea 2: cost ₹2, left ₹8.
  6. Buy samosa 1: cost ₹3, left ₹5.
  7. Buy tea 3: cost ₹2, left ₹3.
  8. Next best is samosa 2 (5), cost ₹3, left ₹0. Budget is used.
  9. Bundle: 3 cups of tea and 2 samosas. Spending = 3×2 + 2×3 = ₹12.
  10. Total utility = (20 + 16 + 12) + (24 + 15) = 48 + 39 = 87 utils.
  11. Check: last tea MU per rupee is 6 and last samosa is 5, the closest possible with whole units.

Answer: 3 cups of tea and 2 samosas, spending the full ₹12, giving total utility of 87 utils.

Exam tips

  • In MCQs, check whether the question gives TU or MU before you calculate. Mixing them is the commonest slip.
  • Always divide by price before comparing goods. Show this step in written answers to earn method marks.
  • Remember: TU is maximum where MU = 0, and TU falls when MU is negative.
  • In written answers, state the assumptions of diminishing MU: same period, constant tastes and income, identical units.
  • When asked to link to demand, explain that falling MU makes the consumer willing to pay less for extra units, so the demand curve slopes down.

Practice questions from Consumer demand and behaviour

Utility Theory and Marginal Utility in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Utility Theory and Marginal Utility: frequently asked questions

What is the difference between cardinal and ordinal utility?

Cardinal utility assumes satisfaction can be measured in numbers, called utils. Ordinal utility only assumes you can rank bundles as preferred, less preferred or indifferent. Marginal utility analysis uses the cardinal view; indifference curve analysis uses the ordinal view.

Can marginal utility be negative?

Yes. If consuming one more unit reduces total satisfaction, MU is negative. Think of eating too much food. A rational consumer stops before MU turns negative, unless the extra unit is free.

What is the equimarginal principle?

It says a consumer maximises total utility when the MU per rupee is equal across all goods bought, and the whole budget is spent. If one good gives more MU per rupee, you should buy more of it and less of the other.

Why does the law of diminishing marginal utility lead to a downward-sloping demand curve?

As you buy more, each extra unit adds less satisfaction. You will pay only a lower price for it. So the quantity demanded rises as the price falls, holding other factors constant.