Utility, diminishing marginal utility and equi-marginal utility
Principles of Economics · Unit II, topics 1 and 2 · Topic 5 of 17 · 6 min read
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Cases checked to exist. Read the judgment before quoting a holding.
Every case cited here has been checked to exist — the citation resolves and names the right parties. The propositions attributed to each case have not been verified line by line against the judgment. Case names link through to the full judgment: read it before quoting a holding in an examination answer, and never carry a proposition from notes into court on trust.
1. Utility and its forms
Utility is the want-satisfying power of a commodity. It is subjective, relative to time and place, and morally neutral. Production creates utility in several forms:
- Form utility: changing the shape of a material (wood into furniture).
- Place utility: moving goods to where they are more wanted (transport).
- Time utility: storing goods until they are more wanted (warehousing grain).
- Service utility: the services of doctors, lawyers and teachers.
- Possession (ownership) utility: transferring goods to those who value them more (the trader).
- Knowledge utility: increasing the utility of a good by information (advertising).
The cardinal approach (Marshall) holds that utility can be measured in units, called “utils”, or in the money a consumer is willing to pay. The ordinal approach (Hicks and Allen) holds that utility can only be ranked; it underlies indifference-curve analysis.
2. Total and marginal utility
Total utility (TU) is the sum of the utility from all units consumed. Marginal utility (MU) is the addition to total utility from consuming one more unit: MUn = TUn − TUn−1.
| Oranges | TU | MU |
|---|---|---|
| 1 | 10 | 10 |
| 2 | 18 | 8 |
| 3 | 24 | 6 |
| 4 | 28 | 4 |
| 5 | 30 | 2 |
| 6 | 30 | 0 |
| 7 | 28 | −2 |
While MU is positive, TU rises; when MU is zero (6th orange), TU is at its maximum, the point of satiety; when MU becomes negative (disutility), TU falls. In the diagram, units are on the horizontal axis and utility on the vertical; the TU curve rises, flattens and turns down, while the MU curve slopes downward and crosses the horizontal axis at 6 units, exactly below the peak of TU.
3. Law of diminishing marginal utility
The law was stated by H.H. Gossen (1854) and is called Gossen’s first law; Marshall gave it its standard form: “The additional benefit which a person derives from a given increase of his stock of a thing diminishes with every increase in the stock that he already has.” The table above illustrates it: each orange gives less satisfaction than the one before.
Assumptions. Units are homogeneous and of a reasonable size (not a spoonful of water); consumption is continuous, without a time gap; the consumer’s tastes, income and habits do not change; the consumer is rational; utility is measurable (cardinal); the marginal utility of money is constant.
Exceptions (apparent).
- Hobbies and rare collections: each additional stamp or coin may give more satisfaction.
- Misers and money: the desire for money may seem to grow with it, though for money too MU diminishes when its uses are considered.
- Drunkards: each glass may seem to increase the craving.
- Music and poetry: repetition may increase enjoyment.
- Very small initial units: the second spoonful may give more than the first.
Most of these break an assumption (tastes change, or units are not of reasonable size), so they are not true exceptions.
Importance. It is the basis of the law of demand (since MU falls, a consumer buys more only at a lower price) and of consumer’s surplus; it explains the diamond–water paradox (water has high total utility but low marginal utility because it is abundant); it justifies progressive taxation and redistribution, since a rupee taken from the rich loses less utility than a rupee given to the poor gains; and it explains why consumers seek variety.
4. Law of equi-marginal utility
Also called Gossen’s second law, the law of substitution or the law of maximum satisfaction. A consumer with limited income and many wants gets maximum satisfaction when he spends his income on different goods in such a way that the marginal utility of the last rupee spent on each good is equal. In symbols:
MUx / Px = MUy / Py = … = MU of money
Marshall: “If a person has a thing which he can put to several uses, he will distribute it among these uses in such a way that it has the same marginal utility in all.”
Assumptions. Utility is cardinally measurable; MU of each good diminishes; the consumer is rational and seeks maximum satisfaction; his income and the prices of goods are given; the MU of money is constant; goods are divisible.
5. Worked illustration
A consumer has Rs. 6 to spend on goods X and Y. Each costs Re. 1 per unit, so MU per rupee equals MU.
| Units | MU of X | MU of Y |
|---|---|---|
| 1 | 20 | 16 |
| 2 | 18 | 14 |
| 3 | 16 | 12 |
| 4 | 14 | 10 |
| 5 | 12 | 8 |
| 6 | 10 | 6 |
Spending each rupee where it adds most: 1st rupee on X (20), 2nd on X (18), 3rd and 4th on X (16) and Y (16), 5th and 6th on X (14) and Y (14). He buys 4 units of X and 2 units of Y; the MU of the last rupee on each is 14. Total utility = (20 + 18 + 16 + 14) + (16 + 14) = 98 utils.
Check any other allocation: 3X + 3Y gives (20 + 18 + 16) + (16 + 14 + 12) = 96; 5X + 1Y gives (20 + 18 + 16 + 14 + 12) + 16 = 96. Both are lower, so 4X + 2Y is the best.
Diagram (described). Draw MUx and MUy as two downward- sloping curves, one in each panel, with rupees spent on the horizontal axis. A horizontal line at MU = 14 cuts MUx at 4 and MUy at 2. Shifting a rupee from Y to X would mean losing 14 utils on Y and gaining only 12 on X; the net loss shows the equilibrium is optimal.
6. Criticism
- Utility cannot be measured cardinally; Hicks replaced it with ordinal ranking.
- Indivisibility: goods like a car or a house cannot be bought in small units, so MU cannot be equalised at the margin.
- Consumers are not always rational: they buy by habit, custom or impulse and do not calculate.
- MU of money is not constant; it falls as spending rises.
- Ignorance of prices and alternatives.
- Fashion and advertising distort choices.
- No clear budget period and many goods are complementary, so their utilities are not independent.
7. Applications
Despite its limits the principle applies widely: in consumption (the household budget); in production, where a producer combines factors so that the marginal product per rupee is equal in each; in exchange; in distribution of national income among factors; in public finance, where the government spends so that the marginal social benefit of each head of expenditure is equal (Dalton’s principle of maximum social advantage); and in dividing income between spending and saving.
How this is examined
“Critically explain the law of equi-marginal utility” was a 16-mark question in January 2026 (100-mark paper) and a 10-mark essay in January 2026 and June 2025 (80-mark papers). Diminishing marginal utility is rarely set alone in recent papers but is the foundation of the equi-marginal, demand and consumer’s surplus answers. Include the table; examiners reward the working that shows why 4X + 2Y is the best allocation.
Now try the real questions: Principles of Economics past question papers.