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According to Marshall, “the excess of price which the consumer would be willing to pay rather than go without it, the thing over which he actually does pay is the economic measure of the surplus of satisfaction. It may be called consumer surplus.”

Assumptions Related to Consumer Surplus

Here are the core pillars of Alfred Marshall’s classical theory of Consumer Surplus:

  1. Cardinal Measurement of Utility - Marshall assumed that utility, i.e, the satisfaction derived from a good, can be measured in quantitative units called "utils." If you can’t put a number on how much you like something, you can’t subtract the price paid from the total satisfaction received. As of reality, in this framework, if you value a coffee at 50 units and pay 20 units (rupees), your surplus is exactly 30 units.
  2. Law of Diminishing Marginal Utility (DMU) - The concept of Consumer Surplus is based on Law of Diminishing Marginal Utility, i.e, as a consumer consumes more of a good, the extra satisfaction from each additional unit decreases. Because the first unit gives you high utility, you’d be willing to pay a high price for it. Since you actually pay a lower market price (determined by the utility of the last unit you buy), you gain a "surplus" on all previous units. 
  3. Constant Marginal Utility of Money - Marshall assumed that as you spend money on a product, the "utility" of the money remaining in your pocket stays the same. If the utility of money changed as you became "poorer" by spending it, the "measuring rod" itself would be shrinking or growing. To use money as a consistent scale for utility, its own value must remain fixed. In truth, the more money you spend, the more "precious" each remaining dollar/rupee becomes, but Marshall ignored this for the sake of simplicity in his calculations.
  4. Absence of Substitutes - The theory assumes that the commodity in question is isolated and has no close substitutes. If a product has a close substitute (like tea and coffee), a change in the price of one would instantly shift the utility and demand for the other. By assuming no substitutes, Marshall ensures that the consumer's willingness to pay is derived solely from that specific product’s utility, making the surplus easier to isolate and measure.

Criticism

These criticisms are primarily directed at the Law/Theory of Consumer Surplus as decribed by Alfred Marshall (The core pillars we discussed above):

  1. Unrealistic Assumptions - Marshall assumed utility could be measured in "utils" or exact numbers. In reality, utility is ordinal (we know we prefer an apple to an orange, but we can’t say we prefer it by exactly "5 units."). Similarly, as the stock of ones' money decreases, each remaining unit of money (Dollar/Rupee) becomes more "precious" to you, which changes your willingness to pay. Consumer surplus for one good (like coffee) depends on the price of substitutes (like tea). Marshall’s model often looked at goods in isolation, which ignores the complex web of market interactions.
  2. Consumer Surplus Concept is Imaginary and Illusionary - Since a consumer doesn't actually receive the difference in cash, it’s just a psychological feeling of "getting a bargain." You cannot go to a store and buy bread using the "surplus" you saved on a shirt. Critics argue it’s a purely subjective mental state that varies so wildly between individuals that it’s impossible to aggregate into a meaningful social statistic.
  3. Consumer Surplus is Meaningless in Case of Necessities – If you are starving, your "willingness to pay" for a loaf of bread might be your entire life savings. Because the utility of water or salt is technically "infinite" (since you'd die without them), the calculated surplus becomes mathematically undefined or nonsensical. We can't measure a surplus against an infinite price.
  4. No Practical Utility – The concept of consumer surplus is based on theoretical principles and has no practical significance; critics argued that because it’s hard to measure and based on "feelings," governments and businesses can't use it. While this was a common early criticism, it is actually the least true in modern economics. Today, the concept is vital for Cost-Benefit Analysis. For example, when a government decides whether to build a new bridge, they don't just look at toll revenue; they calculate the Consumer Surplus (the time and money saved by citizens) to justify the investment.

Measurement of Consumer Surplus

CS = TU – [P*Q]

Here,

  • CS = Consumer Surplus
  • TU = Total Utility
  • P = Price
  • Q = Quantity

Consumer Surplus = Prepared to Pay – Actual Price Paid

The concept of Consumer Surplus is a relative concept because the term utility is relative, i.e., it differs from person to person as from time to time. Even it differs from commodity to commodity.

In case of necessary commodities whose prices are low, consumer surplus will be higher than the high price of luxuries.