7.1 Utility


2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Define and calculate total utility and marginal utility
  2. Explain the law of diminishing marginal utility
  3. Understand and apply the equi-marginal principle
  4. Explain how an individual demand curve is derived using marginal utility theory
  5. Evaluate the limitations of marginal utility theory and its assumptions about rational behaviour

Objective 1 — Total Utility and Marginal Utility

What is Utility?

Utility is the measure of the happiness, pleasure, or satisfaction that a person gets from consuming (using or eating) a good or service. Think of it as a "satisfaction score."

Economists assume that satisfaction can be measured in units — sometimes called utils (units of utility). Just like you can count the number of slices of pizza you eat, you can (in theory) count how many units of satisfaction you got from eating them.


Total Utility

Total utility (TU) is the overall satisfaction you get from consuming all the units of a good over a period of time.

In simple terms: Add up all the satisfaction from every single unit you consumed — that's your total utility.

Example: Suppose you eat slices of pizza one by one. The table below shows how much satisfaction (utility) each slice gives you:

Slices of Pizza ConsumedUtility from Each SliceTotal Utility
12020
21333
3942
4648
5452
6355
7257
  • After 1 slice → Total Utility = 20
  • After 2 slices → Total Utility = 20 + 13 = 33
  • After 3 slices → Total Utility = 33 + 9 = 42
  • And so on…

Notice: Total utility keeps increasing as you eat more slices — but it increases by smaller and smaller amounts each time.


Marginal Utility

Marginal utility (MU) is the extra satisfaction you get from consuming one more unit of a good.

In simple terms: How much extra happiness does the very next unit give you?

Formula:

Marginal Utility = Change in Total Utility ÷ Change in Quantity

Or more simply: MU = New Total Utility − Previous Total Utility

Example:

  • After eating 1 slice, your total utility is 10 utils.
  • After eating 2 slices, your total utility is 15 utils.
  • The marginal utility of the 2nd slice = 15 − 10 = 5 utils

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