How Do Indifference Curves and Budget Lines Work?

An indifference curve shows combinations of two goods that give a consumer the same satisfaction. A budget line shows the combinations they can afford. Together, they help explain which affordable combination a consumer chooses.

For CIE International A Level Economics, this is A Level subtopic 7.2, Indifference curves and budget lines. It builds on the demand concepts in AS topic 2, The price system and the microeconomy.

How the curves work

Put one good on each axis. Every point on an indifference curve is equally preferred: the consumer is willing to give up some of one good to gain more of the other. A curve representing a more preferred set of combinations shows greater satisfaction, assuming the consumer prefers more of both goods.

The budget line depends on income and the prices of the two goods. Points on the line use all available income; points below it are affordable but leave some income unspent. Points beyond it are unaffordable.

For example, suppose income is $60$, good $X$ costs $10$ and good $Y$ costs $5$. The budget line is

$$10X+5Y=60.$$

The consumer could buy $6$ units of $X$ and no $Y$, or $12$ units of $Y$ and no $X$. They could also buy $2$ units of $X$ and $8$ units of $Y$. Which combination they prefer depends on their indifference curves.

The usual model predicts that the consumer chooses the most preferred indifference curve they can reach. With smooth, convex curves, this is typically where a curve just touches the budget line.

Common exam mistake

Do not confuse a change in preferences with a change in affordability. Higher income moves the budget line outwards if prices stay the same; a change in just one good’s price pivots it. Neither change, by itself, means the consumer’s indifference curves have changed.