Econ 101, Part 8: Microeconomics Deep Dive
The Budget Constraint
How economists describe all the combinations of goods a person can afford with a given income and prices, and how changes in income or prices shift the possibilities.
Every consumer faces limits. With a fixed income and given prices, you cannot buy everything you want. Economists describe these limits with the budget constraint.
The idea
Imagine a student with 1,000 rupees a month to spend on two things: books at 200 rupees each and cinema tickets at 100 rupees each. She could buy:
- 5 books and no tickets.
- 10 tickets and no books.
- 3 books and 4 tickets.
- Any other combination costing 1,000 rupees or less.
All the combinations that use exactly her full budget make up her budget constraint. If you imagine a graph with books on one axis and tickets on the other, these combinations lie along a straight line, often called the budget line. Anything on or inside the line is affordable; anything outside is not.
Opportunity cost
The slope of the budget line shows the opportunity cost of one good in terms of the other. Here, buying one more book means giving up two cinema tickets, because a book costs twice as much. This ratio is called the relative price.
When income changes
If the student’s budget rises to 1,500 rupees, she can afford more of everything. The budget line shifts outward, parallel to the old one, because relative prices have not changed.
When prices change
If the price of books falls to 100 rupees, she can now afford up to 10 books. The budget line pivots: it becomes flatter because books are relatively cheaper. Her choices expand, especially for books.
Why it matters
The budget constraint is the starting point for understanding consumer choice. It separates what is affordable from what is not, and shows the trade-offs involved. Governments use similar thinking when analysing how taxes and subsidies change people’s choices.
Suppose the government gives students a discount so books cost half price. The student's budget line pivots outward on the books side. She can now afford more books without giving up as many tickets. Whether she actually buys more books depends on her preferences, but the constraint shows what has become possible.
The budget constraint shows only what people can afford. What they actually choose depends on their preferences, which economists describe separately.
- The budget constraint shows all combinations of goods affordable with a given income and prices.
- Its slope reflects relative prices and opportunity cost.
- Higher income shifts the budget line outward in parallel.
- A price change pivots the line, changing relative prices.
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