A budget line, also known as a budget constraint, shows all possible combinations of two goods that a consumer can buy with their limited income and given market prices.
It acts as a boundary: everything on or inside the line is affordable, while everything outside the line is beyond the consumer’s reach.
The Equation:
P_X × Q_X + P_Y × Q_Y = M
- P_X, P_Y: Prices of good X and good Y.
- Q_X, Q_Y: Quantities of good X and good Y.
- M: Total money income.
Key Properties:
- Downward slope: Because income is limited, choosing to buy more of one good means you must buy less of the other.
- Slope: Represented by P_X / P_Y, this shows the trade-off rate between the two goods in the market.
- Income impact: An increase in income shifts the entire line outward, meaning more options are available.
- Price impact: If the price of one good increases, the line rotates inward on that good’s axis, reflecting reduced purchasing power for that specific item.
Consumption Choices:
- Points on the line: You are spending your entire income.
- Points inside the line: You have money left over.
- Points outside the line: You cannot afford these items with your current budget.