Income elasticity of demand calculator
As incomes grow, people buy more of some things, the same of others and less of a few. Income elasticity puts a number on it: the percentage change in quantity demanded divided by the percentage change in income. Enter income and quantity before and after, and choose the midpoint or simple method. The result tells you whether the good is a necessity, a luxury or an inferior good.
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How to use it
- Choose the midpoint or simple method.
- Enter income before and after.
- Enter the quantity bought before and after.
E = %ΔQ ÷ %ΔI, with %Δ over the average of the two values (midpoint) or over the first value (simple).
- E < 0: inferior good
- 0 < E < 1: necessity
- E = 1: unit elastic, the good keeps its share of the budget
- E > 1: luxury
Worked example
A household’s income rises from 40,000 to 50,000 and it eats out 15 times a year instead of 10.
- %ΔQ (midpoint) = 5 ÷ 12.5 = 40 %.
- %ΔI = 10,000 ÷ 45,000 = 22.22 %.
- E = 40 ÷ 22.22 = 1.8: a luxury.
With the simple method, a 25 % rise in income that moves bread from 100 to 105 loaves (+5 %) gives E = 0.2, a necessity; bus trips falling from 20 to 16 (−20 %) give E = −0.8, an inferior good.
Tips
- Keep prices fixed between the two points, or the price effect mixes in.
- Use real income (after inflation) for comparisons across years.
- Elasticity is not fixed: a good can be a luxury at low incomes and a necessity at high ones.
Frequently asked questions
What is the formula for income elasticity of demand?
E = % change in quantity demanded ÷ % change in income. With the midpoint method: income 40,000 → 50,000 is +22.22 % and quantity 10 → 15 is +40 %, so E = 40 ÷ 22.22 = 1.8.
What is the difference between a necessity and a luxury?
Both are normal goods: demand rises with income. For a necessity it rises more slowly than income (E between 0 and 1), so its share of the budget falls. For a luxury it rises faster (E above 1), so its share grows.
What is an inferior good?
One people buy less of as they get richer, because they switch to something better: E is negative. Which goods are inferior depends on the market and the income level, not on the product itself.
Can I use household spending instead of quantity?
Yes, if prices did not change in between: spending then moves in the same proportion as quantity. With inflation, use quantities or spending in constant prices.