Microeconomics · Unit 2: Supply and Demand · 14 min read · Updated 2026-05-11
Price Elasticity of Demand — AP Microeconomics
AP Microeconomics · Unit 2: Supply and Demand · 14 min read
1. What is Price Elasticity of Demand?★★☆☆☆⏱ 3 min
Price elasticity of demand (abbreviated PED or $E_d$) measures how responsive quantity demanded of a good is to a change in its price, holding all other determinants of demand constant. Unlike the slope of the demand curve (which measures absolute change), elasticity measures proportional change, making it unit-independent, so you can compare elasticity across goods measured in different units.
Per the AP CED, this topic makes up 6-8% of your total AP exam score, appearing regularly in multiple-choice and as a foundation for free-response questions. Because demand curves slope downward, $E_d$ is almost always negative; by convention, we use the absolute value of $E_d$ for simplicity.
2. Calculating PED: Midpoint (Arc) Method★★☆☆☆⏱ 4 min
The general formula for PED is:
E_d = \frac{\% \Delta Q_d}{\% \Delta P}
Simple percentage change has a "base problem": moving from point A to B gives a different elasticity than B to A, because the initial base value changes. The midpoint (arc) method solves this by using the average of the two values as the base, giving the same elasticity regardless of direction. The simplified midpoint formula (after canceling the 1/2 term) is:
Exam tip: If the prompt explicitly asks for the midpoint method, you must use the average base, not simple initial-value percentage change. This is one of the most common point deductions on PED calculation questions.
3. Elasticity Categories and the Total Revenue Test★★★☆☆⏱ 4 min
Once you calculate $|E_d|$, you can categorize demand by responsiveness, and this classification directly tells you how a price change affects total revenue ($TR = P \times Q$). The five standard categories are:
Perfectly inelastic: $|E_d| = 0$, vertical demand curve, quantity does not change with price
Inelastic: $0 < |E_d| < 1$, quantity changes less than proportionally to price
Unit elastic: $|E_d| = 1$, quantity changes proportionally to price
Elastic: $1 < |E_d| < \infty$, quantity changes more than proportionally to price
The total revenue test is a shortcut to determine elasticity without calculation: it uses the direction of TR change when price changes to infer elasticity. The rule: if price and TR move in opposite directions, demand is elastic; if they move in the same direction, demand is inelastic; if TR does not change, demand is unit elastic.
4. Determinants of Price Elasticity of Demand★★☆☆☆⏱ 3 min
The AP exam regularly tests your ability to predict whether demand for a good will be elastic or inelastic based on its core characteristics. There are four key determinants consistently tested:
**Availability of close substitutes**: More close substitutes = more elastic demand. Narrow, specific goods have more substitutes than broad categories.
**Necessity vs. Luxury**: Necessities have inelastic demand; luxuries have elastic demand.
**Time horizon**: Demand is more elastic in the long run than the short run, as consumers have more time to adjust behavior and find substitutes.
**Share of consumer budget**: Goods that make up a smaller share of total consumer budget have more inelastic demand.
Exam tip: When answering AP MCQ about elasticity differences, always match your reasoning to one of the four core determinants above, aligned with the CED framework.
5. Point Elasticity and Elasticity Along Linear Demand Curves★★★☆☆⏱ 4 min
Point elasticity measures elasticity at a single point on the demand curve, rather than over an arc between two points. It is most commonly used to analyze how elasticity changes along a linear (straight-line) demand curve. The point elasticity formula is:
E_d = \frac{dQ}{dP} \times \frac{P}{Q}
A key commonly tested relationship: even though the slope of a linear demand curve is constant, elasticity is *not*. As you move down the demand curve to lower price and higher quantity, $|E_d|$ decreases. At the midpoint of the linear demand curve, $|E_d| = 1$ (unit elastic). Above the midpoint (higher price, lower quantity), $|E_d| > 1$ (elastic). Below the midpoint (lower price, higher quantity), $|E_d| < 1$ (inelastic).
Common Pitfalls
Why: Students remember the basic $\%\Delta Q/\%\Delta P$ formula and forget midpoint requires an average base.
Why: Students forget the negative sign only reflects the downward-sloping demand curve, and classification uses absolute value.
Why: Students confuse slope (absolute change) with elasticity (proportional change).
Why: Students misapply the substitute availability determinant: broad categories have fewer substitutes than specific, narrow goods.
Why: Students forget the TR-elasticity relationship only applies to own-price changes that cause movement along the demand curve.