AP · Equilibrium in the AD-AS Model · 14 min read · Updated 2026-05-10

Equilibrium in the AD-AS Model — AP Macroeconomics Study Guide

For: AP Macroeconomics candidates sitting AP Macroeconomics.

Covers: Short-run macroeconomic equilibrium, long-run macroeconomic equilibrium, full-employment output, recessionary and inflationary output gaps, calculation of equilibrium output and price level, and analysis of demand and supply shocks.

You should already know: Aggregate demand (AD) curve and its shifters, Short-run and long-run aggregate supply (SRAS/LRAS) curve definitions, The natural rate of unemployment.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Macroeconomics style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. What Is Equilibrium in the AD-AS Model?

Equilibrium in the AD-AS model occurs at the intersection of the aggregate demand (AD) curve and the relevant aggregate supply curve. This intersection gives the economy’s equilibrium price level (PL) and equilibrium real output (real GDP, Y). Equilibrium means the total quantity of goods and services demanded in the economy equals the total quantity supplied, so there is no unintended inventory accumulation or depletion, and no inherent upward or downward pressure on the overall price level. Synonyms commonly used include "macroeconomic equilibrium" and "general equilibrium," though the AP exam exclusively uses "macroeconomic equilibrium."

When the economy is out of equilibrium, market forces automatically push it back to equilibrium: if output is above equilibrium, unsold goods pile up, leading firms to cut production and prices; if output is below equilibrium, excess demand leads firms to increase production and raise prices. According to the AP Macroeconomics CED, this topic accounts for 10-15% of Unit 3’s weighting, and it is tested on both multiple-choice (MCQ) and free-response (FRQ) sections of the exam. Mastery of equilibrium conditions is required for nearly all AD-AS related questions on the exam.

2. Short-Run Macroeconomic Equilibrium

Short-run equilibrium occurs at the intersection of the AD curve and the upward-sloping short-run aggregate supply (SRAS) curve. In the short run, nominal wages and input prices are sticky (slow to adjust to changing economic conditions), so firms respond to higher price levels by increasing output, resulting in the upward slope of SRAS. The core mathematical equilibrium condition is: $A D (Y) = S R A S (Y)$ Where $A D (Y)$ is the price level at which aggregate demand equals output $Y$ , and $S R A S (Y)$ is the price level at which short-run aggregate supply equals output $Y$ . Solving this condition gives two equilibrium values: $P L^{*}$ (equilibrium price level) and $Y^{*}$ (equilibrium short-run real GDP). Importantly, short-run equilibrium output does not need to equal full-employment output ( $Y_{F}$ , the output at which LRAS is vertical); it can fall above, below, or exactly at $Y_{F}$ .

Worked Example

Suppose an economy has aggregate demand given by $P L = 150 - 0.5 Y$ , and short-run aggregate supply given by $P L = 30 + 0.25 Y$ , where $P L$ is the price level and $Y$ is real GDP in billions of dollars. Calculate the equilibrium short-run output and price level.

Apply the short-run equilibrium condition: set AD equal to SRAS: $150 - 0.5 Y = 30 + 0.25 Y$ .
Rearrange terms to isolate $Y$ : $150 - 30 = 0.25 Y + 0.5 Y$ , so $120 = 0.75 Y$ .
Solve for $Y$ : $Y = 120/0.75 = 160$ billion dollars.
Plug $Y$ back into the SRAS equation to find $P L$ : $P L = 30 + 0.25 (160) = 30 + 40 = 70$ .
Verify consistency with the AD equation: $150 - 0.5 (160) = 150 - 80 = 70$ , which matches.

Final equilibrium: $Y^{*} = 160$ billion, $P L^{*} = 70$ .

Exam tip: On FRQs, always explicitly label both equilibrium output $Y^{*}$ and equilibrium price level $P L^{*}$ on your graph. AP exam rubrics almost always award 1 full point for correctly labeling both values, and missing one label will cost you a free point.

3. Long-Run Macroeconomic Equilibrium

Long-run equilibrium occurs when short-run equilibrium output equals full-employment (potential) output $Y_{F}$ , meaning AD, SRAS, and LRAS all intersect at the same point. LRAS is vertical at $Y_{F}$ because in the long run, all nominal wages and input prices are fully flexible, so changes in the price level do not change the economy’s maximum sustainable output.

The long-run equilibrium condition adds a requirement that $Y^{*} = Y_{F}$ , so: $A D (Y_{F}) = S R A S (Y_{F}) = P L^{*}$ At long-run equilibrium, the economy is at full employment: the unemployment rate equals the natural rate of unemployment, there are no output gaps, and the economy is operating at its potential output. If short-run equilibrium is not at $Y_{F}$ , the economy is in a gap, and will automatically adjust back to long-run equilibrium over time.

Worked Example

Suppose the economy from the previous worked example has a full-employment output $Y_{F} = 200$ billion. Is the economy in long-run equilibrium? If not, what type of output gap exists, and what is the size of the gap?

Recall we found short-run equilibrium output $Y^{*} = 160$ billion.
Compare $Y^{*}$ to $Y_{F}$ : $160 < 200$ , so $Y^{*} \neq = Y_{F}$ , so the economy is not in long-run equilibrium.
By definition, when short-run equilibrium output is less than full-employment output, the gap is a recessionary output gap.
Calculate the gap size: $Gap = Y_{F} - Y^{*} = 200 - 160 = 40$ billion, so the recessionary gap is $40 billion.

Exam tip: Always remember that LRAS is vertical at $Y_{F}$ , so long-run equilibrium requires all three curves to intersect at the same output level. A common exam mistake is only checking for intersection of AD and LRAS, but SRAS must also intersect at that point for the economy to be in long-run equilibrium.

4. Output Gaps and Automatic Long-Run Adjustment

An output gap is the difference between short-run equilibrium output $Y^{*}$ and full-employment output $Y_{F}$ . There are two distinct types: (1) Recessionary (contractionary) gap: $Y^{*} < Y_{F}$ , where unemployment is above the natural rate, and there is slack in the economy. (2) Inflationary (expansionary) gap: $Y^{*} > Y_{F}$ , where unemployment is below the natural rate, and the economy is overheating.

In the absence of government or central bank intervention, the economy automatically adjusts back to long-run equilibrium through shifts in SRAS driven by changes in nominal wages and input prices:

For a recessionary gap: High unemployment leads workers to accept lower nominal wages, and input prices fall. Lower production costs shift SRAS right, lowering the price level and increasing output until $Y^{*} = Y_{F}$ .
For an inflationary gap: Low unemployment leads workers to demand higher nominal wages, and input prices rise. Higher production costs shift SRAS left, raising the price level and decreasing output until $Y^{*} = Y_{F}$ .

This automatic (self-correcting) adjustment mechanism is frequently tested on both MCQ and FRQ sections of the AP exam.

Worked Example

Suppose an economy initially in long-run equilibrium experiences a positive AD shock (e.g., a surge in consumer confidence that increases consumption spending) that shifts AD right. Describe the automatic adjustment back to long-run equilibrium step-by-step.

Initial long-run equilibrium: $A D_{1}$ intersects $S R A S_{1}$ and $L R A S$ at $Y_{F}$ , with initial price level $P L_{1}$ .
The positive AD shock shifts AD right to $A D_{2}$ . The new short-run equilibrium is at the intersection of $A D_{2}$ and $S R A S_{1}$ , with new output $Y_{2} > Y_{F}$ and new price level $P L_{2} > P L_{1}$ . This creates an inflationary gap of $Y_{2} - Y_{F}$ .
Because unemployment is below the natural rate, workers negotiate higher nominal wages in new long-term contracts, and other input prices rise to reflect the higher price level.
Higher production costs for firms shift SRAS left from $S R A S_{1}$ to $S R A S_{2}$ .
The new long-run equilibrium forms at the intersection of $A D_{2}$ , $S R A S_{2}$ , and $L R A S$ . Output returns to $Y_{F}$ , and the price level rises to $P L_{3} > P L_{2}$ . The inflationary gap is eliminated, and long-run equilibrium is restored.

Exam tip: When the question asks for automatic (self-correcting) adjustment without policy intervention, always shift SRAS, not AD or LRAS. Automatic adjustment works through input price changes that shift SRAS, not through changes in potential output or aggregate demand.

5. Common Pitfalls (and how to avoid them)

Wrong move: Calling a gap where $Y^{*} > Y_{F}$ a recessionary gap, and $Y^{*} < Y_{F}$ an inflationary gap. Why: Students mix up gap names with direction, confusing the label with the policy action needed to close the gap. Correct move: Memorize the rule: the name describes the state of the economy relative to full employment. If output is lower than full employment, the economy is in a recession, so it is a recessionary gap. Write this rule on your scratch paper for reference during the exam.
Wrong move: Shifting AD instead of SRAS during automatic long-run adjustment. Why: Students confuse automatic self-correction with fiscal/monetary policy, which shifts AD. Correct move: If the question explicitly says "no policy intervention" or "automatic adjustment," always shift SRAS to close the gap. Only shift AD if the question asks for government or central bank policy.
Wrong move: Claiming long-run equilibrium occurs at the intersection of only AD and LRAS, with no requirement that SRAS intersects at that point. Why: Students forget SRAS is always relevant even in the long run, and short-run equilibrium is required for the economy to be at rest. Correct move: Always confirm all three curves intersect at $Y_{F}$ when identifying long-run equilibrium, and draw SRAS through the intersection point on FRQ graphs.
Wrong move: Shifting LRAS during automatic adjustment to a demand-side output gap. Why: Students confuse changes in potential output (which shifts LRAS) with the self-correction mechanism, which does not change potential output. Correct move: LRAS only shifts when there is a change in productivity, labor force, capital stock, or a permanent supply shock. It never shifts during automatic adjustment to a demand shock.
Wrong move: Failing to verify equilibrium calculations by plugging $Y^{*}$ into both AD and SRAS. Why: Students rush through algebra and make simple arithmetic errors that are easy to catch. Correct move: Always plug your solved $Y^{*}$ into both equations to confirm you get the same $P L^{*}$ ; this catches 90% of common calculation mistakes.

6. Practice Questions (AP Macroeconomics Style)

Question 1 (Multiple Choice)

An economy has AD given by $P L = 200 - Y$ , SRAS given by $P L = 50 + 0.5 Y$ , and LRAS at $Y_{F} = 120$ . Which of the following correctly describes the economy's current equilibrium? A) Short-run equilibrium at Y = 100, PL = 100, with a $20 billion recessionary gap B) Short-run equilibrium at Y = 120, PL = 80, with no output gap C) Short-run equilibrium at Y = 100, PL = 100, with a $20 billion inflationary gap D) Short-run equilibrium at Y = 140, PL = 60, with a $20 billion inflationary gap

Worked Solution: Apply the short-run equilibrium condition by setting AD equal to SRAS: $200 - Y = 50 + 0.5 Y$ . Rearranging terms gives $150 = 1.5 Y$ , so $Y = 100$ . Plugging $Y = 100$ back into the AD equation gives $P L = 200 - 100 = 100$ . Next, compare short-run output $Y = 100$ to full-employment output $Y_{F} = 120$ . Since $100 < 120$ , the gap is $120 - 100 = 20$ billion recessionary gap, which matches option A. Correct answer: A.

Question 2 (Free Response)

An economy is initially in long-run equilibrium. A sudden decrease in global oil prices (a positive temporary supply shock) shifts the short-run aggregate supply curve to the right. (a) Draw a correctly labeled AD-AS graph showing the initial long-run equilibrium, and the new short-run equilibrium after the supply shock. Label the initial output $Y_{1}$ and price level $P L_{1}$ , and the new short-run output $Y_{2}$ and price level $P L_{2}$ . Identify what type of output gap exists after the shock. (b) Explain how the economy will automatically adjust back to long-run equilibrium in the absence of policy intervention. What happens to output and the price level during this adjustment? (c) Suppose policymakers want to close the output gap using aggregate demand policy after the shock. Should they shift AD right or left? What is the resulting long-run price level compared to the original long-run equilibrium price level?

Worked Solution: (a) The correctly labeled graph has axes "Price Level (PL)" and "Real GDP (Y)", with a downward-sloping AD, upward-sloping SRAS, and vertical LRAS at $Y_{1} = Y_{F}$ . Initial equilibrium is at the intersection of all three curves at $(Y_{1}, P L_{1})$ . After SRAS shifts right to $S R A S_{2}$ , the new short-run equilibrium is at the intersection of AD and $S R A S_{2}$ , with $Y_{2} > Y_{1}$ and $P L_{2} < P L_{1}$ . Since $Y_{2} > Y_{F}$ , the economy has an inflationary output gap. (b) The inflationary gap means unemployment is below the natural rate. Over time, nominal wages and other input prices adjust upward to reflect tight labor markets and increased purchasing power from lower prices. Higher input prices increase firm production costs, shifting SRAS back left to its original position. During adjustment, output decreases back to $Y_{F}$ , and the price level rises back to the original $P L_{1}$ . (c) To close the inflationary gap, policymakers need to reduce aggregate demand, so they shift AD left. After the shift, the new long-run equilibrium is at $Y_{F}$ , with a price level equal to $P L_{2}$ (the lower short-run price after the original supply shock), which is lower than the original long-run equilibrium price level $P L_{1}$ .

Question 3 (Application / Real-World Style)

In 2021, an economy experienced a sharp increase in aggregate demand from stimulus payments and expansionary monetary policy. Potential output (full-employment output) for the year was estimated at $22 t r i l l i o n, an d s h or t - r u n e q u i l ib r i u m o u tp u t a f t er t h e d e man d in cr e a se w a s$ 22.6 trillion. What type of output gap exists here, what is its size, and what would you expect to happen to SRAS and the price level in the long run without policy intervention, per the AD-AS equilibrium model?

Worked Solution: First compare short-run equilibrium output $Y^{*} = 22.6$ trillion to full-employment output $Y_{F} = 22$ trillion. Since $Y^{*} > Y_{F}$ , this is an inflationary output gap with a size of $Y^{*} - Y_{F} = 0.6$ trillion (or $600 bi l l i o n) . W i t h o u tp o l i cy in t er v e n t i o n, t i g h tl ab or ma r k e t s an d r i s in g in p u tp r i ces w i l l s hi f tt h e S R A S c u r v e t o t h e l e f t . T hi ss hi f t in cr e a ses t h eo v er a l l p r i ce l e v e l an d r e d u ceso u tp u t ba c k t o t h e p o t e n t ia l f u l l - e m pl oy m e n tl e v e l o f$ 22 trillion. In context, the AD-AS model predicts that the post-shock demand surge would lead to higher long-run inflation and a return to full employment output without ongoing expansionary policy.

7. Quick Reference Cheatsheet

Category	Formula / Rule	Notes
Short-run equilibrium condition	$A D (Y) = S R A S (Y)$	Gives equilibrium $Y^{}$ and $P L^{}$ ; $Y^{*}$ can be above, below, or equal to $Y_{F}$
Recessionary gap	$Gap = Y_{F} - Y^{}$ , $Y^{} < Y_{F}$	Unemployment above natural rate; downward pressure on wages and input prices
Inflationary gap	$Gap = Y^{} - Y_{F}$ , $Y^{} > Y_{F}$	Unemployment below natural rate; upward pressure on wages and input prices
Long-run equilibrium condition	$Y^{} = Y_{F}$ , $A D (Y_{F}) = S R A S (Y_{F}) = P L^{}$	All three curves (AD, SRAS, LRAS) intersect at the same point
Automatic adjustment: recessionary gap	Shift SRAS right	Lower input prices shift SRAS; closes gap without changing $Y_{F}$
Automatic adjustment: inflationary gap	Shift SRAS left	Higher input prices shift SRAS; closes gap without changing $Y_{F}$
Long-run equilibrium output	$Y = Y_{F}$	Potential output; no output gap; unemployment equals natural rate
Out-of-equilibrium adjustment	Excess supply → cut output/prices; excess demand → raise output/prices	Driven by unintended inventory changes that push the economy back to equilibrium

8. What's Next

Equilibrium in the AD-AS model is the foundational framework for analyzing all macroeconomic shocks and policy effects in AP Macroeconomics, and it is a required prerequisite for the next major topics in the syllabus. Next, you will apply this equilibrium framework to analyze how fiscal and monetary policy shift aggregate demand to close output gaps, a topic that makes up more than 20% of total AP exam points. Without mastering how equilibrium changes after a curve shift, you will not be able to correctly answer FRQs that ask for the effect of policy on output, unemployment, and the price level. This topic also feeds into broader core concepts across the rest of the course, including the Phillips curve and long-run economic growth.

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Equilibrium in the AD-AS Model — AP Macroeconomics Study Guide

1. What Is Equilibrium in the AD-AS Model?

2. Short-Run Macroeconomic Equilibrium

Worked Example

3. Long-Run Macroeconomic Equilibrium

Worked Example

4. Output Gaps and Automatic Long-Run Adjustment

Worked Example

5. Common Pitfalls (and how to avoid them)

6. Practice Questions (AP Macroeconomics Style)

Question 1 (Multiple Choice)

Question 2 (Free Response)

Question 3 (Application / Real-World Style)

7. Quick Reference Cheatsheet

8. What's Next

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