The AD-AS Model: Aggregate Demand and Aggregate Supply Explained

The AD-AS model is the core framework AP Macroeconomics uses to explain output, unemployment, and the price level in a whole economy. It plots three curves on a graph with the price level on the vertical axis and real GDP on the horizontal axis: downward-sloping aggregate demand (AD), upward-sloping short-run aggregate supply (SRAS), and vertical long-run aggregate supply (LRAS). Where these curves cross determines the economy's equilibrium output and price level, and shifting them models recessions, inflation, and the effects of fiscal and monetary policy. This guide breaks down each curve, why AD slopes down, what shifts each curve, and the difference between short-run and long-run equilibrium. You can build and shift every curve yourself in the interactive AD-AS sandbox.

What aggregate demand measures

Aggregate demand is the total quantity of real output (real GDP) that households, firms, the government, and foreign buyers want to purchase at each price level. It is the economy-wide version of a demand curve, and it is built from the same four components as GDP itself, summarized as C + I + G + Xn.

• C, consumption is spending by households on goods and services. It is the largest component of GDP.
• I, investment is business spending on capital such as machinery, factories, and inventories, plus new residential construction.
• G, government spending is federal, state, and local purchases of goods and services (transfer payments like Social Security are excluded).
• Xn, net exports equals exports minus imports. It can be negative when a country imports more than it exports.

Because AD is the sum of these four, anything that changes one of them changes total demand in the economy. That is the foundation for the AD shifters below.

Why the AD curve slopes downward

The AD curve slopes down for three specific reasons, and AP graders expect you to name and explain all three. Note that these explain movement along the curve as the price level changes, not shifts of the whole curve. Do not confuse them with the ordinary "substitution and income" reasoning behind a single-market demand curve, because that logic does not apply at the economy-wide level.

• The wealth effect (real balances effect): When the overall price level rises, the purchasing power of money that people hold in cash and savings falls. Feeling poorer, households cut back consumption, so quantity of real GDP demanded drops. A falling price level does the reverse.
• The interest rate effect: A higher price level means people need more money to make the same purchases, so money demand rises. Higher money demand pushes up interest rates, which discourages interest-sensitive investment and consumption (cars, homes, business capital), reducing quantity demanded.
• The net export effect (exchange rate effect): When the domestic price level rises, home-produced goods become relatively more expensive than foreign goods. Exports fall and imports rise, so net exports and quantity of real GDP demanded decline.

Together these three effects give AD its negative slope. You can see each one described alongside the live curve in the AD-AS graph walkthrough.

What shifts aggregate demand

An AD shift happens when C, I, G, or Xn changes for a reason other than the price level. An increase in any component shifts AD right (more output demanded at every price level); a decrease shifts AD left. Common determinants of AD include:

• Consumer confidence and wealth: optimism, rising home or stock values, and lower taxes raise consumption and shift AD right.
• Business expectations and interest rates: expected profits and lower real interest rates raise investment and shift AD right.
• Government fiscal policy: more government spending or tax cuts shift AD right; spending cuts or tax hikes shift it left.
• Foreign income and exchange rates: stronger foreign economies or a weaker domestic currency raise net exports and shift AD right.
• The money supply: expansionary monetary policy lowers interest rates and shifts AD right.

Because spending changes ripple through the economy, an initial change in spending shifts AD by a multiplied amount. You can size that effect with the spending multiplier calculator and the tax multiplier calculator.

Short-run aggregate supply (SRAS)

Short-run aggregate supply shows the total output firms produce at each price level when input prices, especially nominal wages, are "sticky" and slow to adjust. SRAS slopes upward: a higher price level lets firms sell output for more while their costs stay temporarily fixed, so profit margins widen and they produce more. In the short run, output can therefore rise above or fall below its full-employment level.

SRAS shifts when the per-unit cost of production changes for reasons other than the price level. Determinants of SRAS include:

• Nominal wages and resource (input) prices: cheaper oil, materials, or labor lowers costs and shifts SRAS right; higher costs shift it left. A jump in a key commodity like oil is the classic "supply shock" that shifts SRAS left.
• Productivity: better technology or more efficient workers lower per-unit cost and shift SRAS right.
• Business taxes, subsidies, and regulation: subsidies and deregulation lower costs and shift SRAS right; new taxes or costly regulations shift it left.
• Inflation expectations: if workers and firms expect higher inflation, they build higher wages into contracts, raising costs and shifting SRAS left.

Long-run aggregate supply (LRAS) and full employment

Long-run aggregate supply is a vertical line at the economy's full-employment output (also called potential output or Yf). It is vertical because, in the long run, all prices and wages are flexible, so the price level does not affect how much an economy can actually produce. Output in the long run depends only on real factors: the quantity and quality of resources, the capital stock, and technology.

The output level at LRAS corresponds to the natural rate of unemployment, the rate that exists when the economy uses its resources fully and only frictional and structural unemployment remain (no cyclical unemployment). LRAS shifts only when the economy's productive capacity changes:

• More or better resources: population growth, immigration, more capital, or a more skilled workforce shift LRAS right.
• Technological progress: innovation that raises what the economy can produce shifts LRAS right.
• Loss of resources: a natural disaster or shrinking workforce shifts LRAS left.

An AD shift or an SRAS shift does not move LRAS. That distinction is the key to long-run analysis. Explore all three curves together in the AD-AS sandbox or review the full unit on the macro hub.

Short-run vs long-run equilibrium

Short-run equilibrium is where AD crosses SRAS. Long-run equilibrium is the special case where AD, SRAS, and LRAS all intersect at the same point, meaning the economy produces exactly at full employment. When short-run output differs from LRAS, the economy has an output gap that the economy self-corrects over time.

A recessionary gap exists when short-run output is below full employment (AD or SRAS shifted left), so unemployment is above the natural rate. An inflationary gap exists when short-run output is above full employment, so unemployment is below the natural rate and inflation pressure builds.

How the economy self-corrects

Left alone, the economy returns to full employment through wage adjustment, though slowly. In a recessionary gap, high unemployment pushes nominal wages down, lowering firms' costs; SRAS shifts right until output returns to LRAS at a lower price level. In an inflationary gap, tight labor markets push wages up, raising costs; SRAS shifts left until output falls back to LRAS at a higher price level. This self-correction is a core AP topic because it explains why active policy is sometimes used to close gaps faster than wages would on their own.

Once you understand the mechanics, the model connects to almost every other Macro unit. Fiscal and monetary policy work by shifting AD, supply shocks shift SRAS, and long-run growth shifts LRAS. For fast, structured review, use the macro hub, look up any term in the glossary, and drill graph technique in the graph walkthroughs before test day.