Every macroeconomic headline you have ever read — inflation up, growth slowing, a central bank cutting rates — is a claim about one diagram. The AD–AS model puts total spending and total production on the same axes, and it is the frame your first-year course hangs everything else on. This page builds the model properly; it is also the place to start if you are looking for a macroeconomics tutor who works at university level.
1 · Aggregate demand is not a big demand curve
Start with the most-tested misconception in intro macro. The AD curve looks like the microeconomic demand curve scaled up. It is not, and examiners set questions to catch students who treat it as one.
Aggregate demand is total planned spending on the economy’s output at each price level P: consumption, investment, government purchases and net exports. When the price of coffee rises, you switch to tea. But P here is the price of everything at once — there is nothing left to switch to. Substitution cannot explain the downward slope, so something else must.
Three effects do the work. First, the wealth effect: a higher price level shrinks the real value of the money you hold, and households that feel poorer spend less. Second, the interest-rate effect: a higher P raises the demand for money, pushes interest rates up, and squeezes investment. Third, the net-export effect: when domestic prices rise, exports lose ground abroad and imports gain at home. All three push spending down as P rises. That is the slope. Have the list — wealth, interest rate, net exports — cold in the exam.
What shifts AD? Anything that changes spending at a given price level: government purchases, taxes, interest rates, confidence, foreign incomes. A change in P never shifts the curve. It moves you along it.
2 · Two supply curves, because two horizons
The supply side of the model needs two curves, and the distinction between them carries most of the marks.
The short-run aggregate supply curve slopes upward because wages are sticky. Pay is set by contracts, not repriced by the hour. When the price level rises before wages catch up, the real cost of labour falls and firms expand output. Higher P, higher Y — an upward-sloping SRAS.
The long-run curve is different in kind. Give wages time to adjust fully and the sticky-wage logic dies: firms produce what their labour force, capital stock and technology allow, whatever the price level. LRAS is just an inventory of the economy’s real resources, drawn as a vertical line at potential output Y*. Printing money does not build factories, so no amount of extra spending moves it.
Hold on to the division of labour here. Prices and spending live on the demand side and the short run; real capacity lives on LRAS. Writing that a spending boom “raises potential output” is the error marking schemes punish hardest.
3 · Equilibrium and the output gap
Short-run equilibrium sits where AD crosses SRAS: the one price level at which planned spending equals planned production. At any higher P, unsold output pushes prices down; at any lower P, excess demand bids them up. So far the logic mirrors a single market.
The macro twist is the third curve. Compare short-run output with Y* on LRAS. Below potential, the economy has a recessionary gap: idle workers and idle machines. Above potential — possible for a while through overtime and stretched capacity — it has an inflationary gap. Only when AD and SRAS cross exactly on LRAS is the economy in long-run equilibrium, with no pressure on wages in either direction.
Gaps do not last forever, because wages eventually move. In an inflationary gap, tight labour markets bid wages up, costs rise, and SRAS shifts leftward until output falls back to Y*. In a recessionary gap the same mechanism runs in reverse, slowly. This is the economy’s self-correction, and its speed is the central empirical fight in macroeconomics.
4 · The policy framework — which curve does it shift?
Every macroeconomic policy you will meet slots into this diagram, and the exam skill is classifying each one by the curve it shifts.
Demand-side policy shifts AD. Fiscal policy works through government purchases and taxes; monetary policy works through interest rates, which move investment and consumption. Both can close a gap faster than waiting for wages to adjust — that is the case for stabilisation. Neither moves LRAS. Push AD right at full capacity and the long-run result is a higher price level, not higher output. The worked example below shows that arithmetic exactly.
Supply-side policy shifts LRAS: education, infrastructure, technology, competition reform. It raises potential output itself, but slowly, and it does nothing for a demand shortfall this quarter.
The framework in one sentence: diagnose the gap, decide whether self-correction is fast enough, and match the tool to the curve. Write answers in that order and the marks follow.
Worked example — a £20 billion infrastructure programme
An economy has potential output of £500 billion. Y is real GDP in £ billions; P is the GDP deflator, base value 100.
Step 1 — Aggregate demand. Planned spending is Y = 700 − 2P. At P = 100, demand is 700 − 200 = £500bn. Each 10-point rise in the deflator trims £20bn of spending through the wealth, interest-rate and net-export effects.
Step 2 — Supply. Short-run aggregate supply is Y = 300 + 2P: with wages fixed, each 10-point rise in P draws out £20bn more output. Long-run aggregate supply is vertical at Y* = 500.
Step 3 — Solve the initial equilibrium. Set AD equal to SRAS:
700 − 2P = 300 + 2P → 400 = 4P → P* = 100.
Substitute back: Y = 700 − 2(100) = 500. Check with SRAS: 300 + 2(100) = 500. ✓ Output equals potential, so the economy starts in long-run equilibrium — no gap.
Step 4 — Perturbation. The government launches a £20bn infrastructure programme. With a marginal propensity to consume of 0.75, the multiplier is 1/(1 − 0.75) = 4, so AD shifts right by 4 × 20 = £80bn at every price level: Y = 780 − 2P. At the old P = 100, demand is 780 − 200 = £580bn but firms still supply £500bn — excess demand of £80bn.
Step 5 — Resolve the short run. Solve again: 780 − 2P = 300 + 2P → 480 = 4P → P = 120, and Y = 780 − 2(120) = 540. Check: 300 + 2(120) = 540. ✓ Output sits £40bn above potential — an inflationary gap.
Step 6 — The long run. The gap tightens the labour market and wages rise, shifting SRAS leftward until output returns to potential on the new AD curve. Impose Y = 500: 500 = 780 − 2P gives P = 140. The new short-run supply curve passing through that point is Y = 220 + 2P (check: 220 + 280 = 500 ✓).
Step 7 — Interpretation. The multiplier promised £80bn. The economy delivered £40bn in the short run and nothing in the long run. In the short run the rising price level did two jobs at once: it drew £40bn of extra output along SRAS, and it choked off £40bn of the new demand along AD₂ (580 down to 540) — the whole £80bn shift, accounted for. In the long run wage adjustment handed the rest to prices: the deflator ends 40 points higher, output ends exactly where it began. Demand policy bought a temporary boom and a permanently higher price level. That trade is the model’s central lesson.
Comfortable with each curve but lost when they all move? That is exactly the gap a one-on-one macroeconomics tutor closes in a first session — running your own course’s problem sets until the sequencing becomes automatic. Book a trial session.
Practice
Q1. An economy has AD Y = 900 − 3P, SRAS Y = 300 + 3P, and potential output Y* = 600.
(a) Find the short-run equilibrium price level and output. Is the economy in long-run equilibrium?
(b) A collapse in consumer confidence shifts AD to Y = 840 − 3P. Find the new short-run equilibrium and the size of the output gap.
Q2. An economy has AD Y = 1000 − 4P, SRAS Y = 400 + 2P, and Y* = 600.
(a) Find the short-run equilibrium and confirm it is also the long-run equilibrium.
(b) An export boom raises foreign spending by 18. With a marginal propensity to consume of 2/3, find the new AD curve and the new short-run equilibrium.
(c) Find the price level at the new long-run equilibrium.
Answers: Q1 (a) P = 100, Y = 600 — yes, output equals potential; (b) P = 90, Y = 570, a recessionary gap of 30. Q2 (a) P = 100, Y = 600, on LRAS; (b) multiplier 3, AD shifts by 54 to Y = 1054 − 4P, giving P = 109, Y = 618; (c) impose Y = 600 on the new AD: P = 113.5.
Key takeaways
- AD slopes downward for macro reasons — wealth, interest-rate and net-export effects — not because of substitution between goods.
- SRAS slopes upward because wages are sticky; LRAS is vertical at potential output Y* because real capacity does not depend on the price level.
- Compare short-run output with Y* to name the gap: below potential is recessionary, above is inflationary.
- Demand-side policy shifts AD and can close gaps quickly, but in the long run it moves only the price level. Supply-side policy is the only thing that moves LRAS.
- Solve every equilibrium algebraically, check it in both equations, then narrate the sequence: shift → gap → wage adjustment → long run.
Why students choose our macroeconomics tutoring
- One-on-one format: every session is private and built around your course — your problem sets, your lecture notes, your university’s syllabus and notation.
- University-level specialists: our tutors teach macroeconomics as your department teaches it, from AD–AS foundations through IS–LM and growth models when your course gets there.
- Exam-first preparation: sessions work through past papers with marking schemes in view, because the classify-the-curve discipline in Section 4 is where marks are won.
FAQ
Q: What is the difference between SRAS and LRAS?
A: SRAS slopes upward because wages are sticky in the short run, so a higher price level temporarily makes production more profitable. LRAS is vertical at potential output because once wages fully adjust, output depends only on labour, capital and technology — not on the price level.
Q: Why does the aggregate demand curve slope downward?
A: Three effects: a higher price level reduces the real value of money holdings (wealth effect), raises interest rates and cuts investment (interest-rate effect), and makes exports less competitive (net-export effect). Substitution between goods is not the reason — the price of everything is rising at once.
Q: What is an output gap?
A: The difference between actual output and potential output. Output below potential is a recessionary gap, with idle workers and capacity; output above potential is an inflationary gap, which bids up wages and prices until it closes.
Q: How do you find equilibrium in the AD–AS model?
A: Set the AD equation equal to the SRAS equation, solve for P, then substitute back to get Y and check in both equations. If that Y equals potential output, the economy is also in long-run equilibrium.
Q: Does an increase in government spending raise GDP permanently?
A: In this model, no. It raises output above potential in the short run, but wages then rise, SRAS shifts leftward, and output returns to potential at a higher price level. Only shifts in LRAS — more capital, labour or technology — raise output permanently.
Q: Do I need maths for introductory macroeconomics?
A: Mostly linear equations and diagrams like the ones on this page — solving two equations for equilibrium is the core skill. Calculus arrives at intermediate level with IS–LM and growth theory. If your course is calculus-based, say so when booking and we will match you accordingly.
Book a macroeconomics tutor
Whether you are three weeks from an exam or three weeks into the course, a one-on-one session turns AD–AS from three curves you recognise into a framework you can argue with. Tell us your university and module, and we will match you with the right tutor this week.
