Aggregate Demand vs Gross Domestic Product (GDP)

The same four components, but one is a schedule and the other is a total

Aggregate demand and GDP are both built from consumption, investment, government spending and net exports, which is why students treat them as one idea. They are not. Aggregate demand is a schedule. It answers a conditional question: if the price level were 90, or 100, or 110, how much real output would buyers want to purchase? Every price level gets its own answer, and plotting those answers gives the downward sloping AD curve. GDP is a single measured total for a period that has already happened, and it records what was produced rather than what would have been bought under other conditions. The two meet at one point. Real GDP settles where AD crosses aggregate supply, so a single point on the schedule becomes the period's output while every other point stays hypothetical. That is also why a rightward shift of AD does not raise real GDP by the full width of the shift. When short-run aggregate supply slopes upward, the higher price level walks part of the shift back along the new curve, and measured output rises by less than the horizontal distance you drew. The curve and its determinants are set out at /macro/aggregate-demand.

Measured GDP counts goods that nobody chose to buy

National accounts close the gap between planned spending and production with inventory investment. Anything a firm makes and fails to sell is treated as a purchase by the firm itself, so recorded spending always equals recorded output. Take an illustrative economy that produces 900 billion dollars of final goods in a period while households, firms, government and foreign buyers together choose to buy 860 billion dollars. The unsold 40 billion dollars is booked as inventory investment inside the investment term, so measured GDP still comes to 900 billion dollars even though planned purchases were 40 billion dollars lower. Nothing in the accounts is wrong, because the identity holds by construction. What the identity hides is the pressure building on firms. Warehouses are fuller than intended, so the next production decision is to cut output and usually to cut hours or hiring. That unplanned inventory build is how a fall in aggregate demand turns into a fall in real GDP one step later. The reverse runs the same way: if buyers want 940 billion dollars at that price level, firms draw inventories down by 40 billion dollars and then raise production. Aggregate demand is the plan, GDP is the record, and inventories absorb the difference until output catches up. The accounting rules are at /calculate/gdp.

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