Aggregate supply
Aggregate supply is the total output of goods and services that firms are willing to produce at a given price level.
Demand is only half the story. What an economy can actually produce, aggregate supply, sets the limits within which demand plays out.
Aggregate supply is the total quantity of goods and services that firms in an economy are willing and able to produce at a given overall price level. Paired with aggregate demand, it forms the basic framework for understanding the economy's output, employment, and inflation.
Short run versus long run
Aggregate supply behaves differently over different horizons. In the short run, it slopes upward: higher prices, with input costs slow to adjust, make production more profitable, so firms supply more. In the long run, supply is determined not by the price level but by the economy's productive capacity, its labour, capital, technology, and institutions, and is independent of prices. This distinction is central: demand can move output around in the short run, but long-run output is set by capacity.
What shifts it
Long-run aggregate supply, the economy's potential output, grows with more or better labour and capital, improved technology, and stronger institutions, the same forces that drive economic growth. It can also be shocked: a sudden jump in the price of a vital input, an oil shock, shifts short-run supply adversely, raising prices while cutting output, which is the recipe for stagflation. Supply, in other words, is where the economy's capacity and its cost shocks register.
Why the supply side matters
The split between demand and supply maps onto a deep divide in economic thinking and policy. Demand-side management can stabilise the economy around its capacity but cannot raise that capacity. Lifting long-run living standards requires supply-side improvements, to productivity, skills, investment, and institutions, that expand what the economy can produce. Confusing the two, expecting demand stimulus to deliver lasting growth, or supply reform to provide an instant boost, leads to disappointed expectations.
Aggregate supply completes the picture of the economy as a whole. It is the reminder that demand operates against a ceiling set by capacity, that the ceiling itself can be raised only slowly and by different means, and that supply shocks can confront policymakers with the cruellest trade-offs they face.