What is GDP?
GDP (Gross Domestic Product) is the central indicator of national accounting. It measures the monetary value of all final goods and services produced within a country's borders over a given period. Mankiw presents it as the most closely watched economic statistic in the world, as it summarizes an entire nation's economic activity in a single figure.
Last updated: 31 July 2026
Definition
DefinitionGross Domestic Product (GDP) represents the market value of all final goods and services produced in an economy over a given period (typically a quarter or a year). Each term of this definition matters: "market value" means goods are valued at their market prices; "final" excludes intermediate goods to avoid double counting; "produced" means only current output is counted, not the resale of second-hand goods. One nuance: the non-market output of public administrations (education, security, justice…) has no market price — national accounting values it at its cost of production.
GDP is broken down using the expenditure approach:
GDP = C + I + G + (X − M)
C = Household consumption: all household spending on goods and services (food, housing, leisure, healthcare). It is the largest component, representing slightly more than half of GDP in France (around 53%).
I = Investment: it comprises Gross Fixed Capital Formation (GFCF) — purchases of capital goods by firms (machinery, buildings), residential construction by households — as well as changes in inventories. Investment represents the economy's future productive capacity.
G = Government spending: purchases of goods and services by national and local governments (civil servant salaries, public equipment, infrastructure). Social transfers (benefits, pensions) are not counted here as they do not correspond to production.
(X − M) = Net exports: the difference between exports (goods and services sold abroad) and imports (goods and services purchased from abroad). A positive balance means the country exports more than it imports.
Origins of GDP accounting
Why did humanity start calculating GDP? Until the early 1930s, no government had an overall measure of economic activity: policy was steered using partial indicators (steel output, rail traffic, stock prices). The Great Depression changed everything. To find out how far the American economy had actually collapsed, the United States Congress commissioned an estimate of national income from the economist Simon Kuznets. His report "National Income, 1929-1932" (published in 1934) revealed that U.S. national income had roughly halved in three years. A figure that had simply been unknowable before.
The Second World War provided the second impetus: to plan the war effort without strangling civilian consumption, governments needed to know how much production could be mobilised. Following Keynes's essay "How to Pay for the War" (1940), Richard Stone and James Meade built Britain's national accounts. After the war, reconstruction, the Marshall Plan and the Bretton Woods institutions demanded figures comparable across countries. The United Nations standardised the method in the System of National Accounts (1953), developed under Stone's direction. Kuznets (1971) and then Stone (1984) each received the Nobel Prize in economics for this work.
The founding irony is worth remembering: as early as 1934, Kuznets warned that "the welfare of a nation can scarcely be inferred from a measurement of national income." Born to measure a crisis and then to administer a war, GDP never claimed to measure happiness.
Why it matters
Mankiw compares GDP to an annual health check-up for a patient: it does not tell everything, but it provides an indispensable overview. GDP makes it possible to measure the size of an economy, to assess its growth rate, and to compare performance across countries or periods. Governments use it to calibrate their economic policies, central banks to adjust interest rates, and investors to guide their asset allocation decisions.
The real GDP growth rate is calculated as: g = (RealGDP_t − RealGDP_t-1) / RealGDP_t-1 × 100.
However, Mankiw has highlighted the limitations of GDP since his earliest editions. It does not account for unpaid domestic work, volunteer activities, the underground economy, environmental quality, or income distribution. A country may display a high GDP per capita while exhibiting severe social inequalities.
Key points
The three approaches to GDP yield identical results: the production approach (sum of value added), the expenditure approach (C + I + G + X − M), and the income approach (sum of labor and capital compensation)
Real GDP is the preferred measure for assessing growth, as it neutralizes the effect of inflation on the figures
A GDP growth rate of 2–3% per year is generally considered satisfactory for a developed economy
The Rule of 70: to estimate the time needed for GDP to double, divide 70 by the annual growth rate. At 2% growth, GDP doubles in 35 years
Mind the reporting convention: in the United States, quarterly GDP changes are annualized (+0.5% in Q2 is reported as +2% at an annualized rate); in Europe, INSEE and Eurostat publish the raw quarter-on-quarter change (the same quarter is reported as +0.5%)
Concrete example
ExamplesIn 2025, France's GDP stood at €2,991.1 billion. Its expenditure breakdown: household consumption €1,545.9 bn (51.7%), investment €663.6 bn (22.2%), government spending €719.6 bn (24.1%), net exports −€14.4 bn (−0.5%).
CAREFUL — these four items do NOT add up to GDP: they total €2,914.7 bn, €76.4 bn short. Two lines are missing that the formula C + I + G + (X − M) never names: consumption by non-profit institutions serving households (associations, unions, religious bodies) and the change in inventories. That is also why the shares above sum to 97.5% and not 100%: the remaining 2.6% is exactly those two items. A breakdown that lands neatly on 100% is a sign that something has been rounded away.
On the same basis, if real GDP reaches €3,021 billion in 2026 (in 2025 euros), real growth is 1.0%. If nominal GDP reaches €3,051 billion, the gap between 2.0% nominal growth and that 1.0% real growth measures inflation — about 1.0%.
Mankiw anecdote
MankiwMankiw devotes a recurring box to Robert Kennedy, who declared in 1968 — speaking of GNP, the headline indicator in the United States at the time — that it "measures everything except that which makes life worthwhile." Mankiw uses this quote to introduce complementary indicators such as the HDI (Human Development Index) from the UNDP, which incorporates life expectancy and education levels alongside income.
Further reading
ProgressionThis sheet is a reference: it sets the definitions, gives the orders of magnitude and goes as far as potential output. The course of the same name does something else — it builds the formula step by step, lets you manipulate it in a simulator and tests it on eight worked exercises. The two complement each other: the sheet to revise fast, the course to understand where each term comes from.
Market impact
MarketsThe quarterly GDP release is a major event for financial markets. GDP above expectations generally strengthens the national currency and supports stock indices, as it signals a dynamic economy. Conversely, GDP below forecasts—and especially two consecutive quarters of negative growth (technical recession)—triggers sell-offs in equity markets and a flight to safe-haven assets — the investments considered safest in times of stress — such as sovereign bonds (the debt of major States) and gold.