Nominal GDP and real GDP: what the difference actually tells you

Gross domestic product is the total value of the finished goods and services an economy produces in a given period. The complication is the word value, because value is measured in money and money does not hold still. Whether you adjust for that is the entire difference between nominal GDP and real GDP, and it is the difference between a number that can mislead you and one that usually will not.

Two ways of counting the same output

Nominal GDP values everything at the prices actually prevailing in the year being measured. If an economy produced a million widgets at ten units of currency each, nominal GDP records ten million.

Real GDP values the same physical output at the prices of a fixed reference year, called the base year. If prices have risen twenty per cent since that base year, real GDP strips those increases back out, leaving a figure that reflects how much was actually produced rather than what it happened to cost.

The reason this matters is easiest to see in the extreme case. Suppose an economy makes exactly the same quantity of exactly the same things two years running, and all prices rise ten per cent. Nominal GDP rises ten per cent. Real GDP does not move at all — correctly, because nothing more was produced. Anyone reading the nominal figure alone would conclude the economy had grown, when the only thing that grew was the price tag.

Which one the headlines mean

When a statistical agency announces that an economy grew by a certain percentage last quarter, it is almost always quoting real GDP. This is a convention worth knowing, because it silently answers the question most readers would otherwise have to ask.

Nominal figures still have their uses, and they are not merely the naive version. Anything measured as a share of GDP — government debt, tax revenue, a deficit — is normally compared against nominal GDP, because those quantities are themselves expressed in current money. Comparing a debt in today’s currency against output valued in the prices of a base year some distance in the past would mix two different units and produce a ratio that means very little.

The deflator, and why it is a useful number

Divide nominal GDP by real GDP, multiply by a hundred, and you have the GDP deflator. It is a price index derived from the gap between the two measures, and it is one of the broadest available gauges of inflation across an economy.

Its breadth is precisely what distinguishes it from a consumer price index. A CPI tracks a fixed basket of goods and services that a typical household buys, including imports. The deflator covers everything counted in domestic output — machinery, exports, government services, construction — and excludes imports, because those were not produced domestically. The two therefore answer different questions and routinely give different numbers, and neither is the wrong answer to its own question.

What neither figure tells you

Real GDP is a considerable improvement on nominal GDP, and it remains a narrow instrument. It counts output, not distribution: an economy can post solid real growth while most households see nothing of it, because the aggregate says nothing about who received the gains.

It is also indifferent to what the output consists of. Rebuilding after a flood adds to GDP; so does a longer commute that burns more fuel. Unpaid work — care, childcare, housework — is largely excluded, not because it lacks value but because it is not transacted and so is extremely hard to measure consistently. Depletion of natural resources is generally recorded as production rather than as the running-down of an asset.

There is one further trap worth naming, which catches people comparing countries rather than years. Converting each country’s nominal GDP into a common currency at market exchange rates flatters economies with strong currencies and understates those where the same money buys considerably more locally. Adjusting for purchasing power parity corrects for that gap, which is why international comparisons often quote two very different figures for the same country without either being wrong.

None of this makes GDP a bad statistic. It makes it a specific one, answering a specific question: how much was produced, adjusted for prices. Treating it as a measure of national wellbeing asks it to do a job it was never built for, and most of the criticism aimed at GDP is really criticism of that substitution rather than of the measure itself.