Nominal GDP measures the total value of all final goods and services produced within a country's borders using current market prices. Because it reflects whatever prices happen to be that year, it includes the effects of both inflation and deflation. That makes it useful for comparing the size of different economies at the same point in time (say, 2022 US nominal GDP vs. 2022 Canada nominal GDP), but it can be misleading when you're tracking one economy over time.

Real GDP measures the same output but values everything at constant prices from a chosen base year (for example, 2012 prices). By holding prices fixed, real GDP strips out inflation and isolates actual changes in the quantity of goods and services produced. It's often expressed in chained dollars, which update the base-year weights over time to better reflect the changing mix of goods in the economy.

Why does this distinction matter?

Nominal GDP can overstate growth when prices are rising. Comparing 1990 GDP to 2020 GDP without adjusting for inflation makes the economy look like it grew more than it actually did.
Real GDP lets policymakers assess the true health of an economy. Rising real GDP signals economic expansion; falling real GDP signals contraction.
If nominal GDP grew 5% but prices rose 2%, real GDP growth was only about 3%. That difference is the whole point of the adjustment.

Nominal vs real GDP, Comparing Nominal and Real GDP | Public Economics

Role of the GDP deflator

The GDP deflator is a price index that captures the average price level of all goods and services included in GDP. You calculate it by dividing nominal GDP by real GDP and multiplying by 100:

$GDP\ Deflator = \frac{Nominal\ GDP}{Real\ GDP} \times 100$

In the base year, the deflator equals 100 (because nominal and real GDP are the same). A deflator of 125 means the overall price level has risen 25% since the base year. A deflator below 100 would indicate deflation.

The GDP deflator differs from the Consumer Price Index (CPI) in an important way:

The CPI tracks price changes for a fixed basket of consumer goods and services.
The GDP deflator covers a broader range, including investment goods and government purchases, and its basket automatically updates as the composition of GDP changes.

Both tools track inflation, but they can give slightly different readings because they cover different sets of goods.

Nominal vs real GDP, Comparing Real and Nominal GDP | Boundless Economics

Calculation of real GDP

To convert nominal GDP into real GDP, divide by the GDP deflator and multiply by 100:

$Real\ GDP = \frac{Nominal\ GDP}{GDP\ Deflator} \times 100$

Here's a step-by-step example:

Suppose nominal GDP is $1,000 billion and the GDP deflator is 125 (base year = 100).
Convert the deflator to a decimal-friendly form: 125 ÷ 100 = 1.25.
Divide nominal GDP by that value: $Real\ GDP = \frac{1{,}000}{1.25} = 800 \text{ billion}$
This tells you that in base-year prices, the economy actually produced $800 billion worth of goods and services. The remaining $200 billion of nominal GDP was due to higher prices, not more output.

This adjustment expresses GDP in constant base-year dollars, so you can compare output across years without price changes clouding the picture. For instance, if nominal GDP grew 5% but the deflator rose from 120 to 122.4 (a 2% price increase), real growth was roughly 3%.

Price Indices and Purchasing Power

Price indices are statistical tools that measure how the price level of goods and services changes over time. The most common ones include:

Consumer Price Index (CPI): Tracks price changes for a fixed basket of goods and services that a typical urban household buys. It's the most widely reported measure of inflation.
Producer Price Index (PPI): Measures price changes from the seller's perspective, tracking what producers receive for their output. PPI changes often show up in CPI later, since higher production costs tend to get passed on to consumers.

Purchasing power refers to how much a unit of currency can actually buy. When inflation rises, each dollar buys less. This is why wages and benefits often include cost-of-living adjustments (COLAs), which increase pay to keep up with rising prices and maintain workers' real purchasing power.

The core idea tying this section together: any time you see a dollar figure from a different year, you should ask whether it's been adjusted for inflation. Nominal values tell you what people paid; real values tell you what they actually got.

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