Real vs Nominal: Deflating GDP and Comparing Across Years

You’ll be able to

Convert nominal GDP to real GDP using a price index
Explain why real GDP is the correct measure of growth
Determine from nominal and real changes whether prices or output drove a rise

Nominal mixes two things together

Nominal GDP values output at the prices of the year it was produced, so it moves when output changes and when prices change. Real GDP values output at the prices of a fixed base year, so only quantities can move it. This is why real GDP is the measure of growth: a country whose nominal GDP doubled while prices doubled produced nothing extra, and only the real series reveals that.

Deflating

real GDP = nominal GDP / price index × 100 · deflator = nominal / real × 100

The same equation rearranged. Given any two of nominal, real and the deflator, you can find the third.

Reading the two series against each other

Compare the nominal and real growth rates and the diagnosis follows immediately. If nominal grows faster than real, prices rose — inflation accounts for the gap. If nominal grows while real is flat, the entire increase is inflation and no additional output was produced. If real grows faster than nominal, prices actually fell, which is deflation. The approximation worth knowing is that nominal growth ≈ real growth + inflation, exact enough for exam-scale numbers and a fast way to check an answer.

Which year is the base year

In the base year, nominal and real GDP are equal and the index is 100 — there is nothing to deflate because current prices are base-year prices. This is a free diagnostic: if a table shows nominal and real GDP equal in some year, that year is the base year, and the index there must be 100. Exams routinely ask you to identify it, and the answer is visible without any computation.

Worked example

Nominal GDP is $18 trillion in 2024 with a deflator of 120, and $19.5 trillion in 2025 with a deflator of 125. Compute real GDP for both years and the real growth rate.

1.2024 real = 18 / 120 × 100 = $15.0 trillion.
2.2025 real = 19.5 / 125 × 100 = $15.6 trillion.
3.Real growth = (15.6 − 15.0)/15.0 × 100 = 4.0%.
4.Nominal growth for comparison = (19.5 − 18)/18 × 100 = 8.3%.
5.Inflation = (125 − 120)/120 × 100 = 4.2%, and 4.0 + 4.2 ≈ 8.3. The approximation checks out.

Answer: Real GDP $15.0 trillion then $15.6 trillion, 4.0% real growth. Roughly half the 8.3% nominal increase was output and half was prices.

Watch out

Divide by the index and multiply by 100 — do not divide by 1.20. Both work if you are careful, but mixing the index form (120) with the decimal form (1.20) in the same problem is a reliable way to be off by a factor of 100.

Checkpoint

Nominal GDP is $22 trillion and the GDP deflator is 110. Real GDP is:

Checkpoint

Nominal GDP rises 6% while real GDP is unchanged. It follows that:

Checkpoint

A table shows nominal GDP of $16 trillion and real GDP of $16 trillion in 2021. What can you conclude?

On the exam

Use nominal ≈ real + inflation as a sanity check on every deflation problem. If your three numbers do not roughly satisfy it, one of them is wrong, and you will catch it in seconds rather than lose the question.

Answer the 3 checkpoints as you read.