Which price index is used to convert nominal GDP to real GDP?

The price index used to convert nominal GDP to real GDP is the GDP deflator.

The GDP deflator, also called the implicit price deflator for GDP, removes the effect of price changes from current-dollar or nominal GDP. It helps separate an increase in the economy’s measured dollar value into changes caused by higher prices and changes caused by greater real production. In simplified form, the deflator equals nominal GDP divided by real GDP, multiplied by 100.

Unlike the Consumer Price Index, the GDP deflator is not limited to a fixed basket of household purchases. It covers final goods and services produced domestically, including consumption, business investment, government output, and exports. Imports are excluded because they are not produced domestically, while exports are included.

The base or reference period is assigned an index value of 100. Modern national accounts use chain-type measures rather than relying on one permanently fixed basket, so the composition of the underlying output can evolve over time. The deflator therefore differs from both consumer and producer price indexes.