If nominal gdp is $12 trillion and real gdp is $10 trillion, then the gdp deflator is: 120, and this indicates that the price level has increased by 20 percent since the base year.
GDP deflator reflect the effects of new prices to the product that produced domestically.
It calculated with this equation:
GDP Deflator = GDP Nominal/Real GDP x 100
= 12 Trllion /10 Trillion x 100
= 120
