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The primary function of nominal GDP is to express the total output of a country over a time period (usually a quarter or year) at its present market value. Nominal GDP growth can then be used to find the growth or decrease in output between years with inflation or deflation included. While this growth may not serve as a reliable measure of actual output between years, comparing market values between years can be useful for other purposes. The primary use of nominal GDP growth is to measure inflation between years. Real GDP growth is calculated for the same set of years. Then, the two growth rates are compared to assess inflation. If nominal GDP is rising faster than real GDP, the country's currency is experiencing inflation. If nominal GDP is growing at a slower rate, the country is experiencing deflation. Real GDP adds one more step to the summation of nominal GDP by factoring out inflation or deflation from GDP. The nominal GDP is modified by the GDP deflator, a measure of relative prices, to arrive at real GDP. The GDP deflator is composed of price indexes for the two periods being compared. For example, the price indexes of two years might be 105 for a base year and 120 for the current. The GDP deflator to convert nominal GDP for the current year to real GDP would then be 105120{\displaystyle 105\div 120}, or 0.875. So, if the nominal GDP for that year were $100 billion, real GDP would be 0.875$100billion{\displaystyle 0.875 imes \$100billion}, or $87.5 billion.

Summary:
Figure out the total output of a nation. Assess inflation. Convert to real GDP.