Calculating GDP...

GDP = consumption expenditure + investment expenditure + government expenditure + export - import
As generally, an economy has so many people and companies, there must be deviation between the actual GDP and the calculated figure.
nominal GDP is the GDP include the effect of change of price level, or inflation, real GDP exclude such effect.

Gross Domestic Product (GDP) calculation formula can be written as this:
GDP = R + I + P + SA + W
R = rents
I = interests
P = profits
SA = statistical adjustments (corporate income taxes, dividends, undistributed corporate profits)
W = wages