Gross Domestic Product (GDP) of the United States (US) both nominal and real on an annual and quarterly basis. Annual data is provided since 1930 and quarterly data since 1947. Both total GDP (levels) and annualized percentage change in GDP are provided. Both levels and changes are available both in current dollars (nominal GDP) and in chained 2009 dollars (real GDP). Data is sourced from US Government's Bureau of Economic Analysis (BEA) and provided in standardized CSV.
The calculation of GDP and, in particular, chained measures of GDP involves some complexities. You can read more about the benefits and issues of BEA's Chain Indexes in the BEA's 1997 Survey of Current Business.
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Note we assume source data is public domain as US Federal Government.