Exports plus Imports divided by GDP is the total trade as a percentage of GDP.
For more information and proper citation see http://www.rug.nl/research/ggdc/data/pwt/pwt-7.1
Source Indicator: openc
Source: University of Pennsylvania
Release: Penn World Table 7.1
University of Pennsylvania, Openness at Current Prices for United States [OPENCPUSA156NUPN], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/OPENCPUSA156NUPN, February 13, 2016.