The Census Bureau computes county population estimates, current as of July of each year. Estimates are calculated using birth rates, death rates and numbers of residents migrating from one area to another. (For a more detailed explanation, refer to the Census Bureau's methodology here)
This map shows changes in each county's total population between July 2013 and July 2014. Zoom in and pan the map to view a specific region, then click a county to display its data.