The Iowa Data Center has released 2015 county population estimates. Washington County has grown an estimated 2.5% since 2010, and has an estimated 22,247 people. Of surrounding counties, Henry, Louisa, Keokuk have lost population, while Jefferson, Iowa and Johnson Counties have gained population. Johnson County had the highest rate of growth in the area, at 10.2%. Dallas County, west of Des Moines, had the highest rate of population growth over the past 5 years at 21.2%.