Johnston is located close to the center of Iowa. Johnston is part of Polk County. Johnston has 17.16 square miles of land area and 1.20 square miles of water area. As of 2010-2014, the total Johnston population is 18,874, which has grown 118.22% since 2000. The population growth rate is much higher than the state average rate of 5.19% and is much higher than the national average rate of 11.61%. Johnston median household income is $94,821 in 2010-2014 and has grown by 24.61% since 2000. The income growth rate is lower than the state average rate of 33.56% and is lower than the national average rate of 27.36%. Johnston median house value is $230,100 in 2010-2014 and has grown by 22.20% since 2000. The house value growth rate is much lower than the state average rate of 53.09% and is much lower than the national average rate of 46.91%. As a reference, the national Consumer Price Index (CPI) inflation rate for the same period is 26.63%. On average, the public school district that covers Johnston is much better than the state average in quality. The Johnston area code is 515.