Seattle, WA has a population of 668,849 and is the 20th largest city in the United States. The population density is 7,978 per sq mi which is 8152% higher than the Washington average and 8706% higher than the national average. The median age in Seattle is 36 which is approximately 5% lower than the Washington average of 38. In Seattle, 45% of the population over 15 years of age are married, 78% speak English and 4% speak Spanish. 37% of Seattle residents were born in Washington, 42% were born out of state, 2% were born outside of the United States and 18% were foreign born.