Clayton, CA has a population of 11,655 and is the 3,496th largest city in the United States. The population density is 3,035 per sq mi which is 1206% higher than the California average and 3250% higher than the national average. The median age in Clayton is 46 which is approximately 27% higher than the California average of 36. In Clayton, 65% of the population over 15 years of age are married, 89% speak English and 4% speak Spanish. 63% of Clayton residents were born in California, 26% were born out of state, 1% were born outside of the United States and 10% were foreign born.