Foster City, CA has a population of 32,967 and is the 1,291st largest city in the United States. The population density is 8,685 per sq mi which is 3637% higher than the California average and 9486% higher than the national average. The median age in Foster City is 41 which is approximately 13% higher than the California average of 36. In Foster City, 68% of the population over 15 years of age are married, 49% speak English and 3% speak Spanish. 36% of Foster City residents were born in California, 18% were born out of state, 2% were born outside of the United States and 44% were foreign born.