Thermalito, CA has a population of 7,056 and is the 5,148th largest city in the United States. The population density is 554 per sq mi which is 139% higher than the California average and 512% higher than the national average. The median age in Thermalito is 36 which is approximately 1% lower than the California average of 36. In Thermalito, 46% of the population over 15 years of age are married, 74% speak English and 4% speak Spanish. 69% of Thermalito residents were born in California, 15% were born out of state, 2% were born outside of the United States and 15% were foreign born.