Capital means uppercase letters, money/wealth or the city that serves as the seat of a country’s or state’s government. A capitol is the building where the legislative branch (Congress) convenes. 

Capitalize U.S. Capitol. Also, capitalize the Capitol when referring to the building in Washington, D.C.: Senators gathered at the Capitol. 

Do the same for state capitols: The Arizona Capitol is in Phoenix. 

Use capital (and lowercase) for a city or town that is the seat of government: Phoenix is the capital of Arizona.