Tax identification number

For individuals on personal returns, the federal tax identification number is the taxpayer's Social Security number. For businesses, the federal tax identification number is the employer identification number (EIN). The EIN is a nine-digit number assigned to each business.

Usually, the federal EIN is also used for state income tax purposes. Depending on the state, there may be a separate state tax identification number. Expect to be asked for your tax ID number when you sell to another business.