How can someone determine their federal tax ID?

An Employer Identification Number (EIN) is a nine digit number issued by the Internal Revenue Service (IRS) and is used to identify a business entity. An EIN is also commonly referred to as a federal tax identification number or simply a tax ID. An EIN is used by most business entities as their Tax Identification Number (TIN). However individuals and sole proprietors may use their Social Security number for TIN purposes.