For most individuals, their federal tax identification number is a Social Security number. If lost or forgotten, this number often can be found by inspecting tax documents or Medicaid cards. The Social Security Administration says it can issue a duplicate card if the individual provides acceptable proof of identity.
Individuals who are not eligible for a Social Security number, such as some resident or nonresident aliens, use an ITIN as their federal identification number, according to the Internal Revenue Service. This number, obtained by filing form W-7 with the IRS, is found on the acceptance documents and other tax-related paperwork. It is a nine-digit number, formatted like a Social Security number, but it begins with a "9."
Businesses, trusts and estates use an employer identification number for federal tax purposes, says the IRS. If misplaced, the EIN is found on the computer-generated letter issued by the IRS when the number was applied for. Banks and state licensing agencies may have a copy of the number in their records if it was used on an application. Tax returns and annual reporting documents such as 1099s show the EIN. An authorized person can call the IRS business and specialty tax line on weekdays, and the EIN is given over the phone after the IRS is given sufficient identifying information.