A federal tax ID number is nine digits long. That holds for a Social Security Number, an Employer Identification Number, an Individual Taxpayer Identification Number, and an Adoption Taxpayer Identification Number. The one exception is the Preparer Tax Identification Number, which is eight digits with the letter “P” in front.1eCFR. 26 CFR 301.6109-1 – Identifying Numbers
Telling the Types Apart by Format
All the nine-digit IDs share a length, but the grouping of digits and a few positional rules make each one recognizable at a glance.
- A Social Security Number is written as three digits, two digits, four digits: XXX-XX-XXXX.
- An Employer Identification Number is written as two digits, then seven: XX-XXXXXXX. Businesses use it in place of an SSN on tax filings.2Internal Revenue Service. About Form SS-4, Application for Employer Identification Number (EIN)
- An Individual Taxpayer Identification Number uses the same XXX-XX-XXXX shape as an SSN, but always starts with a 9, and the middle two digits fall in ranges the IRS reserves so ITINs don’t overlap with SSNs.3Internal Revenue Service. Taxpayer Identification Numbers (TIN)
- An Adoption Taxpayer Identification Number is a temporary nine-digit number the IRS issues when you’re adopting a U.S. citizen or resident child and can’t get an SSN for the child in time to file.3Internal Revenue Service. Taxpayer Identification Numbers (TIN)
The fastest way to identify a number you’re looking at: only two digits before the first hyphen means EIN; a 9 at the very start in SSN format means ITIN; a leading letter means PTIN.
Why the PTIN Is the Outlier
The Preparer Tax Identification Number is the only federal tax identifier that isn’t purely numeric. It runs to nine characters total, but the first one is the letter “P” and the remaining eight are digits.4Legal Information Institute. Preparer Taxpayer Identification Number (PTIN) Anyone who prepares or helps prepare federal tax returns for pay needs one, including enrolled agents and CPAs. The renewal fee for 2026 is $18.75.5Internal Revenue Service. Frequently Asked Questions: PTIN Application/Renewal Assistance
How Long the Number Itself Stays Valid
Length is one question; lifespan is a different one, and the answers don’t match across types.
Social Security Numbers never expire. Once assigned, the number stays with you for life. EINs are also permanent. Even after a business closes, the IRS keeps the EIN tied to that entity and never reuses it.6Internal Revenue Service. If You No Longer Need Your EIN
ITINs are the exception most people miss. If an ITIN isn’t used on at least one federal tax return during any three consecutive tax years, it expires on December 31 after that third year of non-use. Renewing means filing Form W-7 again with fresh documentation, and until you do, the IRS may hold up your refund or deny credits you would otherwise qualify for.7Internal Revenue Service. How to Renew an ITIN
PTINs expire every year on December 31 regardless of when they were issued, and paid preparers have to renew before each filing season.5Internal Revenue Service. Frequently Asked Questions: PTIN Application/Renewal Assistance ATINs are temporary by design; the IRS issues them as a bridge during an adoption, and they’re replaced by a permanent SSN once the adoption is finalized.
Which One You’ll Have
Most U.S. citizens and permanent residents already have an SSN and never need any other tax ID for personal filing. An EIN comes in when you run a business that hires employees, operates as a partnership or corporation, or files excise taxes; estates, trusts, and tax-exempt organizations get their own EIN as well.8Internal Revenue Service. Get an Employer Identification Number An ITIN is for people who have a U.S. filing obligation but don’t qualify for an SSN, including nonresident aliens with U.S. income and spouses or dependents claimed on another person’s return who can’t get an SSN. You can’t hold both an ITIN and an SSN at the same time, and if an SSN application is pending, you shouldn’t file for an ITIN.9Internal Revenue Service. About Form W-7, Application for IRS Individual Taxpayer Identification Number