US Health · Validation
Medicare beneficiary identifier (MBI) format check.
Check whether a Medicare Beneficiary Identifier matches the 11-character CMS format. Format only: it cannot tell you whether anyone is enrolled.
The rule: 11 characters, pattern C A AN N A AN N A A N N. C is a digit 1-9, N is a digit 0-9, A is a letter A-Z except S, L, O, I, B and Z, and AN is either. Example: 1EG4-TE5-MK73. Dashes are optional here; on the card they are only for reading.
Inputs
Result
Enter a number and press Check.
How the MBI format works
The Medicare Beneficiary Identifier replaced the Social Security based number on Medicare cards. It has 11 characters, is randomly generated, and its characters carry no meaning. It has no check digit, so a format test is all that can be done offline.
Position by position
- Position 1: digit 1-9
- Position 2, 5, 8, 9: letter A-Z, excluding S, L, O, I, B, Z
- Position 3, 6: digit 0-9 or a letter from the same allowed set
- Position 4, 7, 10, 11: digit 0-9
CMS excludes S, L, O, I, B and Z to avoid confusion with digits, says dashes are not part of the MBI, and converts lowercase to uppercase, which this tool also does.
Format only — not an enrollment lookup
This page checks format only. A string that matches can still be an MBI that was never issued, and a real MBI cannot be confirmed here. Providers check eligibility through their Medicare Administrative Contractor portal.
Sources
- CMS, Understanding the Medicare Beneficiary Identifier (MBI) Format (PDF) (11 characters, position types, example 1EG4-TE5-MK73)
- CMS, New Medicare Card (restates 11 characters and the excluded letters)
Both sources are CMS. No independent second source or maintained reference implementation was found, so this page rests on the CMS specification alone.
Privacy and scope
An MBI is sensitive health-insurance data. This tool runs entirely in your browser: nothing you type is transmitted, stored, or logged, and it checks format only, with no query to Medicare or any state system. States cannot confirm enrollment from this page either. Do not enter numbers belonging to other people that you have no reason to hold.