What is Aadhaar?
Aadhaar is a 12-digit unique identification number issued by the Unique Identification Authority of India (UIDAI) to residents of India. Launched in 2009, it has become the world's largest biometric ID system with over 1.3 billion enrolled members.
Each Aadhaar number is linked to the resident's demographic information (name, date of birth, gender, address) and biometric data (fingerprints, iris scans, and a facial photograph). This creates a unique digital identity that can be used for identification and authentication purposes across India.
Since its introduction, Aadhaar has become an essential document for accessing government services, opening bank accounts, obtaining mobile connections, and completing many other official transactions in India.
The structure of an Aadhaar number
An Aadhaar number consists of 12 digits with minimal structural requirements:
- First digit: never 0 or 1
- Middle digits (2–11): randomly generated with no structure or meaning
- Last digit: a checksum digit calculated using the Verhoeff algorithm
The checksum digit allows the whole number to be validated without access to the central Aadhaar database, which is what this tool does. The Aadhaar number format guide goes into more detail, including masked Aadhaar, Virtual IDs and enrolment IDs.
The Verhoeff algorithm
The Verhoeff algorithm is a checksum formula used to detect errors in identification numbers. Developed by Dutch mathematician Jacobus Verhoeff in 1969, it was the first decimal check digit algorithm that could detect all single-digit errors and all adjacent transposition errors.
Unlike simpler checksums (like the Luhn algorithm used for credit cards), Verhoeff is based on the dihedral group D5 and uses a non-commutative operation, which is what makes it able to detect swapped digits reliably. UIDAI chose it because 12-digit numbers are typed by hand across thousands of systems and every typo class matters.
A full explanation with the lookup tables, a worked example and an interactive step-by-step calculator is on the Verhoeff algorithm page.
How this tool works
Every validator on this site is built on a few principles:
Privacy by architecture, not by promise
The validation code is shipped to your browser and runs there. There is no API endpoint that receives numbers, so there is nothing to intercept, log or breach. The number you enter is never:
- transmitted to our servers or anyone else's,
- stored in any database or browser storage,
- included in analytics events (we record only whether a check passed or failed, never the number), or
- shared with any third party.
You can confirm this yourself: load a page, disconnect from the internet, and the validators keep working.
Real algorithms
- Aadhaar and Virtual ID: full Verhoeff implementation with the first-digit rule.
- GSTIN: the published base-36 check-character algorithm plus state and PAN decoding.
- PAN: structural validation and holder-type decoding (the check character algorithm is not public).
Honest limits
A checksum can confirm that a number is well-formed. It cannot confirm that the number was issued, that it is active, or that it belongs to a particular person. Those questions can only be answered by the issuing authority, and every tool on this site links to the official route. Read how to properly check an Aadhaar number.
Who builds this
Aadhaar Validator is an independent project. It began as a single-page Verhoeff checker and has grown into a small toolkit for developers, compliance teams and ordinary residents who want to check a number without handing it to a stranger.
It is not affiliated with, endorsed by, or connected to UIDAI, the Income Tax Department, GSTN or any government body. Bug reports and corrections are welcome at privacy@aadhaarvalidate.com.