How is an Australian TFN validated?

A TFN is normally 9 digits (older numbers are 8). Each digit is multiplied by a position weight — 1,4,3,7,5,8,6,9,10 for 9-digit TFNs and 10,7,8,4,6,3,5,1 for 8-digit TFNs — the products are summed, and the total must be divisible by 11.

Does a valid checksum mean the TFN is registered to someone?

No. The modulus-11 checksum only confirms the number is mathematically well-formed. It cannot tell you whether the TFN was actually issued or who holds it, and this tool never contacts the ATO or any other system.

Is my TFN sent anywhere?

No. The validator is privacy-first. The checksum runs locally in your browser with JavaScript — there are no network requests and your TFN never leaves your device.

Australia TFN Validator — Tax File Number Checksum

Australia TFN validator

Validate an Australian Tax File Number (TFN) using the ATO’s published weighted modulus-11 checksum. Each digit is multiplied by its position weight, the products are summed, and a valid TFN has a total divisible by 11.

The tool supports both modern 9-digit TFNs (weights 1,4,3,7,5,8,6,9,10) and older 8-digit TFNs (weights 10,7,8,4,6,3,5,1), and shows the weighted sum and modulus-11 result so you can see exactly why a number passes or fails.

Privacy-first: the checksum runs entirely in your browser. Nothing is uploaded, stored or sent to any server, and this tool does not query the ATO or any other database.