A Tax File Number (TFN) is the personal identifier the Australian Taxation Office (ATO) issues for tax and superannuation. This validator checks whether a TFN is mathematically well-formed using the ATO’s published weighted modulus-11 checksum — handy for developers building tax software and for anyone catching a mistyped number before submitting it.
How it works
Each digit of the TFN is multiplied by a fixed positional weight, the products are added together, and the TFN is valid if that total is exactly divisible by 11. The weight set depends on the length:
- 9-digit TFN: weights 1, 4, 3, 7, 5, 8, 6, 9, 10.
- 8-digit TFN: weights 10, 7, 8, 4, 6, 3, 5, 1.
The tool shows the weighted sum and the sum mod 11 result so you can see exactly why a number passes or fails. Spaces are ignored.
Example
For the 9-digit TFN 123 456 782, multiply each digit by its weight:
(1×1)+(2×4)+(3×3)+(4×7)+(5×5)+(6×8)+(7×6)+(8×9)+(2×10) = 1+8+9+28+25+48+42+72+20 = 253.
253 ÷ 11 = 23 with remainder 0, so the checksum is valid.
| Field | Value |
|---|---|
| Weighted sum | 253 |
| Sum mod 11 | 0 (must be 0) |
| Result | Valid |
A valid checksum only means the number is well-formed, not that it was issued or who holds it. Privacy-first: the checksum runs entirely in your browser, with no network requests, and this tool never queries the ATO.