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 the TFN checksum 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.
Worked 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 |
Why a weighted modulus-11 check catches typos
The design is deliberate. Because 11 is prime and every weight is non-zero and distinct modulo 11, changing any single digit changes the weighted sum by a non-multiple of 11 — so every single-digit typo is caught, with no exceptions. Adjacent transpositions (typing 45 as 54) change the sum by the digit difference times the weight difference; since neighbouring weights in the TFN sets differ by values that are non-zero mod 11, these swaps are caught too. That is strictly stronger than a plain “sum of digits” check, which misses all transpositions, and it is why national ID schemes in many countries use some form of modulus-11.
What the check cannot catch is a typo that happens to produce another valid TFN — for example two compensating errors in different positions. Roughly 1 in 11 random 9-digit strings passes the checksum, so a pass filters out ~91% of random garbage but is never proof of a real number.
TFN vs ABN: don’t validate one with the other
A TFN identifies a person (or entity) to the ATO for tax and super. An ABN (Australian Business Number) is a different, public, 11-digit identifier for businesses — and it uses a completely different checksum (subtract 1 from the first digit, apply weights 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, and check divisibility by 89). Running an ABN through a TFN validator fails on length alone. If a contractor gives you an 11-digit number, you are looking at an ABN, not a TFN, and the paperwork differs: employees complete a TFN declaration, while contractors quote an ABN on invoices.
Handling TFNs responsibly in software
TFNs are legally protected identifiers under Australia’s Privacy (Tax File Number) Rule. If you are building systems that touch them:
- Collect only when authorised. Only entities with a tax, super, or specific statutory purpose (employers, super funds, banks paying interest) may collect TFNs — they must not be used as a general customer identifier.
- Store them like secrets. Encrypt at rest, restrict access, and never log a full TFN. Unauthorised use or disclosure of TFNs carries penalties.
- Validate client-side first. The checksum lets you reject transcription errors instantly without transmitting the number anywhere — the pattern this tool demonstrates.
- Never hard-code “test” TFNs that pass. Use clearly reserved dummy values in fixtures, and mask TFNs in screenshots and error reports.
Common failure causes and edge cases
A checksum failure almost always means one of the following:
- Transposed digits — swapping two adjacent digits is the most common data-entry mistake and almost always breaks the weighted sum differently from a simple off-by-one.
- Missing or extra digit — the validator checks length first; a number with 7 or 10 digits is rejected before the checksum even runs.
- OCR or scanning error — numbers copied from PDF or photographed documents frequently produce a
6for an8or a1for a7. If a number is one check away from valid, consider which digit looks ambiguous in the source. - Test or placeholder values — some software generates dummy TFNs (e.g. 111 111 111) for testing that happen to pass the checksum. The ATO reserves certain ranges; a mathematically valid TFN is not necessarily a real or active one.
Who uses TFN validation
Developers building payroll systems, accounting software, or tax-lodgment tools use the checksum to provide early input feedback before the user submits a form, avoiding a round-trip to the ATO for a simple transcription error. HR platforms validate TFNs when employees complete their tax file number declaration (TFN declaration form), catching mistakes on the spot.
The algorithm is also used in reconciliation: if a batch import of employee records contains a malformed TFN, a pre-processing validation pass surfaces the bad records before the payroll run, rather than after the ATO rejects the file.
Privacy note
The checksum runs entirely in your browser with JavaScript — there are no network requests and your TFN never leaves your device. This tool never contacts the ATO or any external system. A valid result here only confirms the number is mathematically well-formed; it cannot confirm it was issued, is active, or belongs to any particular person.
Sources and references
- Australian Taxation Office — Tax file number — what a TFN is and how it is used
- ATO — TFN declaration — the form where employers collect and record TFNs
Maintained by the Gera Tools editorial team. The weighted modulus-11 algorithm and weight sets are the ATO’s published TFN check; this tool validates structure only and never contacts the ATO. Last reviewed 2026-07-02.