TFN Validator
Check whether an Australian Tax File Number is valid
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What the TFN check digit catches
A Tax File Number is nine digits with the last one acting as a check digit. Multiply the digits by the weights 1, 4, 3, 7, 5, 8, 6, 9 and 10, add the results, and a genuine TFN leaves no remainder when divided by 11. That single test catches every single-digit typo and almost every transposition, which between them account for most of the TFNs that arrive wrong on an employment form.
It does not tell you the number belongs to anybody. Around one in eleven random nine digit numbers satisfies the modulus by chance, and the ATO does not publish a lookup service that would let a website confirm ownership even if it wanted to. Employers verify a TFN through the ATO's own channels, not by arithmetic. Treat a pass here as "worth accepting into the form" rather than "verified".
Privacy, because this one matters
A TFN is sensitive information under Australian privacy law, and the Privacy (Tax File Number) Rule limits who may collect one and what they may do with it. This page never sends what you type anywhere: the check runs in JavaScript on your own device, there is no request to any server as you type, and nothing is stored between visits. If you are only testing a form, prefer a number from the TFN generator over a real one, which is the entire reason that tool exists.
Older eight digit numbers
TFNs issued before the 1990s were eight digits and use a different weighting, so this tool reports them as the wrong length rather than guessing. They are increasingly rare, but if you maintain a system with decades of records, that is the case to keep in mind before you make a nine digit check mandatory. Store TFNs as digits and display them grouped in threes; strip spaces before validating, since that is how people paste them.
Frequently Asked Questions
How does the TFN check digit work?
Multiply the nine digits by the weights 1, 4, 3, 7, 5, 8, 6, 9 and 10, add the products, and a valid Tax File Number leaves no remainder when the total is divided by 11. The final digit is chosen when the number is issued so that the sum comes out even, which is what makes single typos and swapped digits detectable.
Can this tool tell me who a TFN belongs to?
No, and nothing on the public internet can. The ATO does not publish a lookup service for Tax File Numbers, because a TFN is protected information under the Privacy (Tax File Number) Rule. All that can be checked outside the ATO is whether the digits satisfy the algorithm.
Is it safe to type a real TFN into a website?
As a habit, no. This particular page does the check locally and sends nothing, but you cannot tell that by looking at a site, and a TFN is exactly the sort of identifier that should not be pasted casually. If you are testing software, generate a number instead of borrowing a real one.
Why does an eight digit number fail?
Tax File Numbers issued before the 1990s were eight digits and use a different weighting, so this tool reports the length rather than pretending to check them. They still exist in long-lived records, which is worth knowing before you make a strict nine digit rule mandatory in a system holding decades of history.