Business Australia
TFN Validator (Check Digit Test)
A tax file number has a built-in check digit that catches most typing errors. This tool applies that test and shows the arithmetic. It runs entirely in your browser, nothing is sent or stored, and it cannot tell you whether a number belongs to anyone.
Free, no signup Runs in your browser
Nine digits. Spaces are fine. Never enter a real TFN into a site you do not trust; this one works in your browser and sends nothing.
Result
—Formatted
—
Weighted total
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Remainder after dividing by 11
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The working, digit by digit
Keep your TFN private. Treat your tax file number as private. This tool does not store or send what you type, but you should still avoid pasting real numbers into any website you do not need to. Developers and testers should use test numbers such as 123 456 782.
How do I check if a TFN is valid?
Multiply the nine digits by the weights 1, 4, 3, 7, 5, 8, 6, 9 and 10, add the results, and the total must divide by 11. The commonly used test number 123 456 782 gives 253, which is 11 × 23, so it passes. A pass shows the number is well formed, not that the ATO issued it.
What passing means
A TFN that passes the check digit test is a possible TFN. It does not show that the ATO issued it, that it belongs to a particular person, or that it is current. Only the ATO can confirm a TFN's validity, and employers and payers can use the ATO's official verification services where they are entitled to.
A fail almost always means a typing mistake, such as a transposed pair or a missed digit.
Keeping your TFN safe
Your TFN is used to commit identity fraud if it falls into the wrong hands. Give it only to your employer, bank, super fund and government agencies, never to anyone who contacts you unexpectedly, and report a lost or stolen TFN to the ATO.
To check a business number, use the ABN validator.
How this is calculated
The rule, step by step, so you can check any figure by hand.
multiply the nine digits by 1, 4, 3, 7, 5, 8, 6, 9 and 10add the nine productsthe TFN is well formed when the total divides exactly by 11
The weights are a standard modulus 11 pattern, a family of check-digit schemes that detect any single mistyped digit and most swapped neighbours. The rule is documented in open-source validation libraries and widely used in software that must accept TFNs; the ATO does not publish a validation service for the public. This page applies the same weighted-sum rule.
Older eight-digit numbers exist and use a different layout, so this tool checks nine-digit numbers only.
A worked example
The test number 123 456 782.
- 1 × 1 + 2 × 4 + 3 × 3 + 4 × 7 + 5 × 571
- 6 × 8 + 7 × 6 + 8 × 9 + 2 × 10182
- Total253
- Total ÷ 1123 exactly
Passes the check: the number is well formed.
Changing one digit, to 123 456 783, breaks it.
Why this exists
Software that collects tax file numbers, such as payroll and onboarding forms, needs to reject typos before they cause a mismatch. Developers and testers use it to confirm their validation matches the algorithm.
What this does not do
It does not look a number up, match it to a name or tell you whether it is real. It cannot recover a lost TFN; the ATO can help with that.
What this calculator does not do
- Nine-digit numbers only
- Shows the digits are well formed, not that the number is issued
- Does not look numbers up
- The weights come from open-source libraries, not an ATO publication
Common questions
How do I validate a TFN?
Multiply the nine digits by 1, 4, 3, 7, 5, 8, 6, 9 and 10 and check the total divides by 11.
Is 123 456 782 a real TFN?
It is a widely used test number that passes the check. It is not a real person's number.
Does a valid result mean the TFN is real?
No. It only means the digits are well formed.
Is the number I type stored?
No. The check runs in your browser and nothing is sent or stored.
What if my TFN is eight digits?
Some older TFNs have eight digits and use a different layout. This tool checks nine-digit numbers.
Where these numbers come from
- ATO — tax file numbers What a TFN is, who can ask for it and how to keep it safe
- python-stdnum — Australian TFN module An open-source implementation of the check-digit rule
Method last reviewed 2026-10-28 This calculator uses fixed rules rather than rates that change, and its method is re-checked against the sources above at least once a year. Out of date? Tell us and we will fix it.
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