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    How TFN Validation Works

    The Australian Tax File Number uses a weighted mod-11 checksum. Here's exactly how it works — with diagrams you can reference.

    Published March 2026 by WorkLens

    What is a TFN?

    A Tax File Number (TFN) is an 8 or 9 digit identifier issued by the Australian Taxation Office (ATO) to individuals and organisations. It is classified as Critical PII under the Australian Privacy Act 1988 — its exposure can directly enable identity fraud and tax fraud.

    The Validation Algorithm

    TFN validation uses a weighted checksum — each digit is multiplied by a fixed weight, the products are summed, and the result must be divisible by 11. This catches typos and random numbers that happen to be 8-9 digits.

    TFN Validation Flowchart
    YesNoYesNoReceive 8 or 9 digit numbere.g. 123 456 782Strip spaces & punctuation→ 123456782Is it 8 or 9 digits?Not a TFNWrong lengthMultiply each digit by weightWeights: 1, 4, 3, 7, 5, 8, 6, 9, 10Sum all productse.g. 1×1 + 2×4 + 3×3 + ...Is sum divisible by 11?sum % 11 === 0Valid TFNChecksum passedInvalid TFNChecksum failed

    Step-by-Step Example

    Let's validate the TFN 123 456 782:

    Position123456789
    Digit123456782
    Weightx1x4x3x7x5x8x6x9x10
    Product189282548427220

    Sum: 1 + 8 + 9 + 28 + 25 + 48 + 42 + 72 + 20 = 253

    Check: 253 ÷ 11 = 23 remainder 0 → Valid TFN

    Why This Matters for PII Detection

    Without checksum validation, any 8-9 digit number could be flagged as a TFN — phone numbers, postcodes, order numbers. This creates massive false positives.

    Detection With vs Without Checksum
    Feature
    Regex Only
    Regex + Checksum
    Detects valid TFNs
    ✓
    ✓
    Rejects phone numbers
    —
    ✓
    Rejects order IDs
    —
    ✓
    False positive rate
    ~35%
    <2%
    Confidence level
    0.70
    0.95+

    The Weights

    TFN Digit Weights
    ×1
    Position 1
    Multiply by 1
    ×4
    Position 2
    Multiply by 4
    ×3
    Position 3
    Multiply by 3
    ×7
    Position 4
    Multiply by 7
    ×5
    Position 5
    Multiply by 5
    ×8
    Position 6
    Multiply by 8
    ×6
    Position 7
    Multiply by 6
    ×9
    Position 8
    Multiply by 9
    ×10
    Position 9
    Multiply by 10
    Important: A valid checksum does not guarantee a TFN is real — it only confirms the number follows the correct mathematical pattern. The ATO does not publish a public registry of issued TFNs. WorkLens validates the format, not the issuance.

    How WorkLens Uses This

    WorkLens applies the mod-11 checksum as Layer 10 (Validators) in its 15-layer detection pipeline. When a regex pattern matches an 8-9 digit number, the validator runs the checksum. If it passes, confidence is boosted to 0.95+ (Critical severity). If it fails, the detection is suppressed — preventing false positives from phone numbers, order IDs, and random digit sequences.

    The same approach is used for Medicare numbers (prefix + format validation), ABN (mod-89 weighted checksum), and credit cards (Luhn algorithm).

    Try it on your own data

    Drop a CSV or XLSX file — WorkLens detects and validates PII in seconds. No signup required.

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