How ABN Validation Works

An Australian Business Number (ABN) is an 11-digit identifier issued by the Australian Business Register (ABR) to businesses and organisations. It appears on invoices, tax documents, and contracts. While public, it is often surrounded by other sensitive data (bank details, names, addresses), so detecting it accurately matters for redaction and de-identification.

ABN validation uses a weighted checksum modulo 89. The quirk: before applying the weights, you subtract 1 from the first digit. Then each digit is multiplied by a fixed weight, the products are summed, and the result must be divisible by 89. This catches typos and random 11-digit numbers.

Let's validate the ABN 51 824 753 556. First, subtract 1 from the leading digit: 5 → 4, giving the working digits 4 1 8 2 4 7 5 3 5 5 6:

Without the mod-89 checksum, any 11-digit number could be flagged as an ABN — phone numbers with area codes, account numbers, reference IDs. The checksum collapses the false positive rate.

WorkLens applies the mod-89 checksum as Layer 10 (Validators) in its 15-layer detection pipeline. When a regex pattern matches an 11-digit number, the validator subtracts 1 from the first digit, applies the weights, and checks divisibility by 89. If it passes, confidence is boosted to 0.95+. If it fails, the detection is suppressed — preventing false positives from phone numbers, account numbers, and random digit sequences.

The same checksum-before-flagging approach is used for TFN (mod-11 weighted checksum), Medicare (prefix + check digit), and credit cards (the Luhn algorithm). Validation runs entirely on your machine — no number ever leaves the browser.