Sample ABN numbers and test ABNs, checked against the register

Every number below was put through the ABR's checksum (ABN Lookup: Format of the ABN (opens in a new tab)) and looked up on the register through this site on 25 October 2026. Select one to check it again now.

The last row tests input with a branch number. The ATO says some entities' branches have an extra 3-digit GST or PAYG registration number (ATO: Checking an ABN (opens in a new tab)); the box here reads the first 11 digits as the ABN.

Because any number that passes the checksum can belong to someone. 99 000 000 000 passes the ABR's check and looks invented, but on 25 October 2026 it was a sole trader's active ABN. The same goes for numbers that sit on some sample-number pages and in some generators: several we looked at belong to real businesses.

A real business's ABN in your test data can end up in demos, screenshots and logs. Use a test value from the tables, or press Generate above: the generator makes a number that passes the official check, then looks it up on the register before you use it.

An ABN is a 9-digit identifier with two leading check digits (ABN Lookup: Format of the ABN (opens in a new tab)), so you can make a valid one from any 9 digits:

1. Multiply the 9 digits by the last 9 weights, 3, 5, 7, 9, 11, 13, 15, 17 and 19, and add them. Call the total R.
2. The two check digits, read as a number P, add P − 10 to the checksum: step 1 takes 1 off the first digit, and that digit's weight is 10.
3. So P = 10 + ((89 − R mod 89) mod 89). It always falls between 10 and 98.

For 123 456 789, R is 615 and 615 mod 89 is 81, so P = 10 + 8 = 18, giving 18 123 456 789. When R divides by 89 exactly, 99 works as well as 10: that is why both 10 000 000 000 and 99 000 000 000 pass.

The same works for a company: run its 9-digit ACN through it and you get the ABN that company would have if its ABN is its ACN with 2 digits in front (ASIC: Australian Company Number (ACN) (opens in a new tab)). ACN 004 085 616 gives 53 004 085 616, the same ABN ASIC's own guide checks (ASIC: Message Implementation Guide v1.5, Appendix D: Algorithms (opens in a new tab)).

The JavaScript and Python below follow the ABR's and ASIC's rules above. We ran both against every 11-digit ABN and 9-digit ACN on this page.

JavaScript

const ABN_WEIGHTS = [10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19];
const ACN_WEIGHTS = [8, 7, 6, 5, 4, 3, 2, 1];

function isValidAbn(abn) {
  const d = String(abn).replace(/\s/g, "");
  if (!/^[1-9]\d{10}$/.test(d)) return false;
  const digits = [...d].map(Number);
  digits[0] -= 1;
  const total = digits.reduce((sum, n, i) => sum + n * ABN_WEIGHTS[i], 0);
  return total % 89 === 0;
}

function isValidAcn(acn) {
  const d = String(acn).replace(/\s/g, "");
  if (!/^\d{9}$/.test(d)) return false;
  const total = [...d.slice(0, 8)].reduce((sum, n, i) => sum + Number(n) * ACN_WEIGHTS[i], 0);
  return (10 - (total % 10)) % 10 === Number(d[8]);
}

// The full ABN, 2 check digits in front, for any 9-digit identifier such as an ACN.
function abnFromNineDigits(nine) {
  const d = String(nine).replace(/\s/g, "");
  const rest = [...d].reduce((sum, n, i) => sum + Number(n) * ABN_WEIGHTS[i + 2], 0);
  return String(10 + ((89 - (rest % 89)) % 89)) + d;
}

Python

ABN_WEIGHTS = [10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
ACN_WEIGHTS = [8, 7, 6, 5, 4, 3, 2, 1]


def is_valid_abn(abn: str) -> bool:
    d = abn.replace(" ", "")
    if len(d) != 11 or not d.isdigit() or d[0] == "0":
        return False
    digits = [int(c) for c in d]
    digits[0] -= 1
    return sum(n * w for n, w in zip(digits, ABN_WEIGHTS)) % 89 == 0


def is_valid_acn(acn: str) -> bool:
    d = acn.replace(" ", "")
    if len(d) != 9 or not d.isdigit():
        return False
    total = sum(int(n) * w for n, w in zip(d[:8], ACN_WEIGHTS))
    return (10 - total % 10) % 10 == int(d[8])


def abn_from_nine_digits(nine: str) -> str:
    """The full ABN, 2 check digits in front, for any 9-digit identifier such as an ACN."""
    d = nine.replace(" ", "")
    rest = sum(int(n) * w for n, w in zip(d, ABN_WEIGHTS[2:]))
    return str(10 + (89 - rest % 89) % 89) + d

A format check runs offline. To know whether a number is on the register, it has to be looked up: in the box above, or on ABN Lookup.