How to Validate Australian Business Numbers (ABN) in Python

1. What is an ABN?

An Australian Business Number (ABN) is a unique 11-digit identifier assigned to all business entities operating in Australia. It is issued by the Australian Business Register (ABR) and is used for tax reporting, invoicing, GST registration, and government interactions.

Every company, sole trader, partnership, trust, and government body in Australia has an ABN. If your application processes Australian business data — whether for invoicing, onboarding suppliers, or compliance checks — you need to validate ABNs correctly.

2. The structure of an ABN

An ABN consists of 11 digits, typically displayed in the format XX XXX XXX XXX:

For example, the ABN 51 824 753 556 has check digits 51 and entity identifier 824753556.

3. The ABN checksum algorithm

The ABN uses a weighted modulus-89 check digit algorithm. Here's how it works:

1. Subtract 1 from the first (leftmost) digit of the ABN.
2. Multiply each of the 11 digits by its corresponding weight: 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19.
3. Sum all 11 weighted products.
4. Divide the sum by 89. If the remainder is zero, the ABN is valid.

# ABN checksum verification (for illustration only)
WEIGHTS = [10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19]


def is_valid_abn(abn: str) -> bool:
    digits = [int(c) for c in abn.replace(" ", "") if c.isdigit()]
    if len(digits) != 11:
        return False

    # Step 1: subtract 1 from the first digit
    digits[0] -= 1

    # Step 2–3: weighted sum
    total = sum(d * w for d, w in zip(digits, WEIGHTS))

    # Step 4: check divisibility by 89
    return total % 89 == 0


is_valid_abn("51 824 753 556")  # True
is_valid_abn("51 824 753 557")  # False — checksum fails

4. Basic validation vs business lookup

There are two levels of ABN verification, and the right choice depends on your use case:

5. The right solution: one API call

Instead of implementing the checksum yourself and integrating with the ABR separately, use the IsValid ABN API. A single GET request handles format validation, checksum verification, and optional ABR lookup — returning entity name, type, GST status, and more.

Get your free API key at isvalid.dev. The free tier includes 100 calls per day — enough for most development and low-volume production use.

Full parameter reference and response schema: ABN Validation API docs →

6. Python code example

Using the isvalid-sdk Python SDK or the requests library. Install with pip install isvalid-sdk or pip install requests.

# abn_validator.py
import os
from isvalid_sdk import IsValidConfig, create_client

iv = create_client(IsValidConfig(api_key=os.environ["ISVALID_API_KEY"]))

# ── Basic validation ────────────────────────────────────────────────────────

result = iv.au.abn("51824753556")
print(result["valid"])      # True
print(result["abn"])        # '51824753556'
print(result["formatted"])  # '51 824 753 556'

# ── With business lookup ────────────────────────────────────────────────────

lookup = iv.au.abn("51824753556", lookup=True)
print(lookup["entityName"])     # 'GOOGLE AUSTRALIA PTY LTD'
print(lookup["entityType"])     # 'Australian Private Company'
print(lookup["gstRegistered"])  # True
print(lookup["state"])          # 'NSW'

7. cURL example

Basic format + checksum validation (fast, no ABR call):

curl -H "Authorization: Bearer YOUR_API_KEY" \
  "https://api.isvalid.dev/v0/au/abn?value=51824753556"

With ABR business lookup (retrieves entity details):

curl -H "Authorization: Bearer YOUR_API_KEY" \
  "https://api.isvalid.dev/v0/au/abn?value=51824753556&lookup=true"

8. Understanding the response

Basic validation response (no lookup):

{
  "valid": true,
  "abn": "51824753556",
  "formatted": "51 824 753 556"
}

Response with lookup=true:

{
  "valid": true,
  "abn": "51824753556",
  "formatted": "51 824 753 556",
  "found": true,
  "entityName": "GOOGLE AUSTRALIA PTY LTD",
  "entityType": "Australian Private Company",
  "abnStatus": "Active",
  "abnStatusFrom": "2000-07-01",
  "acn": "824753556",
  "state": "NSW",
  "postcode": "2060",
  "gstRegistered": true,
  "businessNames": [],
  "dataSource": "abr.business.gov.au"
}

When lookup is omitted or False, only valid, abn, and formatted are returned.

9. Edge cases to handle

# Handle cancelled ABNs
result = iv.au.abn("51824753556", lookup=True)

if result["valid"] and result.get("found") and result.get("abnStatus") != "Active":
    # ABN exists but is no longer active
    print(f"ABN cancelled since {result['abnStatusFrom']}")
    raise ValueError("ABN is not currently active")

if result["valid"] and result.get("found"):
    if result["gstRegistered"]:
        # Include GST on invoice
        apply_gst(invoice)
    else:
        # No GST — entity is not GST-registered
        print("Entity is not registered for GST")

# Check both entity name and trading names
all_names = [result["entityName"]] + result.get("businessNames", [])
all_names = [n for n in all_names if n]

user_input = "Google Australia"
match = any(
    user_input.lower() in name.lower()
    for name in all_names
)

def validate_abn_safe(abn: str, **kwargs) -> dict:
    try:
        return validate_abn(abn, **kwargs)
    except requests.RequestException as exc:
        # Log and decide: fail open (allow) or fail closed (reject)
        logger.error("ABN validation failed: %s", exc)
        return {"valid": None, "error": "validation_unavailable"}