Austrian UID / ATU
Validator
Check whether an Austrian VAT number (UID / ATU) is correctly formatted and passes the official BMF checksum algorithm — entirely in your browser.
About Austrian UID / ATU
Official validator for Austrian UID / ATU with checksum verification and mathematical integrity.
It is mandatory for cross-border EU trade (VIES validation), electronic invoicing, and domestic corporate billing.
Austrian UID / ATU Anatomy
| Position | Field | Length | Meaning & Rule | Example |
|---|---|---|---|---|
| 1 to 3 | Prefix | 3 chars | Fixed "ATU" (Austria VAT) | ATU |
| 4 to 10 | Company Serial | 7 digits | Registration sequence with the Federal Ministry of Finance | 1234567 |
| 11 | Check Digit | 1 digit | Calculated using a modified Luhn algorithm with a +4 constant | 5 |
Official Austrian ATU Algorithm
| Step | Description |
|---|---|
| 1. Take the 7 digits | Take the 7 numeric digits that follow the ATU prefix. |
| 2. Sum the odd-position digits | Sum the digits at positions 1, 3, 5, 7 (1-indexed) as-is. |
| 3. Double and reduce the even positions | Multiply the digits at positions 2, 4, 6 by 2, and if the result is 10 or more, add its digit sum instead. |
| 4. Add the constant | Add the fixed constant +4 to the accumulated total. |
| 5. Compare the check digit | Compute (10 − (Sum mod 10)) mod 10 and compare it to the 8th digit. |
Frequently Asked Questions (FAQ)
Why does it always start with ATU?
"AT" is Austria's ISO country code and "U" stands for Umsatzsteuer (VAT) — the Austrian tax authority fixed this exact 3-letter prefix for every VAT number it issues.
What is the difference between Swiss and Austrian UID?
They share the "UID" name but are unrelated formats: Switzerland's UID (CHE-xxx.xxx.xxx) isn't an EU VAT number, while Austria's UID (ATU + 8 digits) is the Austrian EU VAT identification number, validated through the EU's VIES system.
Are the generated numbers official or real?
No. This validator only checks the format and BMF checksum — it does not look up any real company registration. Confirming a VAT number is actually registered to a business requires the EU's official VIES lookup service.