What format does an Austrian UID number take?

An Austrian UID (Umsatzsteuer-Identifikationsnummer) is the letters ATU followed by exactly eight digits, e.g. ATU13585627. The first seven digits are the serial and the eighth is a check digit.

How is the UID check digit calculated?

Multiply the digits in positions 1, 3, 5, 7 by 1 and positions 2, 4, 6 by 2, replacing each two-digit product with its cross-sum (sum of its digits). Add a constant of 4, and the check digit is (10 minus the last digit of that sum) mod 10.

Does a valid check digit mean the company is VAT-registered?

No. A valid check digit confirms the number is internally well-formed. To confirm a business is actively VAT-registered, verify it through the EU VIES system. This tool performs no online lookup.

Why does my UID number fail validation?

A failed check almost always means a digit was mistyped or transposed, or the number is the wrong length. Austria's UID has exactly eight digits after ATU — re-check against the source document.

Is the same as the Steuernummer?

No. The Steuernummer is the domestic tax file number. The UID (ATU) is the VAT identification number used for invoicing and intra-EU trade. They are different identifiers issued for different purposes.

Austria UID Number Validator

The Austrian UID number (Umsatzsteuer-Identifikationsnummer) is the VAT identifier every VAT-registered Austrian business uses on invoices and in intra-EU trade. Before you trust a supplier’s UID on a B2B invoice, you can confirm it is internally well-formed using Austria’s official check-digit rule. This validator runs that exact check entirely in your browser.

How it works

An Austrian UID is the prefix ATU followed by exactly eight digits. The last of those eight digits is a check digit derived from the first seven:

Take the seven serial digits d1 d2 d3 d4 d5 d6 d7.
Multiply odd positions (1, 3, 5, 7) by 1 and even positions (2, 4, 6) by 2.
For any product that is two digits, replace it with its cross-sum (e.g. 2 x 8 = 16, cross-sum 1 + 6 = 7).
Sum all the resulting values and add the constant 4.
The check digit is (10 - (sum mod 10)) mod 10.

The eighth digit of a valid UID must equal that computed check digit.

Example

Validate ATU13585627 with serial digits 1 3 5 8 5 6 2 and stated check digit 7.

Weighted values: (1x1) + cross(3x2=6) + (5x1) + cross(8x2=16 -> 7) + (5x1) + cross(6x2=12 -> 3) + (2x1) = 1 + 6 + 5 + 7 + 5 + 3 + 2 = 29. Add 4 -> 33. Check digit = (10 - (33 mod 10)) mod 10 = (10 - 3) mod 10 = 7, which matches — so the UID is valid.

Notes

A valid check digit confirms internal consistency only, not active registration. For legal certainty in cross-border invoicing, confirm the UID through the EU VIES service. Everything here runs locally — your number never leaves your device.