Portugal NIF check digit calculator and validator

Give it the first eight digits to calculate the ninth digit, or submit all nine digits to test whether the supplied check digit agrees with the calculation. The result is deterministic and immediate, making it suitable for form checks, imports, accounting workflows, and API validation. Invalid lengths, non-digit characters, and unsupported modes are reported clearly instead of being silently cleaned or guessed.

Choose whether to compute or validate

Use compute mode when you already have the eight-digit base of a Portuguese NIF and need its final check digit. The response includes the calculated digit and the complete nine-digit value, so you can place it directly into a subsequent validation or test workflow. Use validate mode when the source already contains all nine digits. Validation separates the first eight digits from the supplied ninth digit, calculates the expected value, and reports whether both digits match. It also returns the supplied and expected digits, which makes a failed result easy to diagnose. Keep the input as a string rather than a numeric value. A string preserves every character exactly, including a leading zero, and prevents spreadsheet or programming-language number formatting from changing the identifier before the checksum is evaluated. The default mode is validate because checking an existing complete NIF is the most common form and import task, but callers should set the mode explicitly when a workflow may handle both shapes.

How the modulo-eleven calculation works

The algorithm processes the first eight digits from left to right with descending weights from nine through two. It multiplies the first digit by nine, the second by eight, and continues until the eighth digit is multiplied by two. Those eight products are added, and the total is reduced modulo eleven. If the remainder is zero or one, the check digit is zero. For any other remainder, the check digit is eleven minus that remainder. Validation performs exactly the same calculation on the first eight positions and compares the result with position nine. This capability does not strip spaces, dots, dashes, country prefixes, or labels before calculating. That strict behavior is intentional: automatic cleanup can make malformed source data appear trustworthy and can hide errors in upstream field mapping. Submit only ASCII digits from zero through nine. The calculation uses no network request, random value, clock, database, or locale-sensitive conversion, so identical input and mode always produce identical JSON output.

Interpret the result and its limits

A true validation result means the ninth digit is consistent with the modulo-eleven checksum derived from the preceding eight digits. It is a useful structural check for typing mistakes, damaged imports, and incorrectly mapped columns, but it does not prove that the number was issued, remains active, or belongs to a particular person or organization. Those questions require an authoritative registry or an appropriate business verification process. Likewise, this calculator deliberately evaluates the checksum rather than enforcing assumptions about categories encoded in the leading digit. That keeps its scope precise and avoids rejecting a checksum-valid number merely because an external allocation rule changes. In a data pipeline, treat invalid input errors differently from a false validity result: an error means the request had the wrong type, length, characters, or mode, while false means a well-formed nine-digit value was checked and its supplied digit did not match. API requests cost $0.002 each, and the same deterministic core can run in the browser for quick manual checks.