What is the difference between a DNI and a NIE?

A DNI (Documento Nacional de Identidad) is the national identity document issued to Spanish citizens. It consists of 8 digits followed by a single control letter. A NIE (Numero de Identidad de Extranjero) is issued to foreign residents in Spain and starts with a prefix letter — X for the original series, Y for the second, and Z (theoretically) for a third — followed by 7 digits and a control letter. Both documents use the same mod-23 algorithm to derive the control letter.

How is the control letter calculated?

For a DNI: take the 8-digit number, compute its remainder when divided by 23 (modulo 23), then look up that remainder (0–22) in the fixed table TRWAGMYFPDXBNJZSQVHLCKE. The letter at that position is the required control letter. For a NIE: first substitute the prefix letter with a digit (X becomes 0, Y becomes 1, Z becomes 2), giving an 8-digit number, then apply the same mod-23 table.

Why does the control letter table skip some letters?

The 23-letter alphabet used in the table deliberately omits I, O, and U. O was excluded to avoid confusion with the digit 0, I to avoid confusion with 1, and U because it could be misread as V. This leaves exactly 23 usable letters, which is why modulo 23 is the right divisor.

What does a valid result actually mean?

A valid result means the control letter is mathematically consistent with the preceding digits under the official Spanish mod-23 algorithm. It does not confirm that the document was ever issued by the Policia Nacional, that it belongs to a real person, or that it is currently active. Only the Spanish authorities can verify authenticity.

Can I use this tool for GDPR-regulated processing?

Yes — the tool processes everything locally in your browser. No DNI or NIE number is transmitted to any server, logged, or stored anywhere. This makes it safe to use even when handling personal data under GDPR or the Spanish LOPD.

What is a safe fake DNI to use for testing?

00000000T is a mathematically valid DNI because 0 mod 23 = 0, and position 0 in the table is T. You can also construct any test DNI by picking 8 arbitrary digits, computing their sum mod 23, and reading off the control letter from the table. Never use a real person's document number in test environments or public examples.

Spain DNI / NIE Validator

The Spain DNI / NIE Validator checks whether a Spanish personal identification number is structurally correct by re-running the official mod-23 control-letter algorithm and comparing the result to the letter you supplied. It works for both document types:

DNI (Documento Nacional de Identidad) — issued to Spanish nationals. Format: 8 digits followed by one control letter, e.g. 12345678Z.
NIE (Número de Identidad de Extranjero) — issued to foreign residents in Spain. Format: a prefix letter (X, Y, or Z) followed by 7 digits and one control letter, e.g. X1234567L.

The tool shows every step of the derivation — the numeric value used, the mod-23 remainder, the expected control letter, and whether it matches — so you can understand why a number passes or fails, not just that it does.

How the mod-23 algorithm works

The algorithm is defined in Spanish law (Real Decreto 1553/2005 for the DNI and the corresponding NIE regulations). The steps are:

For a NIE only: replace the leading letter with its numeric equivalent. X → 0, Y → 1, Z → 2. This gives an 8-digit number identical in form to a DNI.
Compute modulo 23 of that 8-digit integer. Because the number fits comfortably within a 32-bit integer, no big-number arithmetic is needed.
Look up the remainder (a value from 0 to 22) in the fixed table:

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
T R W A G M Y F P D X B N J Z S Q V H L C K E
The letter at that position must match the final character of the document. If it does, the document number is valid; if not, either the digits or the control letter contain a transcription error.

The table has exactly 23 entries and omits I, O, and U to prevent mix-ups with the digits 1, 0, and a stylised v.

Example (fake numbers safe for testing)

Input	Type	Numeric used	Remainder	Expected letter	Valid?
`00000000T`	DNI	0	0 mod 23 = 0	T	Yes
`00000001R`	DNI	1	1 mod 23 = 1	R	Yes
`00000000A`	DNI	0	0 mod 23 = 0	T expected, A supplied	No
`X0000000T`	NIE	0 + 0000000 = 00000000	0 mod 23 = 0	T	Yes
`X0000000X`	NIE	0 + 0000000 = 00000000	0 mod 23 = 0	T expected, X supplied	No
`Y0000000Z`	NIE	1 + 0000000 = 10000000	10000000 mod 23 = 14	Z	Yes

These numbers are arithmetically constructed and do not correspond to any real person. Always use obviously-fake inputs (all zeros, sequential digits) in test suites and public documentation.

Privacy note

All validation is performed by JavaScript running in your browser tab. No document number, partial result, or timing signal is transmitted to any external server. The page contains no analytics hooks that capture form input. You can disconnect from the internet and the tool will continue to work exactly the same way.