How the control character works

The last character of every NIF is a control character, computed from the rest of the number, so that a typo is caught. This page explains how each one is computed and where each rule comes from. Not every step has the same kind of source: some are law, some only official guidance, and one is a convention. The tiers are explained on the official sources page.

Note

You don’t need to write any of this: computeControlCharacter() returns the control character of a number without its last character, and every validator checks it.

DNI: the number mod 23

The check letter of a DNI is the remainder of dividing its 8-digit number by 23, looked up in this table (DNI-2):

Remainder	Letter	Remainder	Letter	Remainder	Letter
0	T	8	P	16	Q
1	R	9	D	17	V
2	W	10	X	18	H
3	A	11	B	19	L
4	G	12	N	20	C
5	M	13	J	21	K
6	Y	14	Z	22	E
7	F	15	S

For 12345678: 12345678 = 23 × 536768 + 14, and the letter for 14 is Z, so the DNI is 12345678Z.

import { computeControlCharacter } from "nif-dni-nie-cif-validation";

const LETTERS = "TRWAGMYFPDXBNJZSQVHLCKE";

12345678 % 23;                       // 14
LETTERS[12345678 % 23];              // "Z"
computeControlCharacter("12345678"); // "Z"

The letters I, Ñ, O and U are not in the table, so a DNI never ends with them (DNI-3). When a user types one of them, validate() says which rule failed and which letter was expected:

import { validate } from "nif-dni-nie-cif-validation";

validate("12345678I").error?.rule;     // "DNI-3"
validate("12345678I").error?.expected; // "Z"

Source. The format of the DNI is law: Royal Decree 1065/2007, art. 19 (tier T1). The mod-23 algorithm is not in any law: the Ministerio del Interior publishes it, and so does the AEAT’s technical note on the NIF (tiers T2 and T3).

K/L/M NIF: the 7 digits only

A K, L or M NIF uses the same table, over its 7 digits. The letter at the start is not converted to a digit, so K, L and M with the same digits have the same check letter (KLM-2):

import { computeControlCharacter } from "nif-dni-nie-cif-validation";

computeControlCharacter("K1234567"); // "L": 1234567 % 23 is 19
computeControlCharacter("M1234567"); // "L"

Source. The AEAT’s technical note on the NIF (T3). The law calls the 7 characters “alphanumeric”, but the algorithm only works with digits, so the library follows the note and requires digits (KLM-3, a known conflict between sources).

NIE: replace the prefix, then mod 23

For an NIE, the prefix becomes a digit (X → 0, Y → 1, Z → 2), and the 8-digit number that results goes through the DNI table (NIE-2). So Y1234567 becomes 11234567, whose remainder is 10, letter X:

import { computeControlCharacter, isValidNie } from "nif-dni-nie-cif-validation";

11234567 % 23;                       // 10
computeControlCharacter("Y1234567"); // "X"
computeControlCharacter("X1234567"); // "L": the same as the DNI 01234567
isValidNie("Y1234567L");             // false: the prefix changes the letter

The old 10-character NIE (X0 + 7 digits + letter) has the same letter as its 9-character form, because the leading zero doesn’t change the number (NIE-3).

Source. The Ministerio del Interior’s page and the AEAT’s technical note (T2 and T3); the X, Y and Z series are in Orden INT/2058/2008 (T1).

CIF: a digit or a letter, by organisation key

The control character of a CIF is either a digit or a letter, and the organisation key decides which (CIF-3):

• a digit for A, B, C, D, E, F, G, H, J, U and V (Spanish companies and other entities);
• a letter for N, P, Q, R, S and W (foreign entities, local authorities, public bodies, religious institutions, State bodies and permanent establishments).

The value is computed from the 7 digits (CIF-4):

1. Add the digits in even positions (the 2nd, 4th and 6th).
2. Double each digit in an odd position (the 1st, 3rd, 5th and 7th), and add the digits of each result (14 counts as 1 + 4).
3. Add both sums. The control digit is 10 minus the last digit of the total, or 0 when the last digit is 0.
4. When the key takes a letter, the digit becomes a letter: 0 → J, 1 → A, 2 → B, … 9 → I (JABCDEFGHI).

For B1234567: the even positions give 2 + 4 + 6 = 12; the odd ones give 2 + 6 + (1 + 0) + (1 + 4) = 14; the total is 26, so the control digit is 10 − 6 = 4, and the CIF is B12345674. A key that takes a letter would get D, the fifth letter of JABCDEFGHI.

import { computeControlCharacter, isValidCif } from "nif-dni-nie-cif-validation";

computeControlCharacter("B1234567"); // "4"
computeControlCharacter("N1234567"); // "D": N takes a letter
computeControlCharacter("P2807900"); // "B": Ayuntamiento de Madrid, P2807900B
isValidCif("B1234567D");             // false: B takes a digit

Source. The organisation keys are law: Orden EHA/451/2008 (T1). Which keys take a digit and which a letter comes only from the AEAT’s internal technical note (T3). The arithmetic is not published in any official text that we could find: it is a convention (T4), followed by every implementation and checked against real NIFs of public bodies, such as P2807900B (Ayuntamiento de Madrid) and Q2826000H (the AEAT itself).

C, D, F, G, J, U and V

Many libraries, and version 1 of this one, accept either a digit or a letter for the keys C, D, F, G, J, U and V. The AEAT’s note gives them a digit, so version 2 rejects a letter by default. For old data, { cifControl: "lenient" } accepts both: see the FAQ.

Summary of the sources

Step	Rule	Tier
DNI format: 8 digits and a letter	DNI-1	T1 law, T2 official page
DNI letter: mod 23	DNI-2	T2 official page, T3 AEAT note
K/L/M letter: the 7 digits	KLM-2	T3 AEAT note
NIE prefix: X → 0, Y → 1, Z → 2	NIE-2	T2 official page, T3 AEAT note
CIF organisation keys	CIF-2	T1 law
CIF control: digit or letter by key	CIF-3	T3 AEAT note
CIF control arithmetic	CIF-4	T4 convention