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Para calcular la letra del DNI, divide su número de ocho cifras entre 23 y usa el resto para elegir una letra de TRWAGMYFPDXBNJZSQVHLCKE. Para un NIE estándar, sustituye la X, Y o Z inicial por 0, 1 o 2 y aplica la misma operación. El resultado comprueba que la letra coincide con el número según esta regla; no confirma que el identificador se haya expedido ni quién es su titular.
Cómo calcular la letra del DNI
- Usa el número del DNI, sin la letra final. En su formato estándar tiene ocho cifras.
- Divide ese número entre 23 y toma el resto, que estará entre 0 y 22. En programación, la operación es
n % 23. - Cuenta desde cero en la cadena
TRWAGMYFPDXBNJZSQVHLCKE: el resto es la posición de la letra correspondiente.
Por ejemplo, el resto 0 corresponde a T, el resto 1 a R y el resto 22 a E. La cadena tiene 23 caracteres, uno por cada resto posible.
Cómo calcular la letra del NIE
En el cálculo estándar del NIE, primero se reemplaza la letra inicial por un dígito: X por 0, Y por 1 y Z por 2. Se conservan las cifras que siguen, se interpreta el resultado como un número y se aplica el mismo módulo 23 y la misma cadena de letras.
Por ejemplo, un NIE que empieza por X se prepara con un 0 en su lugar antes de calcular el resto. No se debe aplicar el cálculo al NIE dejando la letra inicial dentro del número como si fuera una cifra.
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Cálculo con Python
Esta función acepta un DNI de solo cifras o un NIE con X, Y o Z inicial, ignora espacios exteriores y no distingue entre mayúsculas y minúsculas en el prefijo:
LETTERS = "TRWAGMYFPDXBNJZSQVHLCKE"
NIE_PREFIX = {"X": "0", "Y": "1", "Z": "2"}
def check_letter(identifier: str) -> str:
value = identifier.strip().upper()
if value[:1] in NIE_PREFIX:
value = NIE_PREFIX[value[0]] + value[1:]
if not value.isdigit():
raise ValueError("Expected a DNI number or an X/Y/Z-prefixed NIE")
return LETTERS[int(value) % 23]El ejemplo solo calcula la letra esperada; no valida por completo la estructura del documento. Para usarlo en una aplicación, añade comprobaciones de longitud y formato para cada tipo de identificador. También conviene presentar el resultado como una coincidencia de letra de control, no como verificación de identidad.
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DNI, NIE y NIF: qué diferencia hay
- DNI: documento nacional de identidad español. El Real Decreto 255/2025 regula el DNI físico y digital y lo describe como documento personal e intransferible. Consulta el Real Decreto 255/2025 en el BOE.
- NIE: identificador asignado a personas sin nacionalidad española conforme a las normas aplicables. El formato habitual empieza por X, Y o Z, seguido de cifras y una letra de control.
- NIF: identificador a efectos tributarios. Según el artículo 19.1 del Real Decreto 1065/2007, el NIF de una persona española es el número del DNI seguido de su carácter de verificación en mayúscula. Para personas extranjeras, por regla general se utiliza el NIE asignado; la normativa contempla además un NIF que empieza por M en determinados casos sin NIE. Consulta el Real Decreto 1065/2007, artículos 18–20, en el BOE.
Por tanto, la letra de control forma parte del NIF de una persona española, pero DNI, NIE y NIF no son términos intercambiables.
Qué demuestra y qué no demuestra la letra
Si la letra calculada coincide con la que aparece junto al número, ambos concuerdan con la regla de control. Eso no demuestra que el número exista, que se haya expedido un documento con esos datos, que siga siendo válido ni que pertenezca a quien lo presenta.
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El BOE confirma la función de la letra de verificación del DNI y recoge formatos de identificador, pero las disposiciones citadas no detallan la cadena de 23 letras ni la sustitución X/Y/Z por 0/1/2. El método descrito aquí es el cálculo estándar empleado para obtener la letra, no una consulta oficial de identidad. Consulta la disposición 21783 del BOE de 29 de diciembre de 2021.
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