How to calculate the verification digit of the Tax ID in the country of Paraguay (calcular digito verificador del RUC)

Question

In the country of Paraguay (South America) each taxpayer has a Tax ID (called RUC: Registro Único del Contribuyente) assigned by the government (Ministerio de Hacienda, Secretaría de Tributación).

This RUC is a number followed by a verification digit (dígito verificador), for example 123456-0. The government tells you the verification digit when you request your RUC.

Is there a way for me to calculate the verification digit based on the RUC? Is it a known formula?

In my case, I have a database of suppliers and customers, collected over the years by several employees of the company.
Now I need to run checks to see if all the RUCs were entered correctly or if there are typing mistakes.

My preference would be a Python solution, but I’ll take whatever solutions I get to point me in the right direction.

Edit: This is a self-answer to share knowledge that took me hours/days to find. I marked this question as “answer your own question” (don’t know if that changes anything).

Asked By: Ralf

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Source

Answer 1

The verification digit of the RUC is calculated using formula very similar (but not equal) to a method called Modulo 11; that is at least the info I got reading the following tech sites (content is in Spanish):

I analyzed the solutions provided in the mentioned pages and ran my own tests against a list of RUCs and their known verification digits, which led me to a final formula that returns the expected output, but which is DIFFERENT from the solutions in the mentioned links.

Update march 2023: here is the official documentation from SET (a government agency) https://www.set.gov.py/portal/PARAGUAY-SET/detail?content-id=/repository/collaboration/sites/PARAGUAY-SET/documents/herramientas/digito-verificador.pdf

The final formula I got to calculate the verification digit of the RUC is shown in this example (80009735-1):

Multiply each digit of the RUC (without considering the verification digit) by a factor based on the position of the digit within the RUC (starting from the right side of the RUC) and sum all the results of these multiplications:

 RUC:             8       0       0       0       9       7       3       5

 Position:        7       6       5       4       3       2       1       0

 Multiplications: 8x(7+2) 0x(6+2) 0x(5+2) 0x(4+2) 9x(3+2) 7x(2+2) 3x(1+2) 5x(0+2)

 Results:         72      0       0       0       45      28      9       10

 Sum of results:  164

Divide the sum by 11 and use the remainder of the division to determine the verification digit:
- If the remainder is greater than 1, the the verification digit is 11 - remainder
- If the remainder is 0 or 1, the the verification digit is 0
In out example:
```
 Sum of results:       164

 Division:             164 / 11    ==>   quotient 14, remainder 10

 Verification digit:   11 - 10     ==>   1
```

Here is my Python version of the formula:

def calculate_dv_of_ruc(input_str):
    # assure that we have a string
    if not isinstance(input_str, str):
        input_str = str(input_str)

    # try to convert to 'int' to validate that it contains only digits.
    # I suspect that this is faster than checking each char independently
    int(input_str)

    base = 11
    k = 2
    the_sum = 0
    for i, c in enumerate(reversed(input_str)):
        if k > base:
            # reset to start value
            k = 2
        the_sum += k * int(c)
        k += 1

    _, rem = divmod(the_sum, base)
    if rem > 1:
        dv = base - rem
    else:
        dv = 0

    return dv

Testing this function it returns the expected results, raising errors when the input has other characters than digits:

>>> calculate_dv_of_ruc(80009735)
1
>>> calculate_dv_of_ruc('80009735')
1
>>> calculate_dv_of_ruc('80009735A')
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "<input>", line 8, in calculate_dv_of_ruc
ValueError: invalid literal for int() with base 10: '80009735A'

Answered By: Ralf

How to calculate the verification digit of the Tax ID in the country of Paraguay (calcular digito verificador del RUC)

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