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Validate a Turkish tax number (VKN) in C#

The Turkish tax number (Vergi Kimlik Numarası, VKN) has 10 digits. The last digit is a check digit calculated from the first nine. Companies use a VKN. Sole proprietors use the owner's identity number (TCKN) as tax number. This page explains the algorithm and shows a check that accepts both.

Short answer

using HeroKit.Globalization;
using HeroKit.Globalization.Turkey;

"4840847211".IsValidVkn();                     // true
"4840847212".IsValidVkn();                     // false
"10000000146".IsValidTaxId(Countries.Turkey);  // true: sole proprietor, TCKN
"4840847211".IsValidTaxId(Countries.Turkey);   // true: company, VKN

dotnet add package HeroKit

Rules

  1. The number has exactly 10 digits.
  2. For each of the first 9 digits, with p the position (1–9):
    • a = (digit + 10 − p) mod 10
    • b = (a × 2^(10 − p)) mod 9
    • if a is not zero and b is zero, b = 9.
  3. Add up all b values. The 10th digit is (10 − sum mod 10) mod 10.

Example: 4840847211

Position Digit a 2^(10 − p) b
1 4 3 512 6
2 8 6 256 6
3 4 1 128 2
4 0 6 64 6
5 8 3 32 6
6 4 8 16 2
7 7 0 8 0
8 2 4 4 7
9 1 2 2 4

The sum is 39. The check digit is (10 − 9) mod 10 = 1, which is the last digit, so the number is valid.

Plain C# without a library

static bool IsValidVkn(string? value)
{
    if (value is null || value.Length != 10 || !value.All(char.IsAsciiDigit))
        return false;

    int sum = 0;
    for (int i = 0; i < 9; i++)                         // i = position − 1
    {
        int a = (value[i] - '0' + 9 - i) % 10;
        int b = a * (1 << (9 - i)) % 9;                 // 1 << n = 2^n
        if (a != 0 && b == 0)
            b = 9;

        sum += b;
    }

    return (10 - sum % 10) % 10 == value[9] - '0';
}

This code gives the same result as HeroKit's IsValidVkn for 300,000 random numbers.

Common mistakes

  • Missing the 9 rule. When a is not zero and b is zero, b must be 9. Without this rule, some valid numbers are rejected.
  • Rejecting sole proprietors. If an invoice or customer form accepts only a VKN, sole proprietors cannot enter their TCKN. Use IsValidTaxId(Countries.Turkey) or [TaxId("TR")] for that field.
  • Storing it as a number. A VKN can start with 0. Stored as long, the leading zero is lost. Use string.
  • Taking a well-formed number for a registered taxpayer. The algorithm says the number is well formed. It does not say that a taxpayer with this number is registered. Use the services of the Revenue Administration (GİB) for that.

ASP.NET Core form validation

using HeroKit.Validation;

public sealed class InvoiceForm
{
    [Required, TaxId("TR")]   // VKN or TCKN
    public string? TaxNumber { get; set; }

    [Vkn]                     // VKN only
    public string? CompanyTaxNumber { get; set; }
}

With FluentValidation (dotnet add package HeroKit.FluentValidation):

RuleFor(x => x.TaxNumber).NotEmpty().TaxId(Countries.Turkey);
RuleFor(x => x.CompanyTaxNumber).Vkn();

MERSİS number

The 16-digit MERSİS number is "0", the company's VKN and a 5-digit serial:

"0879001756600379".IsValidMersis(); // true
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