Omocodia in Italian Tax Code: What It Is and How It Works

What is Omocodia

Omocodia is a phenomenon that occurs when two or more people, applying the standard Tax Code calculation algorithm, obtain the same identical code.

This happens because the Tax Code is calculated only based on: surname, first name, date of birth, gender, and municipality of birth. If all these data match for two different people, their codes would theoretically be identical.

The term comes from the Greek "homo" (same) and "code", literally meaning "same code".

When Does Omocodia Occur?

Omocodia is more likely to occur when:

• Common names and surnames: Like Mario Rossi, Giuseppe Russo
• Large cities: Where many people are born on the same day
• Same birth date: Especially January 1st (common for unspecified dates)
• Twins or siblings: With similar names born on the same day

Practical Example

Two men named Mario Rossi, both born on March 15, 1980in Rome, would have the same calculated Tax Code:

RSSMRA80C15H501X

To distinguish them, the Revenue Agency modifies one or more digits by replacing them with letters.

Omocodia Substitution Table

The Revenue Agencyresolves omocodia by progressively replacing certain digits with letters using this table:

Order of Substitution

Digits are replaced from right to left in this order of positions:

1. Position 15 (last digit of municipality code)
2. Position 14
3. Position 13
4. Position 12
5. Position 11 (day - units)
6. Position 10 (day - tens)
7. Position 7 (year)

Example with Omocodia

The Tax Code RSSMRA80C15H501Xwith first-level omocodia becomes:

RSSMRA80C15H50MX

Where the 1 at position 15 is replaced by M.

Omocodia Levels

There can be up to 7 levels of omocodia, progressively replacing more digits:

Practical Implications

For Citizens

• A Tax Code with omocodia is perfectly valid
• It cannot be calculated with standard calculators
• It must be used exactly as issued by the Revenue Agency
• It does not require any additional documentation

For Software Developers

• Validation systems must account for omocodia substitutions
• The control character is recalculated after substitutions
• Both the original and omocodia code should be considered valid

How to Verify an Omocodia Code

To check if your Tax Code has omocodia:

1. Calculate your "base" Tax Code using our calculator
2. Compare it with your official Tax Code on your Health Card or certificate
3. If they differ only in digit-to-letter substitutions, you have omocodia
4. Verify the control character is still valid

Frequently Asked Questions