What is a rodné číslo?

A rodné číslo (RČ, literally "birth number") is the Czech and Slovak personal identification number assigned at birth or on registration. It encodes the holder's date of birth and gender directly in the digits, making it both an identifier and a compact record of basic personal data.

How does the mod-11 checksum work?

Take the first nine digits as a single integer N. Compute N mod 11. If the remainder is 10, the check digit is 0 (the only documented exception). Otherwise the check digit must equal the remainder exactly. A mismatch means the number contains a transcription error or is fabricated.

Why does the month show 51-62 for some people?

Czech law adds 50 to the month value for female holders: January becomes 51, February 52, and so on up to December = 62. Since 2004, when birth numbers for a given day are exhausted, men use 21-32 and women 71-82 as overflow ranges.

What is the difference between a 9-digit and a 10-digit number?

Numbers issued before 1 January 1954 have only nine digits — six for the birth date and three for the sequence. No check digit was assigned. The validator accepts these and confirms structural validity without a checksum test.

Does a valid result mean the person exists in a Czech database?

No. The validator checks mathematical structure only — correct length, valid encoded date, and passing mod-11 checksum. It has no access to the Czech Ministry of Interior registry and cannot confirm whether the number has actually been issued to a real person.

Is the number safe to enter here?

The entire calculation runs inside your browser using JavaScript. No data is sent to any server, logged, or stored. You can verify this with your browser developer tools — there are zero network requests when you type.

Czech Rodné číslo Validator

The Czech rodné číslo (RČ) is the national personal identification number used in the Czech Republic (and historically in Slovakia before the 1993 split). It appears on national ID cards, travel documents, tax forms, health insurance records, and most official correspondence. Unlike a purely arbitrary sequence, the number carries real semantic content: the holder’s date of birth and gender are encoded directly in the first six digits, so a validator can extract both without any external lookup.

This tool verifies the number against the official mod-11 checksum algorithm, confirms the encoded date is a real calendar date, decodes the gender from the month offset, and explains exactly which check — if any — failed.

Structure of a rodné číslo

A modern (post-1953) rodné číslo looks like this:

  Y  Y  M  M  D  D  /  S  S  S  C
  └──┴──┘  └──┴──┘  └──┴──┘  └──┴──┘  └─┘
  birth year  month   day    serial  check
  (last 2)   code             (seq)   digit

YY — last two digits of the birth year. Whether this means 1900s or 2000s depends on the total length and value (see Birth-year resolution below).
MM — encodes both the calendar month and the holder’s gender. Men: 01–12 (base) or 21–32 (overflow since 2004). Women: 51–62 (base) or 71–82 (overflow since 2004). Subtract 50 from a female code, or 20/70 from an overflow code, to get the real month.
DD — day of birth (01–31, validated against the actual number of days in that month and year).
SSS — three-digit sequential serial, distinguishing people born on the same day.
C — check digit, present only on numbers issued from 1 January 1954 onward. Pre-1954 numbers are nine digits and carry no check digit.

The slash between the sixth and seventh digit is conventional formatting — it carries no mathematical meaning and is stripped before validation.

The mod-11 algorithm explained

Take the first nine digits (the six-digit date block plus the three-digit serial) and interpret them as a single integer N.
Compute N mod 11. This gives a remainder between 0 and 10.
Edge case: if the remainder is exactly 10, the check digit must be 0 (the result is taken mod 10 once more). This is an explicit design decision in the Czech standard — otherwise no valid number could end in 0.
For all other remainders, the check digit C must equal the remainder exactly.
If C != (N mod 11 == 10 ? 0 : N mod 11), the number fails validation.

Example with a deliberately fake number (900101/1234, used only for illustration — not a real person’s data):

Prefix nine digits: 900101123
900101123 mod 11 = 4
Check digit must be 4 — and the final digit here is 4, so it passes.

Birth-year resolution

A 10-digit number with YY between 54 and 99 belongs to 1954–1999. When YY is between 00 and 53, the birth year is 2000–2053. A 9-digit (pre-1954) number always maps to 1900–1953.

Why this matters

Because the date and gender are embedded in the number, a single typo can produce a date that does not exist (e.g. 30 February) or a checksum mismatch. Both failures are caught here with an explanation of exactly which rule was violated, making it easy to spot the error position before submitting a form or processing a document.