What a Romanian CNP says, digit by digit

How the last digit is calculated#

Each of the first twelve digits is multiplied by the digit in the same position of a fixed number, 279146358279. The products are added up, the sum is divided by 11, and the remainder becomes the thirteenth digit. The calculation below runs on the code above and is redone at every change.

A remainder after dividing by 11 can be anything from 0 to 10. There is no single digit for ten, so the rule writes 1. The old ten-digit ISBN on books, which divides by 11 in the same way, uses the letter X here; the CNP has no letters, and choosing 1 has two consequences, in the next two chapters.

The rules that define the CNP, approved by Government Decision 1.375/2006, say only that the check digit is "assigned by the computer" and that it "allows the detection of possible errors of substitution or inversion of digits". The constant 279146358279 and the rule for a remainder of 10 are not in the published text. Public CNP validators use them, from encyclopaedias to the code libraries behind online forms.

Why 2 in 11 CNPs end in 1#

For people born between 1900 and 2025 there can be 4,413,597,984 valid codes: every day, every one of the 48 county and sector codes, every sequence number from 001 to 999, for both sexes. Counted exactly, the remainders after dividing by 11 split into eleven parts equal to eight decimal places. The digit 1 gets two of them, remainder 1 and remainder 10, so 18.18% of all codes. Every other digit gets one, 9.09%.

Real codes do not use every sequence number, because few children are born in one county on one day. The share holds: even if sequence numbers stopped at 050, the digit 1 would still come out at 18.18%. Among real CNPs too, 1 should be the last digit twice as often as any other.

Which typos the check digit catches, and which it misses#

The weights 2, 7, 9, 1, 4, 6, 3, 5, 8, 2, 7, 9 have a useful property: a single wrong digit, or two neighbouring digits swapped, among the first twelve always changes the remainder after dividing by 11. If a remainder of 10 had a symbol of its own, like the X in an ISBN, every such error would be caught.

The rule breaks that guarantee in one place. When an error moves the remainder from 1 to 10, or from 10 to 1, the last digit still comes out 1 and the wrong code looks right. This can only happen to codes that end in 1. Try it on the code above:

Of 761,762 simulated typos in 400,000 valid codes, 1.85% of wrong digits and 1.95% of swaps went unnoticed: about one in 53. All of them were in codes ending in 1. For those, one single-digit typo in ten gets through (10.2%). For a CNP ending in any other digit, no error among the first twelve digits gets through.

The county codes: in alphabetical order, with no exceptions#

Codes 01 to 39 follow the counties of 1978 in alphabetical order, from 01 Alba to 39 Vrancea. Wikipedia says the order has "some exceptions". Without diacritics, they would be Brașov (08) before Brăila (09), Satu Mare (30) before Sălaj (31) and Vaslui (37) before Vâlcea (38). But in the Romanian alphabet ă and â come after a, so the list is alphabetical from end to end.

After the counties comes Bucharest: 40 for the city, then 41–46 for sectors 1–6. Codes 47 and 48 belonged to sectors 7 and 8, and 51 and 52 are Călărași and Giurgiu, counties created in 1981.

The county code does not prove the place of birth. The rules say JJ is the code of the county or sector "where the person was born or where they had their domicile or residence when the CNP was given". For someone who got the code as an adult, it can be the county where they lived then. And 51 and 52 appear for nobody who got their code before 1981, even if they were born in Călărași or Giurgiu.

Why codes 47 and 48 are missing#

In 1978, when the CNP was introduced, Bucharest had eight sectors. By State Council Decree 284 of 31 July 1979, from 4 August 1979 six remained: sectors 1 and 8 merged into sector 1, sectors 2 and 3 into sector 2, and 4, 5, 6 and 7 became 3, 4, 5 and 6.

After August 1979 there was no sector 7 or 8 left to assign a code to, so 47 and 48 survive only on codes given before then. The people who received them then still carry them: a CNP does not change when the map does.

The same is true, less visibly, of codes 41–46. If they too followed the sector numbers of the time, as 47 and 48 show, a code 44 given before August 1979 means the old sector 4, today's sector 3, and 47 means today's sector 6. The map above shows the sectors as they have been since 1979.

Since 2024, code 70: the county disappears from new CNPs#

On 11 May 2024, Romania's Interior Ministry announced that CNPs generated by the new civil-status computer system "contain the single code 70 for every registration, whatever the county or place where the birth took place". The system started partly on 8 May 2024 and was due to reach the whole country by 24 September 2024. CNPs given before stay as they are, an official of the population-records directorate told the newspaper Adevărul.

For people registered in the new system, digits 8 and 9 say nothing about place. After forty-six years, the geographical part of the code stops with those who received their CNP before the new system reached their office, between May and September 2024.

Where a CNP comes from: lists calculated in advance#

Codes are generated centrally. The rules give the task to the National Centre for the Administration of Population Records Databases (today a directorate of the Interior Ministry), which "issues and distributes the corresponding lists every year" to the county services and to Bucharest's. A newborn's code "is taken from the precalculated lists" and written into the birth certificate.

The sequence number, digits 10–12, is a "sequence number (between 001 and 999), allotted to points of assignment", which tells apart people of the same sex, born on the same day and registered with the same county code. How the 999 numbers are split between the offices of one county is not published.

Are there enough? In 1967, the year with the most births in the Eurostat series (527,764 births), an average of 1,446 children were born each day in the whole country, about half of them boys, so under 800. The 999 numbers of a single county, for one day and one sex, would have covered the boys born in the whole country on an average day that year; in the peak months after Decree 770 there were probably more. In 2025, with 134,238 births (provisional), the average was 368 a day.

The 1978 decree required the code to be "written in identity papers, civil-status records and other personal documents", and children under 14 to be entered in the records "within two years".

Two people with the same CNP#

It happens, and the rules provide for it: they give the same national centre the task of "resolving cases of duplicate or wrong personal numeric codes", and anyone with a wrong or duplicate code gets a new one, with new papers. In 2009 the newspaper Evenimentul Zilei reported on two men with almost identical names who shared a CNP. A records officer in Iași put such cases down to old documents in which the numbers had been written by hand; the article also mentions codes copied wrongly at civil-status offices.

The check digit cannot catch a duplicate. It is calculated from the first twelve digits, so two codes identical up to there always share the thirteenth. It shows that a code is written correctly; whether it was given to someone else, only the register can tell. How many duplicates exist is not published.

What anyone who sees your CNP learns#

Anyone who has your code learns, with no other document, your sex, date of birth and county code: where you were born or where you lived when you got the code. The sequence number says nothing about the person, and the check digit is worked out from the rest.

The other way round, a CNP is easy to guess. Someone who knows your date of birth, sex and county faces at most 999 possibilities, and for a code 70 the date and sex are enough. The code identifies you; as proof that you are the one giving it, it is worth no more than any fact others can find out.

Elsewhere: the American number that also gave away a place#

The United States has a nine-digit Social Security number. According to the Social Security Administration, from 1972 its first three digits, the area number, reflected the state of the mailing address on the application. On 25 June 2011 the administration began assigning numbers at random, saying the change would help protect the integrity of the number and make the nine-digit format last longer.

Romania's code went the same way thirteen years later: code 70 since 2024 does to the county what randomisation did to the American state. Unlike the American number, the CNP still carries the full date of birth.