What are the two mod-11 control digits and how are they computed?

Digits 10 and 11 of the fødselsnummer are control digits computed with the Norwegian mod-11 algorithm. K1 is calculated by multiplying digits 1-9 by the weights [3, 7, 6, 1, 8, 9, 4, 5, 2], summing the products, taking the result mod 11, and computing 11 minus that remainder (0 stays 0; a remainder of 10 means no valid digit exists). K2 uses the same process over digits 1-10 with weights [5, 4, 3, 2, 7, 6, 5, 4, 3, 2]. Both computed values must exactly match the corresponding digits in the number.

Why does my number pass the format and length checks but fail on K1 or K2?

The mod-11 checksum is independent of whether the digits look reasonable. A transposition of two digits — for example swapping the 8th and 9th — will usually change the weighted sum and produce a different control digit. The validator shows you the expected value so you can spot and correct the error.

How does the validator determine the birth year and century?

The fødselsnummer encodes only a two-digit year. The century is resolved by combining that year with the three-digit individual number (digits 7-9). Individual numbers 000-499 always indicate 1900-1999. Numbers 500-749 with a year of 54-99 indicate 1855-1899. Numbers 500-999 with a year of 00-39 indicate 2000-2039. Numbers 900-999 with a year of 40-99 also fall in 1940-1999. The validator applies these rules and checks whether the resulting date is a real calendar date.

What is a D-number and how is it detected?

A D-number is a temporary identity number issued to people with a short-term relationship to Norway — such as foreign workers, students or seasonal employees. It is identified by the fact that 40 is added to the day of birth, so the first digit of the number is between 4 and 7 instead of 0 and 3. The validator subtracts 40 before date validation and labels the result as a D-number.

An H-number (help number) is a temporary internal identifier used by some Norwegian healthcare and administrative systems when no real fødselsnummer is available. It is detected because 40 is added to the month value, pushing the third digit above 4. Like D-numbers, H-numbers still carry valid checksums — the validator handles both transparently.

Does a passing result mean the number belongs to a real person?

No. A pass confirms structural validity only — the number has the correct length, encodes a plausible calendar date and passes both mod-11 checksums. Whether the number was actually issued, is currently active or belongs to a specific individual can only be confirmed through the Norwegian National Registry (Folkeregisteret). The validator never contacts any external service.

Norway Fødselsnummer Validator

The Norwegian fødselsnummer is the 11-digit national identity number assigned to all residents of Norway by the National Registry (Folkeregisteret). Beyond being a simple reference code, it encodes a person’s date of birth, a gender indicator and two independently computed mod-11 control digits that catch most transcription errors. This validator checks every one of those rules in sequence, tells you exactly which check failed and why, and extracts all embedded information when the number is fully valid.

Structure of the fødselsnummer

The 11 digits divide into three parts:

D D M M Y Y  I I I  K1 K2
↑           ↑      ↑
Birth date  Indiv. Control digits
(6 digits)  (3)    (2)

Date of birth (DDMMYY): The first six digits encode day, month and two-digit year. The century is not stored explicitly — it is derived from the individual number combined with the two-digit year (see the FAQ above for the exact rules).

Individual number (III): A three-digit sequence, allocated sequentially within each birth date by the National Registry. The last digit of this group encodes gender: odd means male (Mann), even means female (Kvinne).

Control digits (K1, K2): Two digits computed from the preceding digits using the Norwegian mod-11 algorithm with two distinct weight vectors. They exist solely to detect errors.

The mod-11 algorithm in detail

Both control digits follow the same calculation pattern:

Take the applicable slice of digits (digits 1-9 for K1; digits 1-10 for K2).
Multiply each digit by its corresponding weight from the official weight vector.
Sum all products.
Compute sum mod 11.
If the remainder is 0, the control digit is 0.
Otherwise the control digit is 11 - remainder.
If step 6 produces 10, no valid digit exists — the input string cannot be a legitimate fødselsnummer.

K1 weights: 3 7 6 1 8 9 4 5 2 (applied to digits 1 through 9)

K2 weights: 5 4 3 2 7 6 5 4 3 2 (applied to digits 1 through 10, including K1)

Because K2 depends on K1, a single digit error will typically fail both checks, while a transposition near the end may only fail K2. The step-by-step trace in the validator makes it immediately clear which control digit diverges and what the correct value is.

Example walkthrough

Consider the (obviously fictional) 11-digit sequence where the date portion reads 010190 (1 January 1990), individual number 124 (last digit 4 — even — female), and then two control digits that the algorithm computes. Applying K1 weights to the first 9 digits gives a certain weighted sum; 11 - (sum mod 11) yields the expected K1. The same process over the first 10 digits yields K2. If the supplied digits match those computed values the validator shows five green ticks and surfaces:

Field	Extracted value
Number type	Ordinary
Date of birth	1 January 1990
Gender	Female (Kvinne)
Individual number	124
Checksums	Both pass mod-11

Changing any single digit — including the control digits themselves — will produce a different checksum and turn at least one check red with an explanation.

Special number types

D-numbers are issued to people with a temporary relationship to Norway (foreign workers, students, seasonal visitors). They are identical in structure to ordinary fødselsnummers except that 40 is added to the day digit, making the first digit 4-7. The validator detects this, subtracts 40 to recover the real day and labels the result as a D-number.

H-numbers are internal help identifiers used by healthcare and administrative systems when a real fødselsnummer is unavailable. They add 40 to the month, shifting the third digit. The validator handles H-numbers the same way.

Both types carry the same two mod-11 control digits and are validated identically.

Privacy

Every calculation runs entirely in your browser using plain JavaScript arithmetic. No network request is made. The number you enter never leaves your device. The demo sample shown in the tool is generated client-side from a fully fictional date and has never been issued to any real person.