What is a Danish CPR number?

CPR stands for Det Centrale Personregister (the Central Civil Register). Every person who lives in Denmark — Danish citizen or foreign resident — is assigned a unique 10-digit identifier in the format DDMMYY-SSSS. The first six digits encode the date of birth, and the four-digit serial both distinguishes people born on the same day and encodes century and gender information.

How is the birth century determined from only two year digits?

The first digit of the 4-digit serial (the 7th overall digit) is the century indicator. The official rule is: digits 0–3 always map to 1900–1999. Digit 4 or 9 maps to 2000–2036 when YY is 00–36, otherwise 1900–1999. Digits 5–8 map to 2000–2057 when YY is 00–57, otherwise 1858–1957. This means the CPR system can represent birth years from 1858 all the way to 2057.

How do you read gender from a CPR number?

The very last digit (the 10th digit, also the 4th digit of the serial) encodes gender. An odd last digit means the person is male; an even last digit means female. This convention has been part of the CPR format since its introduction in 1968.

What is the mod-11 checksum and why does it sometimes fail on valid numbers?

The original CPR design included a modulus-11 control check. Each of the 10 digits is multiplied by a fixed weight (4, 3, 2, 7, 6, 5, 4, 3, 2, 1) and the products are summed; the sum must be divisible by 11 (remainder 0) for the legacy check to pass. However, the Danish CPR Office officially abandoned this rule on 1 October 2007 because the number space satisfying the constraint was exhausted — there were not enough valid combinations left to assign to new citizens. Numbers issued from that date onwards intentionally fail mod-11 yet are perfectly legal CPR numbers. This validator shows the mod-11 result as a historical reference but does not use it to decide overall validity.

What makes a CPR number structurally invalid?

This tool flags a CPR as structurally invalid if the date portion encodes an impossible calendar date — for example month 13, day 32, or 29 February in a non-leap year. A correctly formatted 10-digit number with a valid date is considered structurally valid. The tool does not have access to any civil register, so it cannot tell you whether a structurally valid number has ever actually been assigned to a person.

Is it safe to enter a real CPR number here?

The tool is entirely client-side. No data is transmitted, logged, or stored anywhere. The JavaScript validation runs only in your own browser. That said, a CPR number is sensitive personal data under Danish and EU law; you should treat it with the same care as any government identity number and avoid entering real numbers into any online tool unless you are certain of its privacy practices.

Denmark CPR Number Validator

The Denmark CPR number validator checks every structural rule of the Danish personnummer in your browser without sending a single character to any server. Paste or type a number and get an instant verdict — along with the full decoded birth date, the correct birth century, the gender, and an explanation of whether the legacy mod-11 checksum applies to this number.

What the CPR number is

The Det Centrale Personregister (CPR) is Denmark’s central civil register, and every resident — citizen or foreign national — receives a unique 10-digit identifier. The format is DDMMYY-SSSS: six digits for the date of birth followed by a hyphen and four serial digits. The structure packs four pieces of information into 10 digits: the date of birth, the birth century (via the century-indicator digit), the individual serial within a birth-date cohort, and the person’s gender.

How the date and century work

The first six digits are the date: DD (day), MM (month), YY (two-digit year). Because two digits are ambiguous about the century, the 7th digit — the first digit of the 4-digit serial — acts as a century indicator:

Serial digit 7	YY range	Birth year
0 – 3	any	1900 + YY
4 or 9	00 – 36	2000 + YY
4 or 9	37 – 99	1900 + YY
5 – 8	00 – 57	2000 + YY
5 – 8	58 – 99	1858 + YY

This means the system can represent birth years from 1858 through 2057 — a 200-year window. The validator applies this exact table and then verifies that the resulting calendar date is real, including correct leap-year logic for February 29.

Gender from the last digit

The 10th digit (last digit of the serial) encodes gender: odd = male (mand), even = female (kvinde). This rule has been in place since the CPR system launched in 1968 and has never changed.

The mod-11 checksum — history and current status

The original CPR design included a modulus-11 control digit. The algorithm multiplies each of the 10 CPR digits by a weight from the sequence 4, 3, 2, 7, 6, 5, 4, 3, 2, 1 and sums the products. If the sum is divisible by 11 (remainder = 0) the legacy check passes.

On 1 October 2007 the Danish CPR Office officially retired this requirement. The number space of combinations satisfying mod-11 had simply run out — there were not enough valid 10-digit combinations to assign to new residents. From that date, numbers are issued freely without the constraint, and a failing mod-11 is normal and legal for anyone born after that policy change, or registered in Denmark after that date.

This validator reports the mod-11 result with full workings (weighted sum, remainder) as a historical transparency measure, but it does not use mod-11 to determine overall validity. Only an impossible calendar date makes a number structurally invalid here.

Example (obviously fake number)

The number 010190-1234 decodes as follows:

Day: 01 (the 1st)
Month: 01 (January)
YY: 90
Serial digit 7: 1 — maps to century prefix 1900, so full year = 1990
Date: 1 January 1990 (a valid calendar date)
Last digit: 4 (even) → Female
Mod-11: 0×4 + 1×3 + 0×2 + 1×7 + 9×6 + 0×5 + 1×4 + 2×3 + 3×2 + 4×1 = 3 + 7 + 54 + 4 + 6 + 6 + 4 = 84; 84 mod 11 = 7 → fails (normal for a post-2007 issuance pattern)

All calculation steps are shown live in the tool as you type.