Estonian ID code validator and decoder

The parts of an ID code, using a made-up example

Let us walk through the structure with the code 49007151236. It was invented for this article: the checksum is correct, but the code is not tied to anyone. The calculator returns "Valid", female, born 15.07.1990, and an age of 36 in September 2026.

• 4: century and sex. An even digit means female, and 3 or 4 means born in the 1900s. The calculator also accepts 7 and 8, reserved for people born in the 2100s.
• 900715: birth date as YYMMDD, so year 90, month 07 and day 15. The century comes from the first digit.
• 123: a serial number that separates people of the same sex and century born on the same day.
• 6: the checksum, computed from the first ten digits.

Calculating the checksum step by step

The first ten digits 4, 9, 0, 0, 7, 1, 5, 1, 2, 3 are multiplied in turn by the weights 1, 2, 3, 4, 5, 6, 7, 8, 9, 1:

The sum is 4 + 18 + 0 + 0 + 35 + 6 + 35 + 8 + 18 + 3 = 127. Dividing by 11 gives 11 with a remainder of 6 (11 × 11 = 121, 127 - 121 = 6). Since the remainder is less than 10, the checksum is 6, which matches the last digit of the code.

• 4 × 1 = 4, 9 × 2 = 18, 0 × 3 = 0, 0 × 4 = 0, 7 × 5 = 35
• 1 × 6 = 6, 5 × 7 = 35, 1 × 8 = 8, 2 × 9 = 18, 3 × 1 = 3

When the remainder is 10: the second weight row

A second made-up code, 38502280454, belongs to a man born on 28.02.1985. With the first weight row the sum is 3 + 16 + 15 + 0 + 10 + 12 + 56 + 0 + 36 + 5 = 153, and 153 divided by 11 leaves a remainder of 10. A checksum cannot have two digits, so the calculation is repeated with the weights 3, 4, 5, 6, 7, 8, 9, 1, 2, 3.

The new sum is 9 + 32 + 25 + 0 + 14 + 16 + 72 + 0 + 8 + 15 = 191. 11 × 17 = 187, remainder 191 - 187 = 4, so the checksum is 4. If the second row also left a remainder of 10, the checksum would be 0.

Which errors the check catches and which it misses

The weights are chosen so that a typo in one digit or two swapped neighbouring digits usually break the checksum. If you swap the 2 and 3 in the example 49007151236 (giving 49007151326), the calculator reports "The checksum does not match".

Not every error is caught, though. The first and tenth digits have the same weight in both rows, so swapping them leaves the sum unchanged. 39007151246 is also a valid code, but now for a man with serial number 124. If a code looks suspicious, compare it with the document.

• 48802290313: valid, because 1988 was a leap year and 29 February existed.
• 48902290311: error "The birth date is not valid", because there was no 29 February in 1989. The date is checked before the checksum.
• 99007151236: error, because the first digit must be 1-8.
• 4900715123: error, because it has 10 digits instead of 11. Spaces are fine; the calculator strips them.

What to do with the result, and what not to

A valid code only shows that the digits are consistent with each other. It does not prove that the person giving the code owns it, so the check is not a way to verify identity. In forms and databases, however, it is a good first line of defence against typos.

Because an ID code reveals a person's birth date, age and sex, it is personal data. Do not post it on social media, in photos of documents or in public spreadsheets. If you build software, store the code as text rather than a number so digits are never lost or rounded.