Finnish Henkilötunnus Validator

Henkilötunnus: Finland's personal identity code with a century sign

The Finnish henkilötunnus (HETU) is the national personal identity code, formatted DDMMYY + a century sign + a 3-digit serial + a check character — e.g. 131052-308T. What makes it distinctive is the century separator: a single character that disambiguates whether the two-digit year is in the 1800s, 1900s or 2000s. This tool validates the date, the sign and the mod-31 check character.

The final character isn't a digit but one of 0–9 plus a set of consonant letters — it's the remainder of the 9-digit number (date + serial) modulo 31, mapped through the alphabet 0123456789ABCDEFHJKLMNPRSTUVWXY.

Reading 131052-308T

• DDMMYY (131052): 13 October 1952.
• Century sign: + = 1800s; -, Y X W V U = 1900s; A B C D E F = 2000s.
• Serial (308): individual number; odd = male, even = female.
• Check character (T): the mod-31 control mapped to the 31-symbol alphabet.

Where it matters

• Everything official: healthcare (Kela), tax, banking and the Suomi.fi e-services.
• New century letters: extra signs (Y, X, W…, B, C…) were added so the format wouldn't run out of capacity.
• Sex from the serial: the serial's parity encodes sex.
• Mock data / testing: the mod-31 rule lets you generate valid-format test codes.

Common pitfalls

• Don't ignore the century sign: - and A mean different centuries for the same YY.
• Check char skips letters: the alphabet omits G, I, O, Q and vowels that look like digits — don't assume A–Z.
• Uppercase: normalise both the sign and the check character to uppercase.
• Valid ≠ registered: a well-formed HETU isn't necessarily an assigned one.

FAQ

What is the century sign? A single character that says which century the YY belongs to — essential because the year is only two digits.

How is the check character computed? Take the 9-digit number (DDMMYY + serial) modulo 31 and map it through a 31-symbol alphabet.

Why were new signs added? To expand capacity; the original -/+/A set was supplemented with extra century letters.