Three ages, one result card
Korea historically used three systems at once: Korean age (세는 나이), calendar age (연나이, birth-year based), and international age (만 나이, birthday based). Since June 2023, official documents use international age — but 세는 나이 still rules everyday speech, drinking-age banter and K-pop fandom math.
The result card computes all three from your birth date so you can see exactly where the systems disagree — usually by one or two years, never more.
How Korean age (세는 나이) works
You are 1 on the day you are born — gestation counts as your first year. On 1 January everyone in Korea gains a year simultaneously, regardless of birthday. So a baby born on 31 December is 2 years old (세는 나이) the very next day.
That makes the formula simple: Korean age = current year − birth year + 1. No birthdays involved — which is also why the number feels so inflated compared to international age.