The traditional count
The old way of counting age in Korea, called seneun nai (세는 나이, "counted age"), follows two rules.
- You are 1 at birth. The first year starts on the day you are born, not after it ends.
- Everyone adds a year at the new year. Your birthday does not change your age. The new year does, for everyone at the same time.
Put those two rules together and you get the famous puzzle: a baby born on December 31 is 1 that day and 2 the next morning, when the new year starts. Two days old, two years old.
What changed in 2023
On June 28, 2023, a law took effect that made international age the standard for administrative and civil matters in South Korea. Contracts, official documents and public services now use the age you would give anywhere else in the world: zero at birth, plus one on each birthday. In Korean this is called man nai (만 나이).
A few rules still work by birth year instead, for example when children start school and when military service obligations begin. That is one reason Koreans still group classmates by the year they were born.
Many people also keep using the old count in daily conversation, especially older Koreans. If someone tells you their age, it is fair to ask which count they mean.
How to convert Korean age to international age
The government's own rule of thumb is simple:
- Birthday already passed this year: subtract 1 from the traditional Korean age.
- Birthday not yet this year: subtract 2.
So someone who is "25 in Korean age" is 24 or 23 in international age, depending on the date. Our Korean age calculator does this for any birth date.
Why it lasted so long
The traditional count is easy in a society where birthdays were not always written down: you only need to know the year someone was born. It also puts everyone born in the same year at the same age, which matters in a language where you choose your words by who is older. The 2023 law did not try to change how people talk. It changed which number counts on paper.
FAQ
How old am I in Korean age?
In the traditional count, take the current year, subtract your birth year, and add 1. Since June 28, 2023, official documents in South Korea use international age instead.
Does Korea still use Korean age?
Not for official purposes. Since June 28, 2023, administrative and civil age is international age. Some areas, such as school entry and military service, count by calendar year, and many people still use the old count in conversation.
Why was a baby born on December 31 two years old the next day?
Because the traditional count starts at 1 on the day of birth and adds a year for everyone when the new year starts.
Sources
- Ministry of Government Legislation: "From now on, international age is my age" (Korean)
- Korea Policy Briefing: what changes and what does not under the age law (Korean)
- Korea Policy Briefing: the international age law takes effect on the 28th (Korean)
- Korea Policy Briefing: exceptions to international age (Korean)
Written by the Korea in Time team. Each fact above is checked against the sources listed. Found a mistake? Tell us.
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