One person, three different ages
Ask someone how old they are and the answer seems simple. But the number you get depends on the counting system behind it. In most of the world, age means the number of birthdays you have completed. In traditional Korean age, a newborn is already one year old, and everyone becomes a year older on 1 January. A third method, often called year age, simply subtracts the birth year from the current year.
The same person can therefore be 25, 26 or 27 on the same day. This article explains how each system works, shows you how to calculate them by hand, and covers the 2023 change in South Korea that made "international age" the legal standard.
The international standard: completed years
The most widely used method counts completed years since birth. A baby is 0 on the day they are born, turns 1 on their first birthday, and so on. Your age changes only on your birthday.
How to calculate it by hand
- Subtract the birth year from the current year.
- If this year's birthday has not happened yet, subtract 1.
Example: a person was born on 20 November 2000. In 2026, 2026 minus 2000 is 26. On 26 October 2026 the birthday on 20 November has not yet happened, so the age is 25. From 20 November 2026 it becomes 26.
If you also want months and days, count the full months since your last birthday and then the remaining days. When the day of the month is smaller than your birth day, borrow the length of the previous month. Months have different lengths, so this is why a simple "30 days per month" shortcut can be off by a day or two. The ChronoAge age calculator does this for you using real calendar dates.
Traditional Korean age (세는 나이)
In the traditional Korean counting system, a baby is considered one year old at birth. The first year of life counts as year one. Then everyone adds a year on 1 January, regardless of their own birthday. The formula is:
Korean age = current year minus birth year, plus 1
Example: a baby born on 30 December 2025 is 1 year old at birth. Just two days later, on 1 January 2026, the baby turns 2. In international age the same baby would still be 0 until December 2026.
This means Korean age is always one or two years higher than the completed-years age. For the person born on 20 November 2000, Korean age in 2026 is 2026 minus 2000 plus 1, which is 27. On 26 October that is two years higher than their international age of 25, and after their birthday it is one year higher.
Year age (연 나이)
Year age subtracts the birth year from the current year, with no extra year added and no regard to the birthday. For the same person in 2026, year age is 2026 minus 2000, which is 26. Some laws and regulations use this method because it gives everyone born in the same calendar year the same age, which makes thresholds easier to administer.
So for one person on 26 October 2026, the three numbers are:
- International age (completed years): 25
- Year age: 26
- Traditional Korean age: 27
What changed in South Korea in 2023
For years, South Korea used a mix of all three methods in everyday life and in official rules, which caused confusion in areas such as medical care, insurance and legal documents. Laws were amended so that, from 28 June 2023, the international method of completed years became the standard for legal and administrative purposes.
In daily conversation, some people still use the traditional counting, especially in social settings. In addition, some rules, such as certain age limits tied to the year of birth, may still be defined by year age. When an age limit matters, it is best to check the exact wording of the rule or ask the organisation that applies it.
Similar systems elsewhere
Korea is not the only place with a counting tradition that starts at one. Other East Asian cultures have or had comparable systems, such as the Chinese "xusui", the Vietnamese "tuổi mụ" and the Japanese "kazoedoshi". In most official settings these have given way to the completed-years method, though they can still appear in traditions, ceremonies and zodiac calculations.
Why this matters when you fill in a form
If a form asks for your age "as on" a particular date, such as for an exam, a visa application or insurance, make sure you know which method is meant. Most official processes use completed years on the stated date. The age calculator on this site has an Age on a specific date option for exactly this case: enter your date of birth and the cutoff date, and you will see your age in years, months and days on that day.
Key takeaways
- International age counts completed years and changes only on your birthday.
- Traditional Korean age starts at 1 at birth and increases on 1 January.
- Year age is the current year minus the birth year.
- The same person can have three different ages on the same day.
- Always check which method a form, rule or organisation uses.
This article is for general information. For legal, medical or official purposes, rely on the guidance of the relevant authority.