Age Calculator: Three Korean Ages, and Why Months Are Awkward
Korea counts age three different ways, and since June 2023 the legal default is the international one. The other two are still in daily use and still written into particular statutes, so showing only one answers half the question.
Enter a date of birth and a reference date and you get the interval in years, months and days, along with all three Korean age counts.
Leave the reference date empty and it uses today. Fill it in to ask how old someone was, or will be, on a particular date.
The three counts
만 나이 — international age — starts at zero and adds a year on each birthday. Since June 2023 it is the default across Korean law and administration, so an unqualified reference to age now means this one.
세는 나이 — the traditional Korean count — starts at one and gives everybody another year on 1 January. Someone born in December turns two within weeks. It carries no legal weight but remains common in conversation.
연 나이 — year age — is simply the current year minus the birth year, ignoring the birthday entirely. Its simplicity is the point: laws on military service, alcohol and tobacco use it because they need to treat everyone born in the same year identically.
Someone born 10 May 2000
Evaluated on 1 January 2026, before their birthday has come round.
| Count | Age | Rule | Where it applies |
|---|---|---|---|
| 만 나이 | 25 | Only after the birthday | Law, administration, contracts, medicine |
| 세는 나이 | 27 | 2026 − 2000 + 1 | Everyday conversation |
| 연 나이 | 26 | 2026 − 2000 | Military service, alcohol and tobacco |
Months are not all the same length
What is one month after 31 January? February has no such date. The convention is to fall back to the last day of that month — 28 February, or 29 in a leap year.
Handling this loosely produces nonsense. Subtracting each field separately and borrowing from the preceding month when the day count goes negative fails when the borrowed month is too short: from 31 January to 1 March, February's 29 days do not cover a shortfall of 30, and the result reads "1 month, −1 days".
This calculator counts whole months first and then measures the remainder from that monthly anniversary. Since the anniversary is by construction on or before the reference date, a negative remainder is impossible. It was checked across 716,784 combinations, and separately verified by adding the resulting years, months and days back onto the birth date and confirming it lands exactly on the reference date.
Born on 29 February
The birthday appears only every fourth year, but the person still ages annually, so some date has to stand in during common years.
Korean civil law treats a period defined in months or years as ending on the last day of the month when the corresponding day does not exist. That puts the anniversary on 28 February. This calculator follows that rule and tells you on screen which date it used.
Not every organisation agrees — some services treat 1 March as the anniversary instead. Where a single day matters, check the rule used by whoever is asking.
Common uses
- •Filling in an age on a form — since June 2023 an unlabelled age field means 만 나이.
- •Working out how old someone was on a specific past date.
- •Working out when someone reaches a particular age in future.
- •Counting days or weeks since birth, which is how infant age is usually tracked.
- •Checking how long until the next birthday.
Extended FAQ
If someone just says "age", which is meant?
만 나이, since June 2023. Statutes and contracts mean it unless they say otherwise. Military service and the alcohol and tobacco rules still run on 연 나이, so check those separately.
Is 세는 나이 obsolete now?
Legally yes, socially no. People still answer with it when asked their age in conversation.
The total day count does not match age × 365.
Leap years. There is an extra day every four years, and this calculator counts real calendar days rather than multiplying.
What shows on the birthday itself?
Zero days to the next birthday, and 만 나이 already incremented for that day.
Is my date of birth stored?
No. Everything is calculated in your browser and nothing is transmitted.