How exact age is counted
Age is counted the way a calendar counts, not the way a stopwatch does: full years first, then whole months, then the leftover days. Born on 1990-05-20 and asked on 2026-09-06, you are 36 years old because the 2026 birthday has passed, 3 months because 20 June, 20 July and 20 August have passed too, and 17 days because 20 August was 17 days ago.
The totals underneath come from the same date difference: total days, whole weeks plus the leftover days, and total hours. Two extra figures are averages rather than calendar counts and are labelled as such — months at 30.4375 days each and decimal years at 365.25 days each. Other calculators that show only the averaged month are why two sites can disagree about your age in months.
Everything is whole days, so no clock and no time zone is involved — the only thing read from your device is today’s date. Leave the reference date empty for today, or fill it in to see your age on a past or future date, which is what forms asking for “age as of 31 December” need.
International age, year age and traditional East Asian age
International age — the one this page shows first — increases on your birthday. It is the figure used for legal age in almost every country, and since the 만 나이 통일법 amendments to Korea’s Civil Act and Framework Act on Administration took effect on 28 June 2023, it is the default reading of any age written in Korean law or administrative documents unless the text says otherwise.
Year age (연 나이) is simply the current year minus the birth year, so everyone born in the same year shares it and it changes on 1 January. A handful of Korean statutes still use it — the Military Service Act and the Youth Protection Act among them, which is why alcohol and tobacco sales are decided by birth year rather than by birthday. The tool shows it under “Other age systems”.
Traditional East Asian age (세는나이) starts at 1 at birth and adds a year at each new year, so it runs one or two years ahead of international age. For a solar birth date the tool counts Gregorian new years; for a lunar birth date it counts lunar new years, which is the older reckoning and can differ by a year for anyone born in January or February.
Birthdays on 29 February
A leap-day birth date has no anniversary in a common year, so a convention has to be chosen. This calculator uses 1 March by default and offers 28 February as the alternative; the setting changes both the day your age ticks over and the next-birthday countdown.
Born on 2000-02-29 and checked on 2027-02-28, you are 26 under the 1 March rule and 27 under the 28 February rule. Pick whichever matches the document you are filling in; every other figure on the page — total days, weekday of birth, milestones — is unaffected by the choice.
Lunar (음력) birth dates
Switch the calendar to Lunar and the date is converted to its Gregorian equivalent using the Korea Astronomy and Space Science Institute conversion tables, which cover the years 1000 to 2050. Every age figure is then computed from that solar date, because that is the date on official records and the one legal age is based on.
Tick “leap month” if you were born in a 윤달. A leap month only exists in some years, so in the years without it the anniversary is taken in the regular month of the same number — the convention most Korean families follow when they hold the celebration.
The countdown then tracks the lunar date, so the solar day of your birthday moves each year — usually about eleven days earlier, jumping forward again after a leap month. The sexagenary year name (간지) and zodiac animal (띠) shown beside the result come from the lunar year, which is why someone born in January can have the previous year’s animal.
Next birthday, milestones and 10,000 days
The countdown card shows the date of your next birthday, how many days away it is, and the age you will turn. On the day itself it says so and then points at next year’s date, so the card never goes blank on the one day you are most likely to open it.
The milestones panel dates the birthdays that usually matter on a form — 18, 21, 30 and 65 — and marks the ones already passed. It also dates your 10,000th day, which arrives at about 27 years and 4 months and is a much better excuse for a party than a round birthday.
Limits, and where results differ
- Dates before 1583 are refused. Earlier dates need the Julian calendar or a proleptic extension, and quoting a Gregorian answer for them would be wrong rather than merely imprecise.
- A birth date in the future produces an error rather than a negative age, which is almost always a typo in the year.
- Lunar conversion stops at 2050, the end of the published table. A lunar birth date outside 1000–2050 is rejected instead of extrapolated.
One thing worth knowing before you share: the calculation runs entirely in your browser and nothing is uploaded, but the Share link puts the date of birth into the URL so the page reopens with it. That is what makes the link useful — and a reason not to post it publicly.