How to use
Pick a birth date (and optionally a target date — defaults to today) and the calculator returns four values: **international age** (full years since birth), **Korean traditional age** (international age + 1 or +2 depending on whether the birthday has passed), **year age** (the Korean Conscription Law convention of `current_year - birth_year`), and a year/month/day breakdown of the international age. A countdown to the next birthday is shown alongside.
The Y/M/D decomposition follows the natural convention: months are calendar months, not 30-day blocks, so someone born 2020-01-31 viewed on 2026-05-15 is "6 years, 3 months, 15 days" — March is counted as a full month because the target day (15) is past the birth day-of-month (31 cannot exist in some months, in which case it is treated as the last day of that month). All calculations run in the browser; the date never leaves your machine.
Examples
Birthday already passed this year
Input
birth: 1990-03-15
today: 2026-05-17
Output
International age: 36 years
Korean age: 37 years (international + 1, birthday passed)
Year age: 36 years (2026 - 1990)
Breakdown: 36 years, 2 months, 2 days
Next birthday: 302 days (2027-03-15)
When the birthday has already happened in the current year, the gap between international age and Korean traditional age is +1. The year-age figure happens to match international age this time because the birthday is before the May reference date — for any birthday after the reference date, year age would be one larger than international age.
Born today, before this year's birthday
Input
birth: 1990-11-08
today: 2026-05-17
Output
International age: 35 years
Korean age: 37 years (international + 2, birthday not yet passed)
Year age: 36 years (2026 - 1990)
Breakdown: 35 years, 6 months, 9 days
Next birthday: 175 days (2026-11-08)
Same birth year as the prior example but the birthday is in November, after today. The Korean traditional gap widens to +2 because Korean age counts the calendar year from January 1 even before the birthday in that year. Year age and Korean age now match at 37 — Korean age and year age coincide for everyone whose birthday has not yet arrived in the current year, but diverge once the birthday passes.
Born on Feb 29 (leap year baby)
Input
birth: 2000-02-29
today: 2026-05-17
Output
International age: 26 years
Korean age: 27 years
Year age: 26 years
Breakdown: 26 years, 2 months, 17 days
Next birthday: 287 days (2027-02-28, observed)
February 29 only exists in leap years. The legal convention in most jurisdictions (including Korea, Japan, and the US) is that non-leap years observe the birthday on February 28, so February 28 is when the international age increments in non-leap years. This calculator uses that convention. Note that some countries (Taiwan, until 2010 in some interpretations) used March 1 as the observed birthday — the choice affects only one day of the year but matters for legal age-of-majority and statute-of-limitations calculations on rare cases.
FAQ
Why does Korea have three different age systems?
Historical layering. **Korean traditional age (세는 나이)** comes from East Asian counting tradition where a newborn is "1 year old" at birth (counting the months in the womb) and everyone ages +1 on January 1 (the lunar new year originally, now Solar new year). **International age (만 나이)** is the Western/legal-medical convention — 0 at birth, +1 on each birthday. **Year age (연 나이)** is a 1962 South Korean simplification used only by the Conscription Law and the Juvenile Protection Act: `current year - birth year`, which is easy to compute from an ID card. The June 2023 통일법 (Korean Age Unification Act) made international age (만 나이) the default in administrative and civil law, but the other two systems remain in casual use and a handful of specific statutes.
What changed with Korea's 2023 age unification?
The Civil Act and Administrative Basic Act were amended on June 28, 2023 to require international age (만 나이) as the default in all administrative documents, civil contracts, and legal interpretation unless a specific law overrides. Practical effects: tax brackets, pension eligibility, voting age, criminal-responsibility age, drinking and smoking age (which had been year age before) — all now use international age. Two specific carve-outs remain. The Conscription Law (military service draft) and the Juvenile Protection Act (alcohol, tobacco, certain media access) continue to use year age, because the year-age cutoff (calendar-year-aligned) makes administrative batching easier for those programs. Casual social usage of Korean traditional age (세는 나이) is unaffected by the law — it remains in conversational use.
How is age calculated in Japan?
Japan uses international age (満年齢) for all official purposes and has done so since the 1902 Age Calculation Law (年齢計算ニ関スル法律). One peculiarity: under Japanese law, a person turns one year older on the day *before* their birthday — for example, someone born April 1 is legally considered to turn 18 on March 31, not April 1. This is because the law treats the birthday as the start of the new year of age. Practical effect: a child born April 1 enters the same Japanese school year as children born April 2 of the previous calendar year (Japan's academic year starts April 1). For ordinary social and conversational purposes, Japanese people use international age in the natural way (turning a year older on the birthday). The traditional kazoe-doshi (数え年) system parallels Korean traditional age and is now used only in cultural contexts (shichigosan, yakudoshi).
Why use year-month-day instead of just years?
For ordinary day-to-day age the integer year count is sufficient — "I am 36" tells everyone what they need. The Y/M/D decomposition matters for **infants and toddlers** (a 7-month-old has very different development than an 11-month-old), **legal deadlines** where age-of-majority or statute-of-limitations falls on a specific day rather than a rounded year, **medical contexts** where dosing or developmental milestones are tracked in months, **HR and pension calculations** where service length is measured to the day, and **demographic statistics** where median age is computed at sub-year precision. The breakdown also surfaces the precise next-birthday distance, useful for planning celebrations or visa renewals.
Can I calculate age for a future or past date?
Yes — change the "as of" date. Common uses: planning the age someone will be on a specific future event (school start date, retirement age threshold, statute-of-limitations expiry), or computing the age someone was on a past historical date (when they got married, started a job, signed a contract, the date of a photograph). The calculator works symmetrically for past and future. For dates more than ~200 years away, calendar reform may affect calculations — the Gregorian calendar transition (1582 Catholic, 1752 British, 1873 Japanese, 1918 Russian) means dates before those local cutoffs require careful interpretation; this calculator uses pure Gregorian dates without historical calendar adjustment.
What is the difference between "gestational age" and chronological age?
Gestational age (胎齢, 재태 연령) measures the time since conception (or, by convention, since the last menstrual period — about 2 weeks earlier than actual conception), used in pregnancy and neonatal medicine. Chronological age counts from birth. For premature infants, doctors often quote two figures: chronological age (`postnatal age`) and corrected age (`postmenstrual age − 40 weeks`) to assess development relative to expected term. A child born at 30 weeks gestation in March who is now 6 months chronological is roughly 3.5 months corrected — developmental milestones are evaluated against the corrected age until ~age 2. Outside of medical contexts, gestational age is rarely needed and this calculator does not compute it.
Related concepts
Age calculation seems trivial — subtract two dates and report the years — but the actual definition of "age" is culturally, legally, and biologically layered. Three calendar-based conventions dominate globally: **international age** (years since birth, +1 on each birthday), the Western and now near-universal legal-medical standard; **Korean traditional age** (1 at birth, +1 every January 1), which traces back to ancient East Asian counting and survives in Korean conversational use; and **year age** (`current_year - birth_year`), used as a simplification for specific administrative purposes in Korea. Japan retains a similar traditional system called kazoe-doshi but uses international age for all legal and ordinary purposes since 1902.
Korea's situation is the most layered. Until June 2023 the three systems coexisted in different domains: civil law used a mix, the Conscription Law used year age, public health and medical contexts used international age, and casual conversation almost always used Korean traditional age (so a baby born December 31 would be "2 years old" the next day). The 2023 unification law made 만 나이 (international age) the legal default while leaving year age intact for conscription and youth-protection statutes, and traditional age for cultural use. Adoption is uneven — older Koreans still default to traditional age in conversation, hospitals quote international age, and HR systems use whatever the relevant statute specifies.
Three adjacent concepts intersect age calculation. **Gestational age** (재태 연령) is used in neonatal medicine and counts from the last menstrual period, not from birth; relevant for premature infant care until roughly age 2. **Bone age** is a radiographic measurement comparing a child's skeletal maturation to standardized references (Greulich-Pyle atlas, Tanner-Whitehouse method) used in growth disorder diagnosis. **Biological age** (vs chronological) is a research concept measuring physiological aging through markers like DNA methylation (Horvath clock, GrimAge), telomere length, or epigenetic age — used in aging research and longevity medicine, distinct from any calendar-based age. None of these alternative measures replace chronological age for legal or social purposes; they coexist as complementary indicators in specific contexts.
