Korean Age Calculator — Convert to Western Age

Three age counts have circulated in South Korea and this calculator reports all of them because they answer different questions. Traditional Korean age, se, assigns 1 at birth and increments for everyone on 1 January, so it is simply reference year minus birth year plus 1 and never depends on the birthday. International or full age, man nai, starts at 0 and increments on the birthday, so it subtracts 1 from the year difference when the birthday has not yet arrived. Year age, yeon nai, is the bare year difference with no adjustment at all. The gap between se and man nai is therefore always 1 or 2: exactly 1 after the birthday has passed in the reference year, and 2 before it. A child born on 31 December is 1 in the traditional count on the day of birth and 2 the following morning. The practical caveat is knowing which count a given rule invokes. Since the 2023 reform, full age is the default for civil and administrative purposes, so contractual capacity and the age of majority at 19 are read in full age. Year age was not abolished, and it remains the operative measure in specific statutes - notably conscription under the Military Service Act and the drinking and tobacco thresholds administered through youth protection law, which treat everyone born in the same calendar year identically from 1 January. That is why a Korean can be legally able to buy alcohol months before turning 19 in full age. Traditional se persists socially, governing seniority terms and speech level, and is the count most often meant when someone is asked their age in casual conversation.

Counting a newborn as one year old is a shared East Asian inheritance, generally explained by counting gestation as the first year and by an ordinal rather than cardinal conception of years lived. Parallel systems existed across the region: the Chinese sui, which increments at Lunar New Year rather than 1 January, the Japanese kazoedoshi, and Vietnamese equivalents. Japan legislated against its version early, with an age-calculation statute in 1902 and a further law in 1950 urging the public to adopt full age; China moved to the international count over the twentieth century, retaining sui for traditional and folk purposes. Korea, uniquely, kept traditional se in everyday use long after its neighbours had largely dropped theirs. South Korean law had prescribed full age for formal purposes for decades, but statutes and daily life diverged badly enough that the same person could be three different ages in one afternoon, and the inconsistency generated real disputes over insurance, employment and dosing during the COVID-19 vaccination rollout, when eligibility bands had to specify which count applied. Standardisation became a campaign pledge of Yoon Suk-yeol and the National Assembly passed the enabling amendments to the Civil Act and the Administrative Basic Act in December 2022. They took effect on 28 June 2023, making full age the default reading of any age stated in law or administrative documents unless a statute says otherwise. The reform changed paperwork rather than culture: surveys around the change found most Koreans expected to keep using se when speaking to each other.