Most countries count age from completed years. This guide compares international age reckoning, traditional Korean age and the rules used on official forms.
Ask someone in London how old they are and they will tell you how many complete years have passed since the day they were born. Ask the same question in Seoul a few years ago and you would have got a number one or two higher, not because anyone was mistaken, but because the two answers came from genuinely different systems for counting the same span of time.
Age looks like arithmetic. It is a convention, and the conventions disagree.
The international system: count completed years
The system used across Europe, the Americas, Africa, Australia and most of Asia is sometimes called the international age or the Western age. The rule is one sentence: you are nought years old on the day you are born, and your age increases by one on each anniversary of that day.
This is what our Age Calculator computes, and it is what almost every official form, medical record and passport in the world records. Its logic is that age measures elapsed time. On the day you are born, no time has elapsed, so the count starts at zero, the same way a stopwatch starts at zero rather than at one.
The system has one quiet consequence that trips people up: for a stretch of every year you are a year younger than the simple subtraction of birth year from current year suggests. Born in 1990, in a year like 2026 you are 36 only after your birthday has passed. Before it, you are 35. This is the single most common error in hand-calculated ages.
East Asian age reckoning: count calendar years touched
The traditional East Asian system counts something different. Instead of asking how much time has elapsed, it asks how many calendar years you have existed in.
You are one year old the day you are born, because you exist in your first calendar year. Then everyone’s age increases together at the new year, not on individual birthdays.
The consequences are strange to anyone raised in the other system:
- A baby born on 31 December is one year old on the day of birth and two years old the following day.
- Two people born eleven months apart can be the same reckoned age, or two apart, depending on where the new year falls between them.
- Nobody’s reckoned age depends on their birthday at all.
It is not an error, and it is not primitive. It is a different question being answered. In a society where seniority governs forms of address and social obligation, a system where everyone in a birth-year cohort shares an age is arguably more useful than one where a colleague outranks you for four months of the year and then does not.
The three Korean systems, and the 2023 change
Korea is the case worth understanding in detail, because until recently three systems were in simultaneous legal use.
| System | Age at birth | Increases on | Used for |
|---|---|---|---|
| Korean age (세는 나이) | 1 | New Year’s Day | Everyday conversation, social hierarchy |
| Counting-year age (연 나이) | 0 | New Year’s Day | Conscription, school entry, some laws |
| International age (만 나이) | 0 | Your birthday | Contracts, medical, official documents |
Three answers, all correct within their own domain, differing by up to two years for the same person. A Korean born in late December could reasonably say they were 30, 29 or 28 depending on which system the question implied.
In December 2022 the National Assembly passed legislation standardising on international age for judicial and administrative purposes, taking effect in June 2023. The stated motivation was practical: the ambiguity generated genuine disputes over insurance payouts, contract eligibility and pension entitlements.
The law changed official practice. It did not change conversation. Korean age remains in everyday use, because the social function it serves, establishing relative seniority instantly on meeting someone, is not something an official standard can legislate away.
Japan and China
Japan used the traditional system historically and moved to international age in stages, with a 1902 law establishing it and a 1950 law actively discouraging the traditional count. Today Japanese official and everyday age are both the international count, and the traditional kazoedoshi survives mainly in religious observance and some ceremonial contexts.
China’s transition was similar in direction and less legislated. The traditional xūsuì is still encountered, particularly among older generations and in rural areas, and lunar birthdays remain widely observed alongside Gregorian ones. Official documentation uses international age.
Where the lunar calendar adds a second layer
Separate from how you count is the question of which calendar you count in. A birthday recorded on the lunar calendar falls on a different Gregorian date each year, because a lunar year is roughly 354 days and requires periodic intercalary months to stay aligned with the seasons.
For anyone whose family records a lunar birth date, this means two birthdays: the fixed Gregorian date on their documents, and the moving lunar date the family observes. Neither is wrong. Conversion between them requires astronomical tables rather than arithmetic, which is why no simple calculator, including ours, can do it.
The same issue affects the Chinese zodiac animal, which is determined by the lunar year rather than the Gregorian one. Anyone born in January or early February belongs to the previous year’s animal, a detail our Zodiac Sign Finder flags rather than getting wrong quietly.
Which age goes on the form
Use international age for
- Passports, visas and immigration paperwork
- Medical records, prescriptions and dosing
- Insurance applications and claims
- Employment contracts and pension calculations
- Anything with a legal age threshold attached
- Any form written in English
Watch out when
- A relative quotes an age from a traditional system
- Comparing ages in a Korean or Chinese family record
- Reading a historical document that predates standardisation
- A birth date is recorded on a lunar calendar
- Someone says "Korean age" without saying which of the three
- School-entry rules use a different count from the legal one
The safe rule for any official purpose is to state the date of birth rather than the age. A date is unambiguous; an age is the output of a convention that the reader may not share.
The one case where none of the systems help
There is a category of person for whom all of this is academic: people who do not know their date of birth. Displacement, undocumented birth, destroyed records and adoption without documentation all produce adults with an estimated year and no day.
Administrative systems handle this badly, because most of them require a full date. The common workaround is to assign 1 January of the estimated year, which is why several countries have unusually large numbers of registered residents apparently born on New Year’s Day. It is a reminder that a date of birth is a record, not a fact of nature, and that the record is sometimes a guess someone made on a form decades ago.
The practical takeaway
If you need to know how old someone is for any purpose that matters, work from the date of birth using completed years, the international system. That is what the Age Calculator does, and it is the answer that will match their documents.
If you are comparing ages across cultures, generations or historical records, hold the possibility that the two numbers in front of you were produced by different systems. The gap will usually be one or two years, and it will resolve the moment you go back to the dates.
And if you want the exact gap between two people’s ages rather than the ages themselves, the Age Difference Calculator works entirely from dates and sidesteps the whole problem.