Lunar Age, Explained: Why Families Count Differently
In the traditional count used across much of East Asia, a newborn baby is already one year old, and everyone gains a year at the new year rather than on their birthday. The result is an age that can sit one or two years above the number on a passport, and the difference is not an error by either side.
The practical questions arrive at specific moments. A grandmother gives a child’s age at a family gathering and it does not match the school form. A Korean colleague mentions turning 31 in January and then 30 later the same year. A birth record from Shanghai lists a date that seems to belong to the year before. Each of these is the same arithmetic wearing different clothes.
One at Birth, Two at the New Year
The rule is short. A baby is counted as one year old on the day they are born. On the first day of the new lunar year, every person adds a year, including that baby, who turns two without having had a first birthday. From then on the whole family ages together on the same date, which is why the count feels natural to anyone who grew up with it.
The old explanation for starting at one is that the months before birth are counted too, so a newborn has already lived through part of a year. Whether that is history or folk etymology is debated, and it does not change the arithmetic. What matters is the two rules: start at one, add one at the new year.
Where the Custom Still Applies
China calls the traditional count xusui, and it survives alongside the international count in everyday speech, particularly in rural areas and among older relatives. Vietnam has its own version. Korea had the most visible form: a newborn was one, and everyone added a year on 1 January rather than at the lunar new year, which produced the much-reported gaps between Korean age and international age.
That changed in law in June 2023, when South Korea began using the international method on official documents. The traditional count did not vanish; it moved to the places law does not reach, such as introductions at work and questions from relatives. If you are filling in a Korean form today, the number expected is the one you would give anywhere else.
In practice, families use both counts without much friction. The international age goes on forms and school records; the traditional age turns up in conversation, in the wording of a birthday greeting, and in the arithmetic an elder does without thinking. Nobody is confused by the double bookkeeping, because everyone knows which context they are in.
How to Convert Between the Two Counts
The reliable method uses lunar years, not birthdays. Find the lunar year you were born in, subtract it from the current lunar year, and add one. Someone born in 1990, in lunar year 1990, is 37 in lunar year 2026, because 2026 minus 1990 is 36, and one more is 37.
There is a shortcut that works for most people. If your birthday has already passed since the last new year, add one to your international age. If it has not, add two. The reason is timing: the lunar count gives everyone a year at the new year, while the international count waits for the birthday. In the weeks between the two events you are carrying a year the other count has not given you yet.
The shortcut fails for one group: anyone born in January or early February, before that year’s new year. They belong to the previous lunar year, so the subtraction gives one more than the shortcut predicts. Someone born a few days before the new year in 1990 counts as 38 in lunar year 2026, not 37.
The lunar age calculator handles both cases if you would rather not check a calendar. It needs the birth date and the new year date for the relevant year, and it returns the traditional count next to the international one.
The New Year Problem
Lunar new year moves. It falls between 21 January and 20 February, which means a birthday in that window can sit on either side of the boundary depending on the year. A child born on 5 February may have a new year before their birthday in one year and after it in another, and the count changes accordingly. This is not a flaw in the system; it is what happens when a lunar calendar is mapped onto a solar one.
Time zones add a second layer. The date is fixed by astronomical calculation for one meridian, so a family in Toronto and relatives in Seoul can mark the new year on different local dates. Vietnam and China have occasionally celebrated a day apart for the same reason. If two branches of a family disagree about an age by exactly one, a calendar boundary is a better explanation than a mistake.
Which Number Belongs on a Form
Official documents want the international count. Passports, tax records, school admissions and medical files all run from the date of birth, and a lunar age entered where a birth age is expected will look like an error of one or two years. The exception is a form that explicitly asks for the traditional count, which happens in some family records and in cultural paperwork.
Family records are the place to keep both. A note that says “born 14 March 1990; lunar age 37 in 2026” prevents the argument that starts when two documents disagree. It also preserves the thing that makes the custom worth keeping: it is a shared date, not a private one, and it gives every generation the same small event to celebrate.
For the international count, the age calculator gives the exact years, months and days. The lunar figure is a second, older layer on top of it, and it is worth knowing which one a question is asking for.
FAQ
Is Korean age the same as lunar age?
No, and the difference is the date of the bump. The Chinese and Vietnamese traditional counts add a year at the lunar new year. The Korean count added a year on 1 January, so it drifted out of step with both the lunar calendar and the birthday. Since 2023 the legal count in Korea is the international one.
Why am I two years older in the traditional count?
Because the two counts add a year at different moments, and the traditional count also starts you at one. If the new year has passed but your birthday has not, you are two ahead: one from the start, one from the new year. Once your birthday arrives, the gap drops to one.
Do I put lunar age on a visa application?
No. Visa and immigration forms ask for the date of birth, and some ask for age in years as of a stated date. Use the international count and the date on your passport; the traditional count has no standing on those forms unless the form says otherwise.
Last updated: October 26, 2026. This article is for general information only and is not medical, veterinary or legal advice. For decisions about your own health, your pet, or a legal matter, consult a qualified professional.
Sources
- National Institute of Standards and Technology, Time and Frequency Division: nist.gov
