How this instrument works
Several East Asian cultures traditionally count age using a 'nominal age' system: a newborn starts at age 1 rather than age 0, and everyone born in the same lunar cycle gains a year together at the same point, rather than each person aging up on their own birthday. The formula collapses to two whole years and a plus-one: nominal age = current lunar year minus lunar birth year, plus one — no birth month or day needed, since the traditional system doesn't track age at that finer resolution.
The detail that sets this calculator apart from a simple Gregorian subtraction is the word 'lunar.' Traditionally, this age system rolls over at the Lunar New Year — a date that shifts annually, typically falling somewhere between late January and mid-February on the Gregorian calendar — rather than at the fixed Gregorian January 1st some modern usages have shifted to. Because of that, someone born in early Gregorian January can belong to a different lunar cycle than their Gregorian birth date would suggest, depending on exactly when the holiday fell that particular season.
This calculator deliberately takes the lunar birth year and current lunar year as direct inputs, rather than trying to derive them automatically from a Gregorian date — doing that conversion correctly would require a lookup table of Lunar New Year dates spanning many decades, which isn't something this tool attempts. If you already know your lunar birth year (from a Chinese, Vietnamese, or other traditional lunisolar calendar record), enter it directly; if you only know your Gregorian birth date, you'll need to look up the corresponding lunisolar figure separately before using this calculator.
- Determine your lunar-calendar birth year — from a family record, a Chinese/Vietnamese lunar calendar app, or another lunisolar-calendar source, not simply your Gregorian birth year.
- Enter that figure into Lunar/Chinese-calendar birth year.
- Enter the current lunar year into Current lunar year.
- Read Traditional East Asian nominal age for the result: current lunar year minus birth year, plus one.
- Remember this is a cultural figure only, tracked by lunar calendar year rather than exact birth date — it isn't used for any legal or administrative purpose.
Worked example — born lunar year 1990, evaluated in lunar year 2026
Someone's family record states they were born in lunar 1990, and the current lunar year is 2026. Entering 1990 for Lunar/Chinese-calendar birth year and 2026 for Current lunar year gives Traditional East Asian nominal age = 2026 − 1990 + 1 = 37.
That 37 is the age they'd traditionally be addressed as within families and cultural contexts that still track age this way — potentially different from their Gregorian-calendar international age, since the traditional nominal count starts a year ahead at birth and increments at the Lunar New Year rather than on an individual birthday.
Questions
Why does this calculator ask for a 'lunar' year specifically, not just a birth year?
Because the traditional East Asian age-counting system rolls over at the Lunar New Year, a date that moves around the Gregorian calendar annually rather than falling fixed on January 1st. Someone born in early Gregorian January could fall into a different lunar cycle than their Gregorian birth date alone would suggest, so this calculator asks for the lunar figure directly rather than assuming it matches the Gregorian one.
How is this different from a Korean-age calculator that just asks for a birth year?
Some modern usages of East Asian nominal age — including South Korea's common cultural usage — have shifted to rolling over on the Gregorian January 1st rather than the Lunar New Year, in which case a plain Gregorian birth year and current figure are enough, and this site has a dedicated calculator for that version. This lunar-age calculator instead follows the older, still widely used tradition (common in Chinese and Vietnamese practice among others) that rolls over specifically at the same holiday, which is why it asks for lunar-calendar figures rather than Gregorian ones.
I only know my Gregorian birth date — how do I find my lunar birth year?
You'll need to look it up separately, typically using a Chinese or Vietnamese lunar calendar converter or app that maps a Gregorian date to its corresponding lunisolar cycle — this calculator doesn't do that lookup itself, since it would require a multi-decade table of shifting Lunar New Year dates. Once you have your lunar birth year from that lookup, enter it here directly.
Why does a newborn already count as age 1 under this system?
The traditional system counts time already spent before birth as part of one's first year, so a baby is considered 'age 1' from the day of birth rather than 'age 0.' Combined with everyone aging up together at the Lunar New Year rather than on individual birthdays, this can make a very recently born baby 'age 2' within weeks if that holiday falls shortly after their birth.
Is this nominal age used for anything official, like legal documents?
No — like other traditional East Asian nominal age systems, this is a cultural convention used in everyday conversation and some social or family contexts, not an official or legal age standard anywhere. Legal and administrative ages across the region are based on the Gregorian-calendar international age system (age zero at birth, incrementing on the individual's own birthday), separate from what this calculator computes.