The convention used here
This calculator uses a common nominal-age convention called xu sui: age one at birth, then add one at each Lunar New Year. The formula is comparison lunar year minus birth lunar year plus one. It is a calendar convention, not a measurement of elapsed time. Other families or historical records may follow another convention, so confirm which system a document uses.
A boundary example
A baby born on February 16, 2026 has nominal age one that day. Lunar New Year arrives on February 17, so nominal age becomes two while the baby is still only one day old. The completed-years result remains zero. This is why simply adding one to Western age does not always reproduce a lunar nominal age.
What “completed years” means
The comparison uses Gregorian birthdays. It subtracts one when the birthday has not yet occurred in the comparison year. For a February 29 birth, this tool advances completed years on March 1 in non-leap years. Legal and administrative rules may differ; this calculation should not decide eligibility for a service or replace an official age record.
Use a comparison date
Choose the date relevant to the question: today, the date of a family record, or a planned celebration. Both dates must be within the supported Gregorian range, and the comparison date cannot precede birth. The result describes the chosen calendar method. It does not predict fertility, a baby’s sex, health or life expectancy.
Sources and calculation method
Hong Kong Observatory conversion tables · 2026 calendar reference · lunar-javascript calculation library
Calculated locally with lunar-javascript 1.7.7. NineHound’s examples and boundary tests are described in our methods; this does not imply independent validation of every date in the supported range.