What is lunar age (Chinese nominal age / xu sui)?
Lunar age, also called Chinese nominal age or xu sui, counts you as one year old on the day you are born and adds a year every Chinese New Year. That makes lunar age one to two years higher than Western age, most visibly right after the Lunar New Year.
How is nominal age calculated?
Formula: nominal age = current year − lunar birth year + 1. The trick is the lunar birth year. If you are born before Chinese New Year, your lunar year is one less than the Western year. A January 15, 1990 birthday belongs to lunar year 1989, because the 1990 Lunar New Year fell on January 27.
Nominal age vs. Western age
Western age ticks up on your birthday. Nominal age ticks up on Lunar New Year, and you start at one on the day you are born. As a result, nominal age runs two years higher than Western age if your birthday falls before Chinese New Year, and one year higher if after.
Western paperwork, passports, and legal documents use Western age. Traditional contexts (Chinese New Year greetings, matchmaking, feng shui, astrology readings) use nominal age. Chinese-speaking communities carry both numbers in parallel.
Chinese New Year edge cases
Chinese New Year lands somewhere between January 21 and February 20, varying each year. A birthday in late January or early February needs the actual Lunar New Year date to confirm the lunar birth year. Premature babies count by actual birth date, not due date: a baby due in mid-February but born in late January belongs to the previous lunar year, with a different zodiac sign.
Worked example
A person born on March 15, 1990 is past the 1990 Lunar New Year (January 27), so the lunar birth year is 1990, which is a Year of the Horse (庚午). In 2026 their nominal age is 2026 − 1990 + 1 = 37; their Western age is 2026 − 1990 = 36.