How Is Age Calculated? Methods Used Around the World

While chronological age (years since birth) is the international standard, different cultures and contexts use different methods.

Chronological age is the standard international method: the number of completed years since birth. It is used for legal purposes, medical records, education, and most official contexts worldwide. The calculation accounts for the current month and day relative to the birth month and day — someone born in December 1990 is 34, not 35, in November 2025.

Korean age was the traditional system in South Korea until June 2023. Under this system, a person is 1 year old at birth and gains a year on January 1 regardless of their actual birthday. A baby born on December 31 would be 2 years old the very next day. South Korea officially switched to the international chronological system in 2023.

East Asian reckoning (used traditionally in China, Japan, and Vietnam) counts a person as 1 at birth and increments on the Lunar New Year. This is now largely replaced by chronological age for official purposes but persists in cultural contexts.

Gestational age (used in pregnancy) counts weeks since the first day of the last menstrual period, not from conception. A "12-week pregnancy" means roughly 10 weeks since conception.

For all practical legal, medical, and administrative purposes, use chronological age. Tooler's Age Calculator provides precise chronological age in years, months, and days, handling leap years and month boundaries automatically.