How Age Is Calculated: Years, Months, and Days
Age calculation sounds trivial but quickly becomes subtle once you need exact results across years, months, and days — especially when leap years are involved. Our age calculator handles all of this automatically, but understanding the logic behind it helps you interpret the results correctly.
The basic calculation
Age in complete years = today's year minus birth year, minus 1 if today's month-day is before the birthday month-day. Beyond complete years, the remaining months and days are counted separately. A person born on March 15, 1990 turns 35 on March 15, 2025 — but on March 14, 2025 they are still 34.
Leap year edge cases
A person born on February 29 in a leap year has a birthday that only exists every 4 years. Legally, in most jurisdictions they are considered to turn a year older on February 28 in non-leap years, or on March 1 depending on local law. The calculator handles this correctly for both interpretations.
Korean vs. Western age systems
In the traditional Korean age system, a person is "1 year old" at birth and gains a year on each New Year's Day — not their birthday. This means a Korean person can be 1–2 years older in Korean count than in the Western system. South Korea officially switched to the Western system in 2023, but the traditional system is still culturally used.