How age is calculated
Calendar age is the difference between two dates expressed in years, months, and days. The standard way is to subtract the birth year from the current year, then adjust if the birthday has not yet occurred this year.
years = currentYear − birthYear − (birthdayPassedThisYear ? 0 : 1)To get months and days as well, subtract the components one at a time and borrow from the next-larger unit when negative — the same way you would subtract base-60 minutes from a clock time.
A worked example
Birthday: 1990-07-15. Today: 2026-05-14. The birthday is later this year, so subtract one extra year and borrow:
- Years:
2026 − 1990 − 1 = 35 - Months:
5 − 7 + 12 = 10(borrowed from the year) - Days:
14 − 15 + 30 = 29(borrowed from the month — June has 30 days)
Result: 35 years, 9 months, 29 days.
Why month length matters
A naive "days since birth" calculation works perfectly when expressed only in days. The complication appears when you want a human-readable answer in years and months because months have 28 to 31 days. Different libraries make different choices when the start date is, for example, the 31st of January and the end date is the 28th of February — some report "1 month, 0 days," others "0 months, 28 days." Both are defensible.
Our calculator follows the convention that an age "rolls over" only on the same calendar day of the month, matching how most legal documents and birthday cards count.
Other meanings of "age"
- Chronological age — calendar age, what most calculators show.
- Biological age — estimated from biomarkers (DNA methylation, fitness, telomere length). Can be years above or below chronological age.
- Gestational age — for a fetus, weeks since the mother's last menstrual period. A full-term baby is born around 40 weeks.
- Korean age — traditionally, everyone gains a year on January 1 and starts life at 1 instead of 0. South Korea formally abandoned this system in 2023 for official documents.
- Astrological age — for charts, the age is measured from the precise birth time in the natal location's time zone.
Useful derived numbers
- Days lived: total calendar days, useful for fun stats. A typical 40-year-old has lived roughly 14,600 days.
- Hours / minutes / seconds: multiply days by 24, 1440, or 86,400.
- Days to next birthday: useful for countdowns and planning.
- Day of the week you were born: Zeller's congruence gives an exact answer from the date alone.
- Generation label: Born 1965–1980 (Gen X), 1981–1996 (Millennial), 1997–2012 (Gen Z), 2013+ (Alpha). Boundaries vary slightly by researcher.
Leap-year edge cases
People born on February 29 are called leaplings and exist on the calendar only every four years. The common civil convention is to celebrate on February 28 in non-leap years (the last day of February) — but legal definitions vary. In the UK and Hong Kong, a leapling's official birthday for legal-age purposes is March 1; in New Zealand and the US, it is February 28 of the year they turn the relevant age.