Home › Guides › How Age Is Calculated (Leap Years)
EducationAge looks like the simplest calculation in the world: today's date minus your birthdate. And mostly it is — but leap years, February 29 birthdays, and even time zones create edge cases that confuse forms, legal systems, and trivia nights alike.
Here's exactly how age is computed, why leap years exist, and what happens if you were born on February 29.
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The basic rule: completed years
Your age in years is the number of birthdays you've completed. Born March 10, 2010? On March 9, 2026 you're 15; on March 10, you turn 16. Legal systems worldwide use this "completed years" convention — which is why "in your 16th year" and "16 years old" mean different things.
Why leap years exist
Earth orbits the sun in about 365.2422 days, not 365. Without correction, the calendar would drift ~6 hours a year — after a century, summer would start in what the calendar calls spring. The Gregorian fix:
- A year is a leap year if divisible by 4…
- except century years (divisible by 100), which are not leap years…
- unless also divisible by 400.
So 2024 was a leap year, 2000 was one, but 1900 was not — and 2100 won't be either. This system keeps the calendar aligned to within about a day per 3,000 years.
Born on February 29?
Leap-day babies (~1 in 1,461 births) face a real question in non-leap years: when is the birthday? Legally, most jurisdictions recognize March 1 (or February 28 in some places) as the official birthday for age purposes — the exact rule varies by country and even by state. Socially, most leaplings just pick a day and celebrate. Either way, age still counts in completed years: a person born Feb 29, 2020 turns 4 in 2024, 5 in 2025, and so on.
Age in days, months, and edge cases
For infants, medicine and official forms often use months or days instead of years. Age in days is simply the count of days between the two dates — leap days included automatically. One genuine edge case: babies born near midnight across time zones can have different birth dates on documents issued in different zones — rare, but it happens with international births and flights.
Age on other planets (yes, really)
A "year" is planet-specific: Mercury orbits in 88 Earth days, Venus in 225, Mars in 687. A 20-Earth-year-old is 82 in Mercury years but not even 11 in Mars years. It's a fun illustration that age is just completed orbits since birth — the unit is arbitrary, the counting method is universal.
Korean age: a real-world variant
Until June 2023, South Korea traditionally counted newborns as 1 year old and added a year every New Year — so a baby born December 31 was "2" the next day. Korea has since standardized on international (completed-years) age for legal purposes. It's a good reminder that "how old are you?" has a cultural component, even if the math underneath is the same subtraction.
Milestones use completed years — strictly
Voting, driving, drinking, contracts: every legal age threshold means completed years, no rounding. Someone born June 1, 2008 cannot vote in a May 2026 election — they're 17 years and 11 months old, which is 17. The law doesn't do "almost."
Why 18? (And why it varies)
There's nothing mathematically special about 18 — it's a legal convention, and it varies: voting is 18 in most democracies but 16 in a few (Austria, Brazil); driving ranges from 16 to 18; drinking from 16 to 21. The completed-years math is universal, but societies choose different thresholds for different responsibilities. When a form asks your age, it's this completed-years number — computed the same way everywhere — that it wants.
Frequently asked questions
Because a year is ~365.2422 days, not 365. Adding Feb 29 every 4 years (with century exceptions) keeps the calendar aligned with the seasons; without it, the calendar would drift about 6 hours per year.
No. Century years are leap years only if divisible by 400 — 2000 qualified, 1900 did not, and 2100 will not either.
Usually February 28 or March 1 in non-leap years — the legal rule varies by jurisdiction. Their age still advances in normal completed years regardless.
In completed years since the birthdate: you are N years old from your Nth birthday until the day before your (N+1)th. This is the standard used for voting age, drinking age, and contracts worldwide.
Disclaimer: Legal recognition of Feb 29 birthdays and age thresholds varies by jurisdiction. Check local law for anything binding, like contracts or eligibility.