Leap Year Rules, and Why 1900 Was Not One

A year in the Gregorian calendar is a leap year if it is divisible by 4, except for century years, which are leap years only if they are also divisible by 400. That single exception is the part most people never learn. It is why 1900 was an ordinary 365-day year, why 2000 was a leap year, and why 2100 will not be one — even though all three are divisible by 4.

The rule exists to keep the calendar aligned with the seasons. A tropical year — one full cycle of seasons — is about 365.2422 days. Adding a leap day every four years assumes the year is exactly 365.25 days, which overshoots slightly. The century exceptions shave off that overshoot. The result is a calendar that stays within a day of the sun for thousands of years, without anyone having to intervene.

The three rules, in order

Apply these in sequence and any year resolves without ambiguity:

• If the year is not divisible by 4, it is a common year.
• Otherwise, if it is not divisible by 100, it is a leap year.
• Otherwise, if it is not divisible by 400, it is a common year.
• Otherwise (divisible by 400), it is a leap year.

Walk 1900 through it: divisible by 4, yes; divisible by 100, yes; divisible by 400, no — so 1900 is a common year. Now 2000: divisible by 4, yes; by 100, yes; by 400, yes — a leap year. And 2100: divisible by 4 and by 100 but not by 400, so it too is a common year. Only one century in four earns its February 29.

The astronomy behind the exceptions

The older Julian calendar used only the first rule: a leap day every four years, making the average year 365.25 days. Compared with the tropical year of 365.2422 days, that is an overshoot of about 0.0078 days — roughly 11 minutes and 14 seconds too long, every single year.

Eleven minutes sounds harmless, but it compounds. Over 400 years the Julian calendar drifts about 3.1 days ahead of the sun. By the sixteenth century the accumulated Julian error had pushed the spring equinox roughly ten days off its intended date, which is what prompted the 1582 reform. The Gregorian fix removes exactly three leap days every 400 years by demoting three of the four century years. That drops the average year to 365.2425 days, within about 27 seconds of the true tropical year.

The arithmetic is worth doing once. In 400 years there are 100 years divisible by 4. Remove the four century years (1700, 1800, 1900, 2000 in one such span), then add back the one divisible by 400 (2000). That leaves 97 leap days across 400 years: (400 × 365 + 97) ÷ 400 = 365.2425. The three demoted centuries are precisely the ~3 days of Julian drift, corrected on schedule.

February 29 birthdays

A person born on February 29 — a “leapling” — gets a calendar birthday only once every four years, and in century common years such as 1900 or 2100 they wait eight years. On the roughly three non-leap years in between, the date simply does not exist, which raises a practical question: on which day does someone born on February 29 legally turn 18 or 21?

The answer varies by jurisdiction, and the two common conventions land on adjacent days:

• The last day of February. Some statutes treat the anniversary as complete at the end of February 28 in common years, so the person ages up on February 28.
• March 1. Others treat a person as not having reached the anniversary until the day after February 28, making March 1 the operative date in common years.

Which convention applies depends on local law and the specific statute — driving age, voting age, and contractual age can even differ within one country. For anything with legal weight, check the rule that governs the particular right rather than assuming. For everyday purposes, most leaplings simply pick February 28 or March 1 and celebrate there in off years.

Leap years you will actually meet

Counting forward from 2026, the next leap years are 2028 and 2032, then every fourth year after that — 2036, 2040, and so on. The next century test is 2100, which will break the every-four-years rhythm by not being a leap year. Anyone who lived through 2000 witnessed the rarer case: a century year that was a leap year, something that had not happened since 1600 and will not happen again until 2400.

Try it

Not sure whether a given year makes the cut? Check whether 2028 is a leap year, or if you were born on February 29 and want your exact completed age, run the age calculator. For the related question of why some years carry a 53rd week, see the sibling guide on ISO week numbers.