The rule, in full
A year is a leap year if it is divisible by 4 — except years divisible by 100, which are not — except years divisible by 400, which are. So 2024 and 2028 are leap years, 1900 was not, and 2000 was.
That third clause is the one everybody forgets, and it is why software written without it broke on 29 February 2000 and will break again on 29 February 2100, which is not a leap year.
Why the rule is this complicated
A tropical year is about 365.2422 days. Adding a day every four years overcorrects slightly, to an average of 365.25 — about 11 minutes a year too long. Skipping three leap days every four centuries brings the average to 365.2425, which is within about 27 seconds a year of the true figure. That is the Gregorian reform of 1582, and it is accurate enough to drift by one day in roughly 3,200 years.
Leap years 2020 to 2060
2020 · 2024 · 2028 · 2032 · 2036 · 2040 · 2044 · 2048 · 2052 · 2056 · 2060. Every four years without exception this century, because 2100 is the next century year and it is not divisible by 400.
What a leap year changes
- The year has 366 days rather than 365.
- Every day number from 1 March onward shifts up by one — see day of the year.
- A date advances by two weekdays rather than one across the year.
- Any span containing 29 February is one day longer than a naive 365-day calculation gives.
29 February birthdays
Roughly one person in 1,461 is born on 29 February. In common years, most jurisdictions treat 1 March as the day they become a year older, though some use 28 February. Our age calculator uses 1 March and says so on the result.
Working it out without a calculator
For any year in this century the rule collapses to one step: divide by 4, and if there is no remainder it is a leap year. The century clauses do not fire again until 2100. For years outside 2001–2099 the full test is three steps in order — divisible by 400 is a leap year; otherwise divisible by 100 is not; otherwise divisible by 4 is. Applied to the years people usually ask about: 1900 fails at step two and is not a leap year, 2000 passes at step one and is, 2100 fails at step two and is not, and 2400 passes at step one and is.
Why 1900 and 2100 matter more than they look
A great deal of software has implemented only the first two clauses, or only the first. A system that knows nothing but "divisible by 4" was wrong about 1900 and will be wrong about 2100. A famous spreadsheet lineage deliberately treats 1900 as a leap year for backwards compatibility with a bug in a competitor's product from the 1980s, which means date arithmetic spanning 28 February 1900 in a spreadsheet is off by one against every correct implementation — a defect preserved on purpose for over forty years. If you are reconciling dates against a spreadsheet and everything before March 1900 is a day out, that is why.
Worked example — what a leap day does to a span
From 1 January 2028 to 1 January 2029 is 366 days rather than 365, because 29 February 2028 sits inside it. Any per-day rate applied across that span is one day's worth larger than a 365-day assumption gives. The same effect moves weekdays: a date advances by one weekday from year to year across a common year and by two across a leap year, which is why your birthday appears to skip a weekday every four years rather than moving steadily. 1 January 2028 is a Saturday and 1 January 2029 is a Monday.
The arithmetic behind the rule
A tropical year is about 365.2422 days. A leap day every four years gives an average of 365.25 — about 11 minutes 14 seconds a year too long, which is a full day every 128 years and was the accumulated ten-day error the Gregorian reform of 1582 removed. Dropping three leap days per four centuries gives 365.2425, an error of about 27 seconds a year, or one day in roughly 3,200 years. There is no further correction in the rule; when the drift matters, some future calendar committee will have to decide what to do about it.
Leap years and related pages
The next several are 2028, 2032, 2036, 2040, 2044, 2048, 2052, 2056, 2060 and 2064. For an individual year with its full set of facts — day count, weekday of 1 January, ISO week count and whether it carries a 29 February — see is 2028 a leap year or how many days in 2028. For what a leap day does to the day-of-year numbering, see day of the year: every date from 1 March onward shifts up by one. For the effect on an age or a span, the age calculator and days between dates both flag leap days explicitly rather than leaving them to be discovered.