What is a leap year?
A leap year is a calendar year that contains one extra day — 29 February — bringing the total to 366 days instead of the usual 365. The extra day exists because Earth doesn't take exactly 365 days to orbit the Sun. The real orbital period is about 365.2422 days. If we ignored the extra ~0.2422 of a day, our calendar would drift out of sync with the seasons by nearly 25 days every century.
The rule (once and for all)
A Gregorian year is a leap year if all three of these conditions are met:
- The year is divisible by 4, AND
- The year is not divisible by 100, OR
- The year is divisible by 400.
Worked examples
- 2024 — divisible by 4, not by 100 → LEAP ✅
- 2025 — not divisible by 4 → NOT leap ❌
- 1900 — divisible by 4 AND 100 but NOT by 400 → NOT leap ❌ (the sneaky one!)
- 2000 — divisible by 4, 100 AND 400 → LEAP ✅
- 2100, 2200, 2300 — divisible by 100 not 400 → NOT leap ❌
- 2400 — divisible by 400 → LEAP ✅
Why the 100/400 exception?
Adding a leap day every 4 years overcorrects. Four years of drift is 4 × 0.2422 = 0.9688 days, not exactly one full day. Over centuries this over-correction adds up.
The Gregorian calendar (introduced by Pope Gregory XIII in 1582) fixes this by removing the leap day in years divisible by 100 — except those also divisible by 400. This tunes the average year length to 365.2425 days, matching Earth's real orbit to within 27 seconds per year.
Julian vs Gregorian leap years
Before 1582 the Julian calendar used a simpler rule: every year divisible by 4 was a leap year. This over-shot Earth's orbit by about 11 minutes per year, drifting nearly 10 days by the 16th century. When Pope Gregory reformed the calendar, ten days were skipped: 4 October 1582 was followed immediately by 15 October 1582. Some Eastern Orthodox countries didn't adopt the Gregorian reform until the 20th century.
What if I'm born on February 29?
You're a "leapling" or "leap-day baby". There are roughly 5 million leaplings alive worldwide. Legally, most countries treat 1 March as the official birthday in non-leap years. Culturally, leaplings usually celebrate on 28 February or 1 March depending on preference.
Leap seconds — a different beast
Occasionally the International Earth Rotation Service adds a leap second to UTC to keep atomic time synchronised with Earth's slightly-slowing rotation. Since 1972 there have been 27 leap seconds. These are unrelated to leap years but they solve a similar problem: bridging the gap between human calendars and physical reality.
Fun facts
- The chance of being born on Feb 29 is roughly 1 in 1,461 (~0.068%).
- The Olympic Summer Games happen in leap years (with rare exceptions).
- Ireland has a tradition called "Bachelor's Day" — women propose to men on 29 February.
- In the Chinese calendar, entire leap months (not just leap days) are inserted 7 times every 19 years to keep lunar months aligned with the solar year.
- The next century leap year skip is 2100. If you're alive to see it, you'll experience something only ~1% of humans ever will.
FAQs
How often is a leap year?
Approximately every 4 years, with the exceptions above. On average 97 leap years occur every 400 years — so the Gregorian calendar averages 365.2425 days per year.
Was 2000 a leap year?
Yes. 2000 is divisible by 400, so it followed the "every 400 years" rule. This is why many programmers who only implemented the "divisible by 4" and "divisible by 100" rules had bugs on Feb 29, 2000.
Will 2100 be a leap year?
No. 2100 is divisible by 100 but not by 400, so it will NOT be a leap year. The same applies to 2200 and 2300. The next century leap year will be 2400.
How many days are in a leap year?
366 days. February has 29 days instead of 28.