DateDelta

Leap Year Calculator

The three-part divisibility rule behind why some century years (like 1900) aren't leap years.

Year

Leap year?

A year is a leap year if divisible by 4, except centuries not divisible by 400.

Days in year: 365

What you entered

Year: 2026

1
Divisible by 4?
2026 ÷ 4= no
2
Divisible by 100?
2026 ÷ 100= no
3
Divisible by 400?
2026 ÷ 400= no
4
Leap year rule: div by 4 AND (not div by 100 OR div by 400)
apply the rule= Not a leap year

Result

Leap year?: No

2026 is not a leap year, so it has 365 days.

Leap years and the century rule

A year is a leap year if divisible by 4, except centuries, which must also be divisible by 400.

Year	Leap year?	Days	Which rule applies
1900	No	365	Century not divisible by 400
2000	Yes	366	Century divisible by 400
2020	Yes	366	Divisible by 4
2023	No	365	Not divisible by 4
2024	Yes	366	Divisible by 4
2025	No	365	Not divisible by 4
2026	No	365	Not divisible by 4
2100	No	365	Century not divisible by 400

1900 and 2100 are the rows that catch people, and they caught a good deal of software too: both are divisible by 4 but are centuries not divisible by 400, so neither is a leap year. 2000 was, which meant a generation of programmers wrote code that never had to handle the exception and some of it will fail in 2100. The rule exists because the tropical year is about 365.2422 days - adding a day every four years overcorrects slightly, and skipping three leap days per four centuries brings the calendar back to within about one day in 3,200 years.

Why century years are the exception

A pure 'every 4 years' rule would add too many days over centuries, drifting the calendar out of sync with the seasons. The Gregorian calendar corrects for this by skipping leap years in century years not divisible by 400 — so 1900 wasn't a leap year, but 2000 was.

Why leap years exist at all

Earth takes about 365.2422 days to orbit the sun, not exactly 365. Without leap years, the calendar would drift about 1 day off from the seasons every 4 years, and after a few centuries, seasons would shift entirely.

Frequently asked questions

Is 2100 a leap year?

No. While 2100 is divisible by 4, century years must also be divisible by 400 to be leap years. 2100 ÷ 400 = 5.25, so 2100 is NOT a leap year. The same applies to 1900 and 2200. But 2000 and 2400 ARE leap years (divisible by 400).

Why not just make every 4th year a leap year?

Because Earth's orbit is 365.2422 days, not 365.25. Adding a day every 4 years (= 365.25) over-corrects by about 3 days every 400 years. The century-year exception (skip 3 out of 4 century years) brings the average to 365.2425 — close enough to stay in sync for thousands of years.

What happens if you're born on February 29?

You only get a 'real' birthday every 4 years. Legally, most jurisdictions treat March 1st as your birthday in non-leap years for age-related purposes (voting, drinking age, etc.). Some people celebrate on February 28th instead — there's no universal standard.

How did the Julian calendar handle leap years differently?

The older Julian calendar made every 4th year a leap year with no exceptions — no century-year skip rule. This overshot by about 11 minutes per year, accumulating a 10-day drift by 1582, when Pope Gregory XIII introduced the Gregorian calendar to fix it.