Leap Year Checker
Test any year against the Gregorian leap-year rules.
Enter any year (1 to 9999). The checker applies the divisible-by-4, not-by-100, divisible-by-400 rule.
How is this calculated?
Gregorian rule: a year is a leap year if it is divisible by 4, except for years divisible by 100 unless they are also divisible by 400. So 2000 was a leap year, 1900 was not. The rule keeps the calendar aligned with the solar year of about 365.2425 days. Source: Inter Gravissimas papal bull (1582), establishing the Gregorian calendar.
About this Leap Year Checker
A leap year is a calendar year that contains 366 days instead of 365, with the extra day added as February 29. The Leap Year Checker on this page applies the official Gregorian rule (Inter Gravissimas, 1582) to any year from 1 to 9999 and returns yes or no, the reason, and the next five leap years that follow. Leap years exist because the Earth takes about 365.2425 days to orbit the Sun, so a fixed 365-day calendar would drift by roughly one day every four years without correction.
How the Leap Year Checker works
Gregorian leap rule: divisible by 4 AND NOT divisible by 100 -> leap divisible by 400 -> leap any other year -> not leap isLeap(y) = (y % 4 === 0 AND y % 100 !== 0) OR (y % 400 === 0)
- Step 1. Is the year divisible by 4? If no, it is not a leap year. Done.
- Step 2. Is it divisible by 100? If no, it is a leap year. Done.
- Step 3. Is it divisible by 400? If yes, it is a leap year. If no, it is not.
- Average year length: 97 leap years per 400-year cycle gives (400 x 365 + 97) / 400 = 365.2425 days, accurate to within 26 seconds of the tropical year.
- Boundary correction: the rule trims three leap days every 400 years (1700, 1800, 1900 are not leap; 2000 is) to compensate for the Julian over-count.
Worked example: is 2100 a leap year?
Walk through the Gregorian rule on the year 2100, a common trick question because it looks like it should be a leap year.
- Step 1 - divisible by 4? 2100 / 4 = 525 with no remainder. Yes.
- Step 2 - divisible by 100? 2100 / 100 = 21 with no remainder. Yes.
- Step 3 - divisible by 400? 2100 / 400 = 5.25. No.
- Verdict: divisible by 100 but not 400, so the century exception applies and 2100 is NOT a leap year.
- Compare with 2000: 2000 / 400 = 5 cleanly, so 2000 WAS a leap year. The next century leap year after 2000 is 2400.
Leap years 2000 to 2100 reference
Every fourth year is a leap year except 2100, which is divisible by 100 but not 400. There are 25 leap years in this stretch (the full 2001 to 2100 century has 24, since 2100 is excluded).
| Year | Leap? | Reason |
|---|---|---|
| 2000 | Yes | Divisible by 400 |
| 2020 | Yes | Divisible by 4, not by 100 |
| 2024 | Yes | Divisible by 4, not by 100 |
| 2026 | No | Not divisible by 4 |
| 2028 | Yes | Divisible by 4, not by 100 |
| 2032 | Yes | Divisible by 4, not by 100 |
| 2050 | No | Not divisible by 4 |
| 2096 | Yes | Divisible by 4, not by 100 |
| 2100 | No | Divisible by 100 but not 400 (century exception) |
| 2400 | Yes | Divisible by 400 (next century leap after 2000) |
Common pitfalls when checking leap years
- Assuming the rule is just "divisible by 4." That was the Julian rule. The Gregorian century exception kicks in three times every 400 years and trips up most quick mental checks. 1700, 1800, 1900, 2100, 2200, and 2300 all fail despite being divisible by 4.
- Confusing Julian and Gregorian dates before 1582. Countries adopted the Gregorian calendar at different times (Catholic Europe in 1582, Britain in 1752, Russia in 1918). For historical dates, specify which calendar you mean. The Julian calendar is now 13 days behind Gregorian.
- The 4000-year correction is not official. Astronomers since John Herschel (1864) have proposed dropping the leap day at years divisible by 4000, but no standards body has adopted it. The Gregorian rule drifts about one day every 3,236 years, which is below current concern.
- Year zero does not exist. The historical sequence runs 1 BCE then 1 CE, with no year 0. ISO 8601 does introduce a year 0 (equal to 1 BCE) for arithmetic, but most historical sources skip it. Leap year math still works in either system.
- Leap seconds are a separate fix. Leap years correct the orbital mismatch; leap seconds (added irregularly by the IERS) correct the slowing of Earth's rotation against atomic time. They are unrelated.
Related date and time tools
Frequently asked questions
Why was 2000 a leap year but 1900 was not?
Both years are divisible by 100, which normally cancels the leap rule. The Gregorian exception restores leap status when the year is also divisible by 400. 2000 / 400 = 5 with no remainder, so 2000 was a leap year. 1900 / 400 = 4.75, so 1900 was not. The same pattern means 2100, 2200, and 2300 will not be leap years, but 2400 will.
Is 2026 a leap year?
No. 2026 is not divisible by 4 (2026 / 4 = 506.5), so February 2026 has 28 days and the year has 365 days. The next leap year is 2028, followed by 2032, 2036, 2040, and 2044.
What is the difference between the Julian and Gregorian leap-year rule?
The Julian calendar (introduced 45 BCE) treated every year divisible by 4 as a leap year, which left the calendar drifting 11 minutes per year. Pope Gregory XIII fixed this in 1582 by skipping 10 days and adding the century exception: centuries are not leap years unless divisible by 400. The Gregorian rule has an average year length of 365.2425 days, within 26 seconds of the true tropical year.
Will there ever be a 4000 rule?
It has been proposed but never adopted. John Herschel suggested in 1864 that years divisible by 4000 should also be excluded from leap status, bringing the average year to 365.24225 days. No standards body (ISO, IERS, or national calendar authority) has implemented this. The Gregorian calendar still drifts about one day every 3,236 years, but no correction is officially scheduled.
Sources
- Inter Gravissimas (1582) - the papal bull introducing the Gregorian calendar and the 400-year century rule.
- ISO 8601:2019 Date and time - Representations for information interchange - the modern standard for civil calendar arithmetic.
- IANA Time Zone Database (tzdata 2026a) - canonical zone definitions, including pre-1972 local mean time fallbacks.
- US Naval Observatory Astronomical Applications - tropical year length 365.2422 days, used to justify the 365.2425 Gregorian average.
- Herschel, John (1864) Outlines of Astronomy - the original proposal for a 4000-year leap-day correction.