How to use the Leap Year Calculator
- Enter the year as a four-digit number.
- Read the yes or no answer.
- Check the days in the year — 366 for a leap year, 365 otherwise.
- Check February's length, which is the only month affected.
- Read the next leap year, which is usually but not always four years later.
How the calculation works
The Gregorian rule has three tiers: a year divisible by 4 is a leap year, unless it is divisible by 100, unless it is also divisible by 400. So 2024 and 2000 are leap years while 1900 and 2100 are not. The century exceptions are what people forget, and they are the reason a great deal of software failed its first test in the year 2000.
The complexity exists because the tropical year is about 365.2422 days, not a tidy 365.25. Adding a day every four years overcorrects by roughly 11 minutes a year, which had pushed the Julian calendar ten days out of alignment by 1582. Skipping three leap days every 400 years brings the average calendar year to 365.2425 days, leaving an error of about one day in 3,030 years.
The consequences reach well beyond February. Interest accrual on an actual/365 basis is short a day in leap years, salaried staff on a daily divisor work an extra day for the same annual pay, and 29 February birthdays and anniversaries need an explicit rule in any system that renews annually. Date libraries that assume 365-day years produce quiet drift rather than loud errors, which makes it worse.
leap = (year mod 4 = 0 AND year mod 100 ≠ 0) OR year mod 400 = 0Source: Gregorian calendar reform, papal bull Inter gravissimas (1582); codified in ISO 8601:2019.
Worked example
Check 2100, a year many systems will get wrong.
- 2100 ÷ 4 = 525, so the first test passes.
- 2100 ÷ 100 = 21, so the century exception applies and it is excluded.
- 2100 ÷ 400 = 5.25, so the 400-year rescue does not apply.
- The next leap year is therefore 2104.
2100 is not a leap year — February has 28 days — despite being divisible by four. Any system using the naive divisible-by-four test will be a day out from March 2100 onwards.
Frequently asked questions
Why was 2000 a leap year but 1900 was not?+
Both are divisible by 100, but only 2000 is also divisible by 400, which reinstates the leap day under the third rule.
How often does the rule get it wrong?+
The Gregorian year averages 365.2425 days against a true 365.2422, so it drifts by about one day every 3,030 years.
When do people born on 29 February have a birthday?+
Legally it varies. Many jurisdictions treat 1 March as the anniversary in common years; others use 28 February. Systems should pick one and apply it consistently.
Do leap seconds relate to leap years?+
No. Leap seconds adjust for irregularities in Earth's rotation and are announced individually; leap years correct the calendar against the orbital year.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.