About the Leap Year Calculator
Every few years February gains a 29th day and the year grows to 366 days. Most people know the "every four years" rule, but the full rule has two exceptions that decide whether century years such as 1900, 2000 and 2100 are leap years — and they catch out people and software alike.
This leap year calculator tells you whether any year is a leap year and why, step by step. It gives the number of days in the year and the previous and next leap years. In range mode, it counts and lists the leap years between two years, gives the total days, and names any century years that are skipped.
How to Use the Leap Year Calculator
Choose Check a year or Leap years in a range.
Enter a year, or a from and to year. Years from 1583 onwards are supported, covering the Gregorian calendar.
The answer and the reasoning appear immediately.
The Leap Year Rule
1. If the year is divisible by 400 → leap year (2000, 2400)
2. Otherwise, if it is divisible by 100 → common year (1900, 2100)
3. Otherwise, if it is divisible by 4 → leap year (2024, 2028)
4. Otherwise → common year (2026, 2027)The order matters: check 400 first, then 100, then 4.
Step-by-Step Examples
2028. Not divisible by 400 or by 100, but divisible by 4 (2028 ÷ 4 = 507), so it is a leap year with 366 days. The previous leap year is 2024 and the next 2032.
1900. Divisible by 100 (a century year) but not by 400 (1900 ÷ 400 = 4.75), so it is not a leap year.
2000. Divisible by 400 (2000 ÷ 400 = 5), so it is a leap year, even though it is a century year.
2000 to 2100. Every fourth year from 2000 to 2096 is a leap year — 25 in all — but 2100 is skipped. The 101 years contain 36,890 days.
Why Leap Years Exist
The Earth takes about 365.2422 days to go round the Sun relative to the seasons (a tropical year). A calendar of exactly 365 days would drift by almost a quarter of a day each year — about one day every four years, and a whole month in roughly 120 years — until summer fell in the calendar's winter months. Adding a leap day every four years fixes most of the drift.
Why Century Years Are Different
Adding a day every four years makes the average year 365.25 days, which is about 11 minutes too long. Over centuries that adds up to about three days every 400 years. The Gregorian calendar, introduced by Pope Gregory XIII in 1582, corrects this by dropping the leap day in three century years out of every four: 1700, 1800 and 1900 were common years, 2000 was a leap year, and 2100, 2200 and 2300 will be common years again. The result is an average year of 365.2425 days, which drifts by only about one day in 3,000 years.
The Julian Calendar
Before 1582, Europe used the Julian calendar, introduced by Julius Caesar in 46 BC, which had a leap year every four years without exception. By the 16th century it had drifted about ten days away from the seasons, so when the Gregorian calendar was introduced, ten days were skipped: Thursday 4 October 1582 was followed by Friday 15 October. Britain and its colonies did not switch until 1752, by which time 11 days had to be dropped. This is why the calculator starts at 1583.
Leap Day Birthdays
People born on 29 February — "leaplings" — have a true birthday only in leap years. The chance of being born on the day is about 1 in 1,461. In common years most celebrate on 28 February or 1 March; laws on which date counts for legal age differ between countries. A leapling born in 2000 celebrated only their seventh true birthday in 2028.
Leap Years and Software
The century rule is a classic source of software bugs. Programs that treat every year divisible by 4 as a leap year work until 2100, when they will wrongly add 29 February. Others that treat every century year as common wrongly rejected 29 February 2000. When writing date code, use a well-tested date library rather than a home-made rule.
Understanding Your Result
The headline says whether the year is a leap year, or how many leap years are in the range.
The why line explains which divisibility test decided it.
The days line gives the number of days in the year or range.
The nearby leap years line gives the previous and next leap years.
The leap years and century years lines list the leap years in a range and any skipped centuries.
When Should You Use This Calculator?
Use it to check whether a particular year has 29 February.
Use it to find the next leap year.
Use it to count leap years and days over a period.
Use it to test date-handling code.
Common Mistakes
Treating every century year as a leap year. 1900 and 2100 are not.
Treating every century year as a common year. 2000 was a leap year.
Checking the rules in the wrong order. Test 400, then 100, then 4.
Applying the rule before 1582. Older dates used the Julian calendar.
Assuming the Olympics prove a leap year. The two often coincide, but not always, as the postponed 2020 Games showed.