Leap Year Checker

How to check a leap year

1. Type a Year from 1 to 99999, for example 2028. The current year is filled in when the page opens, and This year brings you back to it.
2. Click Check (or press Enter). The answer updates as you type.
3. Read the verdict, then the details: Days in the year (365 or 366), Days in February, the weekday that February 29 falls on, the Previous leap year, the Next leap year and the answer under the old Julian calendar.
4. Under Why: the Gregorian rule, step by step, the tool shows the divisions it made, so you can follow the reasoning or reuse it in a homework answer.
5. To get a list, fill in From and To in List the leap years in a range and click List leap years. The tool counts them, names the century years that were skipped, and Copy copies the list. A range can cover up to 10,000 years.

The leap year rule

The Gregorian calendar, used almost everywhere today, has three tests:

• A year divisible by 4 is a leap year (2024, 2028)…
• …except a year divisible by 100, which is a common year (1900, 2100)…
• …unless it is also divisible by 400, which makes it a leap year again (2000, 2400).

In code, that is (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0. Over a 400-year cycle, the rule gives 97 leap years instead of 100, so the average calendar year lasts 365.2425 days. The solar (tropical) year lasts about 365.2422 days, so the Gregorian calendar drifts by only about one day every 3,000 years or so.

Julian and Gregorian calendars

The Julian calendar, introduced by Julius Caesar in 46–45 BC, used a simpler rule: every year divisible by 4 is a leap year, century years included. Its average year of 365.25 days is about 11 minutes too long, and by the 16th century the calendar was 10 days behind the seasons. Pope Gregory XIII fixed this in 1582: Thursday, October 4 was followed by Friday, October 15, and the century exception was added.

Adoption was gradual. Catholic countries such as Spain, Portugal and Italy switched in 1582, and France in December of the same year. Great Britain and its colonies, including the future United States and parts of Canada, waited until 1752, when Wednesday, September 2 was followed by Thursday, September 14. Russia changed in 1918 and Greece in 1923. When the tool says the two calendars disagree (1700, 1800, 1900, 2100…), that is exactly the kind of year that makes old dates tricky. For years before 1582, the tool applies the Gregorian rule backwards (the proleptic Gregorian calendar), which is the convention of ISO 8601 and most software.

February 29 facts

• A leap year has 366 days and 52 weeks plus 2 days, so a date moves two weekdays forward the year after February 29 (instead of one).
• A person born on February 29, a “leapling”, has a real birthday only once every four years (sometimes eight, around 1900 or 2100). Roughly 1 person in 1,461 is born on that day. Most people celebrate on February 28 or March 1 in common years; the legal date for reaching an age depends on local law.
• The 400-year Gregorian cycle has exactly 146,097 days, which is a whole number of weeks (20,871). That is why the calendar repeats every 400 years, weekdays included.
• The next skipped century year is 2100: there will be no February 29, 2100.