Leap Year Calculator
Whether a year is a leap year, and the leap years before and after it.
Good to know. Someone born on 29 February 2000 had their fifth real birthday in 2020, aged twenty.
How it works
In plain words
A year is a leap year if it divides by 4, except century years, which must also divide by 400. So 2024 and 2000 are leap years, but 1900 and 2100 are not.
The formula
leap if (year mod 4 = 0 and year mod 100 ≠ 0) or year mod 400 = 0
Worked example
2028 divides by 4 and isn’t a century year, so the answer is Yes: it has 366 days, with 29 February.
Sources
- ISO 8601, Date and time format Calendar dates in the Gregorian calendar, and weeks that start on Monday.
Sources checked September 30, 2026. 3 automated tests check its answers on every change.
About the leap year calculator
A year is really about 365.2422 days: the time the Earth takes to go round the Sun. A calendar of 365 days would drift by almost a day every four years, so every fourth year gets an extra day, 29 February.
Adding a day every four years makes the average year 365.25 days, slightly too long. The Gregorian calendar, introduced in 1582, fixes that by skipping three leap days every 400 years: century years are only leap years if they divide by 400. So 1600 and 2000 were leap years, while 1700, 1800 and 1900 were not, and 2100 won’t be. That makes the average year 365.2425 days, off by only a day in about 3,000 years.
The calculator uses the Gregorian rules for years from 1583, the first full year of the calendar. Countries adopted it at different times: Britain and its colonies in 1752, Russia in 1918. Before that, the Julian calendar had a leap year every four years without exception.
People born on 29 February, sometimes called leaplings, usually celebrate on 28 February or 1 March in other years. In the UK and many other countries, they legally come of age on 1 March.
Questions people ask
When is the next leap year?
2028, then 2032 and 2036. Leap years come every four years, except century years not divisible by 400.
Is 2100 a leap year?
No. It is a century year that doesn’t divide by 400. The next century leap year is 2400.
Why do we have leap years?
Because a year is about 365.2422 days. Adding a day almost every four years keeps the calendar in step with the seasons.