A leap year is any year with 366 days instead of the usual 365, with the extra day, 29 February, added to keep our calendar lined up with the Earth’s journey around the Sun. It happens roughly every 4 years, though not quite as simply as “divide by 4 “: century years like 1900 don’t count unless they also divide evenly by 400.
For UK primary pupils, this sits neatly across two parts of the curriculum: KS2 maths, where leap years are a real-world example of division and remainders, and KS2 science, where they connect to the Earth’s orbit within the “Earth and Space” unit.
This guide sets out exactly what a leap year is, why the extra day exists, and how to work out whether any given year qualifies, using the same 100/400 rule that trips up plenty of adults, let alone Year 5 pupils meeting it for the first time. It also covers the history behind the rule, from Julius Caesar’s original attempt to Pope Gregory XIII’s correction in 1582, along with a handful of UK-specific traditions that rarely get a mention outside Britain.
Whether you’re a teacher looking for a classroom-ready explanation or a parent fielding “why does February get an extra day?” over breakfast, the aim here is the same: a clear answer first, followed by enough detail to actually understand it, rather than just memorise it.
A leap year is a calendar year containing 366 days rather than 365, with the additional day, 29 February, inserted specifically to stop the calendar drifting away from the astronomical seasons. Without it, spring would slowly creep later and later in the calendar until, after enough centuries, the Northern Hemisphere would be marking Christmas somewhere near midsummer.
The reason comes down to the Earth’s orbit. It takes the Earth about 365.242189 days, or 365 days, 5 hours, 48 minutes and 45 seconds, to complete one full trip around the Sun. Our Gregorian calendar rounds this down to a neat 365 days a year, which means every ordinary year falls about a quarter of a day short of the real solar year. Adding an extra day every four years corrects most of that shortfall.
Most children first meet the term “leap year” through the rhyme about the extra day in February, but the underlying idea, that our tidy calendar needs the occasional adjustment to match the messier reality of orbital mechanics, is genuinely useful groundwork for later science lessons on the solar system.
Leap years exist because a calendar year of exactly 365 days is very slightly shorter than the time the Earth actually takes to orbit the Sun. That leftover 5 hours, 48 minutes and 45 seconds each year sounds trivial, but it adds up fast enough to matter within a single human lifetime.
Left uncorrected, this shortfall accumulates at about one day every 4 years. After a century without any adjustment, the calendar would have drifted roughly 24 days away from the seasons it’s supposed to track. After 400 years, that drift approaches 100 days, enough to push a summer month into what the calendar still calls winter.
The table below shows how quickly this becomes a problem if leap years didn’t exist.
| Time period without leap years | Approximate seasonal drift |
|---|---|
| 4 years | About 1 day |
| 25 years | About 6 days |
| 100 years | About 24 days |
| 400 years | About 97 days |
Adding a leap day every four years brings the calendar back into close alignment, though not a perfect one, which is exactly why the 100/400 rule exists as a further correction.
Not every year divisible by four is a leap year. Centuries ending in “00” are the exception: they’re only leap years if they also divide evenly by 400. This single rule is the part most commonly left out of simpler explanations, and it’s the detail worth spending real classroom time on.
Working through it step by step: first check whether the year is divisible by 4. If it doesn’t, it’s not a leap year, end of story. If it does, and the year ends in “00”, check whether it also divides evenly by 400. Only then is it a leap year. Every year divisible by 4 is a leap year, so no extra check is needed.
| Year | Divisible by 4? | Divisible by 100? | Divisible by 400? | Leap year? |
|---|---|---|---|---|
| 1900 | Yes | Yes | No | No |
| 2000 | Yes | Yes | Yes | Yes |
| 2024 | Yes | No | – | Yes |
| 2100 | Yes | Yes | No | No |
| 2400 | Yes | Yes | Yes | Yes |
This is why 2000 was a leap year, but 1900 wasn’t, and why 2100 won’t be one either, even though both 1900 and 2100 pass the simple “divide by four” test. The correction exists because a leap year every four years without exception slightly overcorrects the calendar, and skipping three leap years every 400 years brings it back into much closer alignment with the real solar year.
Working out whether a year is a leap year is a genuinely useful piece of division practice for Year 4 to Year 6 pupils, using real numbers rather than abstract ones. The method is always the same three checks, applied in order.
First, divide the year by 4. If there’s a remainder, it isn’t a leap year. If it divides exactly, move to the second check: does the year end in “00”? If not, it’s a leap year. If it does end in “00”, apply the final check: divide by 400. Only years that pass this last test are leap years.
Take 2032 as an example. It divides by 4 with no remainder, and it doesn’t end in “00”, so the second check is enough on its own: 2032 is a leap year. Now take 2200. It divides by 4 exactly, but it does end in “00”, so the third check applies: 2200 divided by 400 leaves a remainder, so 2200 is not a leap year. Working through examples like these out loud, rather than just stating the rule, is usually what makes it stick.
“Fractions and remainders click for children when they’re attached to something real rather than a worksheet of random numbers. Leap years give you an actual reason to care whether a division comes out exactly even, which is half the battle with division at this age,” says Michelle Connolly, Founder of LearningMole and former teacher with over 15 years of classroom experience.
The leap year rule we use today was introduced in two stages, roughly 1,600 years apart. Julius Caesar, working with the Alexandrian astronomer Sosigenes, introduced the first leap day into the Roman calendar in 46 BC, adding an extra day every four years without exception.
Caesar’s version was close but not quite right. Because it added a leap day every four years with no century exception, the Julian calendar drifted from the solar year by roughly one day every 128 years. By the 1500s, that drift had built up to around ten days, enough that the spring equinox was landing on the wrong date entirely. Pope Gregory XIII, advised by the mathematician and astronomer Christopher Clavius, corrected this in 1582 by introducing the 100/400 rule still in use today. Not every country adopted the change straight away. Britain and its colonies didn’t switch from the Julian to the Gregorian calendar until 1752, a shift that involved skipping 11 days in September to catch up.
There’s also a distinctly British footnote to this history. St Oswald of Worcester, Archbishop of York in the tenth century, died on 29 February 992 and is sometimes referred to as “the leap year saint” because his feast day falls on 28 February in other years. It’s a small but genuine piece of UK history that rarely makes it into leap year explainers written for a general audience.
Leap years have picked up a fair number of customs and quirks over the centuries, some better documented than others. A person born on 29 February is known as a “leapling”, and since the date only appears once every four years, leaplings usually mark their birthday on either 28 February or 1 March in other years.
One of the better-known traditions has Irish roots. According to a long-standing legend traced to St Brigid of Kildare, women were traditionally permitted to propose marriage to men on 29 February, a custom that spread to Scotland and beyond, and is still referenced in Ireland and the UK today, even though its historical origins are more folklore than documented fact.
Leap years also throw up genuine legal oddities. In the UK, when a leaping year reaches a milestone birthday, such as turning 18, in a year that isn’t a leap year, the law treats 1 March as the relevant date, a position confirmed in legislation covering the age of legal capacity. It’s a small but real example of how an unusual calendar quirk ends up written into law.
Not every fourth year is a leap year. To keep precision tight, Pope Gregory XIII established a rule in 1582 for the Gregorian calendar:
Example: 2000 was a leap year (divisible by 400), but 1900 was not, and 2100 will not be either.
Dating back to a 5th-century Irish legend, St. Bridget complained to St. Patrick that women had to wait too long for men to propose. St. Patrick declared that on Leap Day, women could turn the tables and propose to men.
In Scotland, Queen Margaret allegedly passed a 1288 law stating that any man who turned down a Leap Day proposal had to pay a fine—often in the form of a silk gown, a pair of leather gloves, or 100 kisses!
In parts of southern Germany, young men traditionally place a small birch tree decorated with ribbons outside the home of their crush on May Day. However, during a Leap Year, the tradition flips: women place the decorated birch tree for the men.
While some cultures celebrate the date, others tread carefully:
Leap years work well as a cross-curricular topic, giving teachers a single real-world example that supports both KS2 maths and KS2 science in one lesson. In maths, the 100/400 rule is a natural fit for teaching division with remainders and conditional reasoning (“if this, then check that”). In science, the underlying cause, the Earth’s orbit not matching a whole number of days, connects directly to the “Earth and Space” strand of the National Curriculum.
A simple classroom activity that works well: give pupils a list of years (some ordinary, some century years) and ask them to apply the three-step check themselves before revealing the answer. This turns an abstract rule into something they’ve actually worked out rather than been told.
To keep it accessible to primary learners, frame the concept as a “leftover fraction” of time.
For teachers planning a maths or science lesson around this topic, LearningMole’s cover calendar and time calculations in more depth, and the links to the leap year rule to wider orbital mechanics topics. Parents looking to reinforce this at home can find the same explanations in a form that works well for evening homework conversations, without needing any specialist knowledge themselves.
Leap years typically come up from Year 3 onwards as part of work on time, calendars and calculation, with the 100/400 rule usually introduced later, around Year 5 or Year 6, once pupils are confident with division and remainders. LearningMole’s maths resources are structured to match this progression by year group.
A year is a leap year if it is divisible by 4. The exception is century years (ending in “00”), which are leap years only if they are also divisible by 400. This is why 2000 was a leap year, but 1900 and 2100 are not.
Yes. The most recent leap year was 2024, and the next is 2028, since it divides evenly by 4 and isn’t a century year, so the 100/400 exception doesn’t apply.
A leapling is someone born on 29 February. Because that date only appears on the calendar once every four years, leaplings usually celebrate on either 28 February or 1 March in other years.
Julius Caesar introduced the first version in 46 BC, adding a leap day every four years without exception. Pope Gregory XIII corrected this in 1582 by adding the 100/400 rule, creating the Gregorian calendar most of the world still uses.
The simplest starting point is Earth’s orbit: it takes slightly more than 365 days, so every four years we add a day to keep the calendar in sync. Working through a couple of years together using the three-step check (divide by 4, check for “00”, divide by 400) turns the explanation into a shared activity rather than a lecture.
In the UK, when a leaper reaches a milestone birthday in a non-leap year, the law treats 1 March as the relevant date for legal purposes, such as reaching a set age. This is set out in legislation covering the age of legal capacity.
Effective teaching combines the maths (division and the 100/400 rule) with the science (the Earth’s orbit and why the calendar needs correcting). LearningMole’s Time and Measurement and Earth and Space resources are designed to be used alongside each other for exactly this kind of cross-curricular lesson.
Leap years are a small quirk of the calendar with a surprisingly rich amount packed into them: real division practice, a genuine link to orbital science, thirteen centuries of history from Julius Caesar to Pope Gregory XIII, and a handful of UK-specific traditions that most general explainers skip entirely. For a topic that takes up one extra day every four years, it touches an unusual number of different subjects.
For teachers, the 100/400 rule in particular is worth building into a proper lesson rather than a passing mention, since it’s one of the few places where a calendar fact and a genuine maths skill overlap so cleanly. For parents, the same explanation works just as well around the dinner table, especially with an actual year or two to test out together rather than the rule stated in the abstract.
LearningMole’s curriculum-aligned resources cover both the mathematical and scientific sides of this topic in more depth, with videos and activities designed for KS2 classrooms and home learning alike. To take this further, whether that’s planning a lesson or helping with homework this term, see our subscription plans to unlock everything KS1 and KS2 teachers need for primary education.
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