Leap Year Calculator

The rule in one sentence: 4, 100, 400

A year is a leap year if it divides evenly by 4, unless it also divides evenly by 100, in which case it is a leap year only if it divides evenly by 400. That is the whole rule. It gives 2024 a February 29, denies one to 1900, and grants one to 2000.

The three tests run in order and each one can override the last. Divisible by 4 puts you in the running. Divisible by 100 knocks you out. Divisible by 400 puts you back in. Nothing else matters, and no other calendar exception exists in the Gregorian system.

The arithmetic is easy enough to do in your head once you know the trick for the last two digits. A year ending in 24, 28, 32 and so on divides by 4. A year ending in 00 divides by 100, so it needs the 400 test, which comes down to whether the first two digits divide by 4. That is why 1600 and 2000 pass, and 1700, 1800, 1900 and 2100 fail.

The calculator above applies the same three tests in code, then adds the parts a rule alone cannot tell you: how many days the year holds, when the next leap year arrives, and what a February 29 birthday looks like on a calendar.

Why the rule exists: the 365.2422 day year

Earth does not take exactly 365 days to go around the sun. It takes about 365.2422, which is 365 days, 5 hours, 48 minutes and 46 seconds. A calendar of flat 365 day years drifts against the seasons by roughly one day every four years, and after a few centuries the solstice lands in the wrong month.

Julius Caesar fixed the drift in 46 BC with a plain four year rule, and that calendar stayed close for 1,600 years. It was not exact: 0.2422 is a little less than a quarter, so the Julian year ran about 11 minutes long, which added up to roughly one day every 128 years. By the 1500s the spring equinox had slipped ten days and the church calendar was out of step with the sky.

The Gregorian reform of 1582 dropped those ten days and added the century exceptions. Skipping three leap days every 400 years removes about 18 hours per century, which brings the average year to 365.2425 days. The remaining error is about 26 seconds a year, or one day every 3,300 years, which is close enough for any calendar that has to plan a harvest.

So the odd 100 and 400 rules are not bureaucratic fussiness. They are the correction terms that keep a 365 day calendar aligned with a 365.2422 day orbit, and the 97 leap years in each 400 year cycle are the exact number needed.

Testing any year in three steps

Run the tests in order and stop when one of them decides the answer. The table below shows the same five years put through the same three questions, which is the fastest way to see how the century rule overrides the four year rule.

Notice how 1900 passes the first test and fails the second. That is the trap. A person born in 1900 lived through a year with 365 days and no February 29, and anyone using the simple four year rule would have marked it wrong. The same applies to 2100, which is the next century year most people alive today will not see, and to 2200 and 2300 after it.

The 400 test is the one people forget, and it is the reason 2000 was a leap year when 1900 was not. A quick check: take the first two digits of the year and divide by 4. For 2000 that is 20 divided by 4, which is 5 with nothing left over, so it passes. For 2100 that is 21 divided by 4, which leaves a remainder, so it fails.

February 29 birthdays: calendar age against the real count

Someone born on February 29 has a legal birth date and a calendar problem. In most US states the legal age advances on February 28 or March 1 in non leap years, and the rule varies by state and by purpose: driver licenses, voting, drinking age and contracts are not always handled the same way. For everyday purposes, a leap day baby turns a year older on March 1 in common years, and most families celebrate on February 28.

The fun number is the count of real birthdays. A person born February 29, 1996 and still going in 2026 is 30 years old by the calendar, but has only seen seven actual February 29 dates since the day they were born: 2000, 2004, 2008, 2012, 2016, 2020 and 2024. The next one falls in 2028, which is two years out.

Every leap day birthday lands on a different weekday each time, and the gap is usually four years but sometimes eight. A century year that fails the 400 test creates an eight year gap, which last happened around 1900 and will happen again around 2100. Someone born on February 29, 2096 would wait until 2104 for a second birthday, an eight year stretch that no living person has experienced.

For paperwork, the safe habit is to write the date as it appears on the birth certificate and let the agency apply its own rule. Do not pre-convert it to February 28 or March 1 on a form, because a mismatched date can stall a passport or a benefits application.

Leap years and day count interest

Interest is charged per day, and the day count convention decides which denominator a lender uses. Three conventions cover most consumer loans and savings accounts, and a leap year moves each of them by a different amount.

Actual/365 divides the annual rate by 365 every year, so a 366 day year produces one extra day of interest. On $10,000 at 5 percent that is $1.37, which is trivial on a savings account and less trivial on a $400,000 mortgage, where the same convention adds about $55 in a leap year. Actual/366 does the reverse and charges slightly less in a common year.

Actual/actual is the fair version: the daily rate is the annual rate divided by the number of days in the year being charged, so the total comes out to the stated annual rate either way. Many mortgages and most Treasury securities use it. Credit cards usually use a daily periodic rate built on 365, which means a leap year quietly costs a little more.

The convention is written into the note, not chosen year by year. Look for a line like "interest computed on a 365 day year" in the truth in lending disclosure. If the loan documents are silent, ask the servicer which convention applies before you compare two offers, because a 366 denominator and a 365 denominator are not the same loan.

Leap years in payroll and hourly work

A salaried employee earns the same annual salary in 366 days as in 365, so the daily rate dips slightly in a leap year. Divide $78,000 by 365 and you get $213.70 a day. Divide the same salary by 366 and you get $213.11. The paycheck does not change, but the cost per working day does, which matters to anyone billing a client by the day.

Hourly workers see the opposite effect. A leap year contains one extra calendar day, and if that day is a scheduled workday, it is one more day of pay. Whether it is scheduled depends on where the extra day falls: 2028 begins on a Saturday, so the added day is a Saturday, which most hourly workers do not work. A leap year that starts on a Monday adds a Monday, which most do.

Biweekly payroll has its own arithmetic. Twenty six pay periods cover 364 days, so every year leaves one or two days unpaid in the cycle, and a leap year leaves two. That is why a few employers run 27 paychecks in a year every so often, and why the third paycheck in a month shows up on a schedule that drifts.

None of this changes the annual total for a salaried worker. It changes the timing, the daily rate and the overtime calculation, since overtime is usually derived from an hourly rate that itself comes from a daily or weekly figure. A payroll system that divides an annual salary by 366 instead of 365 produces a rate that is 0.27 percent lower, which is small per hour and visible across a large payroll.

The century years people get wrong

Century years are where the rule earns its keep, and they are also where most online answers go wrong. The table lists every century year from 1700 to 2400 with its status, so you can check the pattern instead of trusting a memory.

The pattern is one leap century year out of every four, which is exactly the 97 in 400 count. Three century years are skipped, then one is kept, and the cycle repeats. That is why the year 2000 mattered: it was the first kept century year since 1600, and the next one is 2400.

Software gets this wrong in a specific way. A check written as year divisible by 4 passes 1900 and fails the correct answer, and a check written as year divisible by 400 without the 4 test fails 2024. The reliable form tests 400 first, then 100, then 4, and returns as soon as one of them settles it.

Excel and Google Sheets both handle the rule correctly through their date functions, but a spreadsheet that stores a year as text and compares it to a four year pattern will not. If you keep a lease schedule or an amortization table that runs past 2100, test the formula on 2100 before you trust it.

Leap seconds are a different thing

A leap second is not a leap day, and the two get mixed up constantly. A leap day corrects the calendar against the seasons. A leap second corrects clock time against the rotation of the Earth, which is not perfectly steady. Since 1972 the world has inserted 27 leap seconds, the last one in 2016, each one making a minute 61 seconds long.

The reason is that a day based on Earth's rotation is about 86,400.002 seconds, so atomic clocks and the sky drift apart by roughly a second every 18 months. Without correction, noon would eventually fall at night. The NIST time and frequency division maintains the US contribution to the global timescale and explains how the correction is decided.

Leap seconds are being phased out. A 2022 decision by the body that coordinates world time calls for stopping new leap seconds by 2035, largely because they break software that assumes every minute has 60 seconds. The US Naval Observatory publishes the official timekeeping data and the earth orientation measurements behind the decision.

For a calendar question, ignore leap seconds entirely. They never add a date, never move February 29, and never change a day count in a loan or a payroll run. For interest reporting, the IRS guidance on interest income works from calendar dates, so a leap day is just another day in the accrual period.

Calendar rules are stable, but payroll and lending conventions vary by contract. Confirm the day count convention in your own documents before relying on a figure from this page.