# Why 2100 is not a leap year (and 2000 was)

> Every fourth year gets a 29 February. Except some do not. The full rule has three parts, and it exists because a year is not a whole number of days.

Published 2026-10-08 by Toolcloak. 3 minute read. Original: https://toolcloak.pages.dev/articles/why-2100-is-not-a-leap-year
Related free tool: [Leap Year Checker](https://toolcloak.pages.dev/tools/leap-year-checker)

Most people learn: a leap year is any year divisible by 4. It works for your whole life, so it feels complete. But 1900 was not a leap year, 2000 was, and 2100 will not be. The shortcut is nearly right, and the gap is the interesting part.

> **In short**
> - Leap year if divisible by 4, except century years, unless also divisible by 400.
> - So 2000 was a leap year, but 1900 and 2100 are not.
> - The rule gives an average year of 365.2425 days, very close to the real 365.2422.

## The full rule

1. If the year is divisible by **400**, it is a leap year. (2000, 2400)
2. Otherwise, if it is divisible by **100**, it is **not** a leap year. (1900, 2100, 2200)
3. Otherwise, if it is divisible by **4**, it is a leap year. (2024, 2028, 2032)
4. Anything else is a normal 365-day year.

Try it: [Leap Year Checker](https://toolcloak.pages.dev/tools/leap-year-checker). Type a year to see whether it is a leap year, and which rule applies.

| Year | Divisible by 4 | By 100 | By 400 | Leap year? |
| --- | --- | --- | --- | --- |
| 2024 | Yes | No | No | Yes |
| 2026 | No | No | No | No |
| 1900 | Yes | Yes | No | No |
| 2000 | Yes | Yes | Yes | Yes |
| 2100 | Yes | Yes | No | No |

*Century years are the only ones where the “every four years” rule is overruled.*

## Why a leap year exists at all

Earth takes about **365.2422 days** to go round the Sun. A calendar with only 365 days falls behind by nearly a quarter of a day each year. After four years it is almost a whole day out, and the seasons slowly drift against the dates. The extra day on 29 February pulls the calendar back.

## Adding a day every four years is too much

A leap day every fourth year makes the average year 365.25 days. That is a hair too long, by about 0.0078 days a year. Small, but it builds up: roughly **one extra day every 128 years**.

**Three calendars, three average years**

|  | Value | Note |
| --- | --- | --- |
| Plain year | 365 | Drifts by about 1 day every 4 years |
| Leap day every 4 years | 365.25 | Drifts by 1 day every ~128 years |
| The full rule | 365.2425 | Drifts by 1 day in roughly 3,000 years |

*The real tropical year is about 365.2422 days. The Gregorian rule is off by only around 0.0003 days a year.*

The fix is to skip the leap day in century years, except every fourth century. In 400 years that is 100 years divisible by 4, minus 4 century years, plus 1 for the one divisible by 400: **97 leap years**. And 365 + 97/400 = 365.2425.

**Leap years in each 400-year cycle**

|  | Value |
| --- | --- |
| Every 4th year | 100 |
| Minus centuries | 96 |
| Plus 400s | 97 |

*Start with 100, remove the 4 century years, then add one back for the year divisible by 400. That leaves 97 per cycle.*

## A bit of history

**How the calendar got fixed**

| When | What |
| --- | --- |
| 45 BC | The Julian calendar. A leap day every four years, no exceptions. Average year of 365.25 days. |
| 1582 | The Gregorian reform. Ten days were removed: 4 October was followed by 15 October. The century rule began. |
| Later | Countries switched at different times. Britain and its colonies changed in 1752. Others waited until the twentieth century. |

*Because countries changed on different dates, old records from different places can disagree by days.*

## Useful facts

- **Next leap years:** 2028, 2032 and 2036. The year 2100 will skip it.
- **A 29 February birthday** comes up once in 1,461 days, because four years hold 365 × 4 + 1 days.
- **Software bugs.** Programs that only check “divisible by 4” fail on 2100. Programs that forget the 400 rule fail on 2000, and some did.
- **Days in a leap year:** 366, so a year has 52 weeks and 2 days, which is why dates shift weekdays by two in a leap year and one in a normal year.

## Frequently asked questions

### Is 2100 a leap year?

No. It is divisible by 100 but not by 400, so the century rule removes it.

### Was 2000 a leap year?

Yes. It is divisible by 400, which overrides the century exception.

### What is the leap year rule?

Divisible by 4 is a leap year, except years divisible by 100, unless they are also divisible by 400.

### Why are there leap years?

Earth’s orbit takes about 365.2422 days, so a 365-day calendar drifts. A leap day keeps dates aligned with the seasons.

### How many leap years are there in 400 years?

97. That gives an average calendar year of 365.2425 days.