C Leap Year Program

What Is a Leap Year?

A leap year has 366 days (February has 29). In the Gregorian calendar, leap years follow the 4 / 100 / 400 rule.

year is leap  ⇔
  (year % 4 == 0 AND year % 100 != 0)
  OR
  (year % 400 == 0)

Examples:
  2024 → divisible by 4, not by 100     → leap
  1900 → divisible by 100, not by 400   → not leap
  2000 → divisible by 400               → leap
  2023 → not divisible by 4             → not leap

How to Check a Leap Year in C

Four clear steps — then write it as one expression or nested ifs.

1. If the year is not divisible by 4, it is not a leap year.
2. If it is divisible by 4 but not by 100, it is a leap year.
3. If it is divisible by 100, it must also be divisible by 400.
4. Otherwise it is not a leap year.

Pseudocode

function IsLeapYear(year):
    if year mod 400 == 0:
        return true
    if year mod 100 == 0:
        return false
    if year mod 4 == 0:
        return true
    return false

Cheat Sheet

Keep these facts handy while writing the program.

Live Preview

Enter any year to see each divisibility check and the final verdict.

year = 2024
2024 % 4 = 0
2024 % 100 = 24
2024 % 400 = 24
Divisible by 4 and not a century year
Verdict: LEAP YEAR

Worked Walkthrough — year = 2000 and 1900

Century years look similar until you apply the % 400 test.

year = 2000
  2000 % 400 = 0  → leap  ✓

year = 1900
  1900 % 400 = 300 ≠ 0
  1900 % 100 = 0      → century, not leap  ✗

year = 2024
  2024 % 4 = 0
  2024 % 100 ≠ 0      → leap  ✓

C Programs

Start with a one-line boolean expression. Use nested ifs when you want to explain each rule out loud.

#include <stdio.h>
#include <stdbool.h>

bool is_leap_year(int year)
{
    return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
}

int main(void)
{
    int year = 2024;

    if (is_leap_year(year))
    {
        printf("%d is a leap year\\n", year);
    }
    else
    {
        printf("%d is not a leap year\\n", year);
    }
    return 0;
}

2024 is a leap year

#include <stdio.h>
#include <stdbool.h>

bool is_leap_year(int year)
{
    if (year % 400 == 0)
    {
        return true;
    }
    if (year % 100 == 0)
    {
        return false;
    }
    if (year % 4 == 0)
    {
        return true;
    }
    return false;
}

int main(void)
{
    printf("%s\\n", is_leap_year(2000) ? "true" : "false");
    printf("%s\\n", is_leap_year(1900) ? "true" : "false");
    printf("%s\\n", is_leap_year(2024) ? "true" : "false");
    return 0;
}

true
false
true

Time and Space Complexity

A single-year check is a few modulus operations. Scanning a year range is O(range size).

Key Takeaways