How to Check If a Year Is a Leap Year (With Simple Logic)

Most people know a leap year means February has 29 days instead of 28. But when you actually try to remember how to tell if a year is a leap year, the rules can feel confusing — especially with weird exceptions like 1900 and 2000.

Let’s fix that. Here’s the exact logic, explained clearly and without the usual fluff.

🌍 Why Leap Years Exist

A normal year has 365 days, but Earth actually takes 365.2422 days to orbit the sun. That’s a little more than 365 days — about 6 extra hours each year.

If we did nothing about it, our calendar would slowly drift out of sync with the seasons:

• After ~4 years, we’d be about 1 day off.
• After ~100 years, we’d be about 25 days off.
• Eventually, summer would land in December.

So, to fix this, we add an extra day every 4 years. That’s February 29 — the leap day.

But that simple “every 4 years” rule alone isn’t quite perfect.

⚙️ The Real Leap Year Logic

Here’s the full, accurate rule that keeps our calendar aligned for centuries:

A year is a leap year if: 1️⃣ It’s divisible by 4, and 2️⃣ It’s not divisible by 100, unless 3️⃣ It’s also divisible by 400.

In plain English:

• Every 4 years → leap year.
• Every 100 years → skip it.
• Every 400 years → add it back.

🧮 Examples

💡 Why So Complicated?

Each rule corrects a small error from the one before it:

1. Every 4 years — fixes the drift from 365 → 365.25
2. Skip every 100 years — corrects for the fact that 365.25 is a bit too much
3. Add back every 400 years — balances the overcorrection perfectly

The result? Our Gregorian calendar stays accurate to within one day every 3,300 years — a near-perfect system.

🧠 The Logic Formula

If you like logic expressions, here’s how it looks mathematically:

Leap year ⇔ (divisible by 4) ∧ (¬(divisible by 100) ∨ divisible by 400)

And in code (Python example):

def is_leap_year(year):
    return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)

✅ Quick Summary

• Every 4 years → leap year
• Every 100 years → not a leap year
• Every 400 years → leap year again

So:

• 2000 → ✅ Leap year
• 1900 → ❌ Not a leap year
• 2024 → ✅ Leap year
• 2100 → ❌ Not a leap year

⚡ Bottom Line

Leap years exist because the Earth’s orbit isn’t perfectly clean in whole days. The 4 / 100 / 400 rule isn’t random — it’s a finely tuned system that keeps our seasons, calendars, and clocks in sync over thousands of years.

Simple rule of thumb:

“If it’s divisible by 4, it’s a leap year — except centuries, unless divisible by 400.”