Student learning the century exception
Understand why 1900 and 2100 are common but 2000 and 2400 are leap years.
Enter each century case and inspect divisibility by 4, 100 and 400.
Can state the complete rule rather than “every four years.”
Leap Year Checker
Apply the full Gregorian 4/100/400 leap-year rule to a bounded whole year and see its century exception, February length, and total days.
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Use this result well
A quick decision brief for this specific tool
Inputs that matter
One whole Gregorian year from 1600 through 9999
Output to expect
Leap/common classification, divisibility by 4/100/400, rule outcome, February length and total year length
How it works
Applies the complete Gregorian rule: divisible by 4, except century years must also be divisible by 400
- Use the 400-year exception for century years: 1900 and 2100 are common; 2000 and 2400 are leap years.
- Confirm the governing calendar for historical or specialized records; this classification does not create reminder, recurrence, anniversary or legal-date policy.
Choose your path
Built around the job you need to finish
Apply the Gregorian 4/100/400 rule to one bounded whole year and expose the exact century exception, February length and total year length.
Developer testing calendar input
Use explicit bounded fixtures for date logic.
Test ordinary, century and non-century years and retain the displayed rule outcome.
Receives a deterministic classification without accepting fractional or unbounded years.
Family planner checking a February date
Know whether the selected Gregorian year includes February 29.
Enter the exact year and inspect February/year lengths before recording the milestone.
Treats the result as calendar classification, not a recurrence/reminder policy.
Authoritative checks for this tool
Outputs and checklists are planning aids. Review the linked current authorities and the records, terms, instructions, and requirements that apply to your exact situation before a consequential decision.
- Leap YearsU.S. Naval Observatory · Primary 4/100/400 Gregorian leap-year rule with 1900, 2000, 2100 and 2400 century examples.
- Introduction to CalendarsU.S. Naval Observatory · Gregorian civil-calendar history, month structure and leap-year context.
- ISO 8601-1:2019 — Date and time representationsInternational Organization for Standardization · Current basic rules for Gregorian calendar-date representation in information interchange.
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Projects using Leap Year Checker
This tool is part of these guided projects. Each project provides step-by-step instructions with checklists and all the tools you need in one place.
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Reference & details
How it works
Updated August 2026
How it works
Updated August 2026Whole-Year Validation
The input must be a whole Gregorian year from 1600 through 9999. Fractions, non-finite values, and out-of-range years are rejected.
4/100/400 Rule
Divisibility by 4 creates the ordinary leap case; divisibility by 100 invokes the century exception; divisibility by 400 restores leap status.
Leap = divisible by 4 AND (not divisible by 100 OR divisible by 400)Calendar Consequence
A Gregorian leap year has February 29 and 366 days; a common year has 28 days in February and 365 days. No recurrence policy is inferred.
Updated: August 2026
Example Scenarios
A student compares 1900, 2000, 2100, and 2400 and explains each result from the displayed divisibility checks.
A developer records ordinary and century cases as deterministic test fixtures while keeping the supported range explicit.
A family planner checks whether the selected year contains February 29, then chooses reminder handling outside the calculator.
Common Mistakes to Avoid
Common Mistakes to Avoid
Using only the divisible-by-four shortcut for century years
Apply the full 4/100/400 rule: century years are common unless they are also divisible by 400.
Treating leap-year classification as a recurrence policy
Set legal, birthday, anniversary, payroll, or reminder handling for February 29 outside this calendar classification.
FAQ
A year divisible by 4 is a leap year, except a year divisible by 100 is common unless it is also divisible by 400.
1900 is divisible by 100 but not by 400, so the Gregorian century exception makes it a common year with 28 days in February.
2000 is divisible by 400, so it passes the century exception and has February 29 and 366 total days.
Not every fourth year. Century years such as 1700, 1800, 1900, and 2100 are common unless divisible by 400.
No. It classifies the year only. Recurrence, legal anniversary, birthday, contract, and reminder policies for non-leap years must be set separately.
About Leap Year Checker
Check one whole Gregorian year and inspect every decision in the 4/100/400 rule. The result explains why 1900 and 2100 are common years while 2000 and 2400 are leap years.