Find Common Years
List all common (365-day) and leap (366-day) years in a range. Counts, ratios, download. Free, offline, client-side, instant and secure.
Enter a Gregorian year range (1-9999). The tool returns every common year (365 days, no Feb 29) and every leap year (366 days) in that range, with counts, percentages, and an option to view both. Auto-swaps the bounds if you put the bigger year first.
How to Use Find Common Years
- Type a Start year and an End year (1-9999). Order doesn't matter - if Start > End the tool auto-swaps the bounds and notes it in the stats line.
- Pick a mode. "Common years only" (default) lists 365-day years. "Leap years only" lists 366-day years. "Both, grouped" shows both lists in one go.
- Click Find years or press Enter in either year field (or Ctrl/Cmd + Enter from anywhere). The output and stats line update immediately.
- Read the stats line - range bounds, span (total years), counts of common + leap with percentages, active mode, and whether the bounds were auto-swapped.
- Sanity check the ratio. Across any long enough range you should see roughly 97 leap years per 400, so leap years should be ~24% - slightly less than 1 in 4 because of the century rule.
- Copy or download. Copy uses the Clipboard API with an
execCommandfallback. Download TXT saves<mode>-years-START-END.txt.
Frequently Asked Questions
What is considered a common year?
A common year is a standard calendar year with exactly 365 days. It is any year that is NOT a leap year according to the Gregorian rules, meaning it does not contain a February 29th.
What’s the leap-year rule again?
A year is a leap year if it’s divisible by 4, EXCEPT century years (divisible by 100), UNLESS they’re also divisible by 400. So 2024 is leap, 2100 is common (century, not divisible by 400), and 2000 was leap (century AND divisible by 400). The tool’s predicate is (y % 4 === 0 && y % 100 !== 0) || y % 400 === 0.
What’s the ratio of common to leap years?
Over any 400-year cycle exactly 97 years are leap and 303 are common – so 24.25% leap, 75.75% common. The Gregorian calendar’s average year length is 365.2425 days, which matches the solar year to within ~26 seconds. Across small ranges the ratio fluctuates; the tool’s stats line shows the percentage for whatever range you entered.
What happens if I enter Start > End?
The tool auto-swaps the bounds so you never get an empty result by mistake. The stats line includes “bounds auto-swapped” so you know what happened.
What about the Julian calendar?
Before October 1582, most of Europe used the Julian calendar, which had a simpler leap rule (every 4 years, no exceptions). This tool uses the Gregorian rule for all years – proleptic for pre-1582 dates. For historical research before 1582, the year classification may differ from what a contemporary calendar would say.
Why is the range limited to 1-9999?
It’s a UI bound, not a math limit. The algorithm is pure modulo arithmetic with no Date involved, so it works for any integer in theory. The cap keeps inputs sensible – a million-year range would produce a huge text dump but technically work.
What does the “Both” mode show?
Two grouped lists: “Common (N):” followed by every common year separated by commas, then “Leap (M):” followed by every leap year. Useful for side-by-side comparison without running the tool twice.
Is my data secure?
Yes, completely! This Common Year Finder operates 100% locally directly from your browser’s (client-side) engine. No calculation data is transmitted across the internet to our servers preserving absolute privacy.