What is Week Number Finder?
Week Number Finder tells you which ISO week number (W01–W53) a date falls in, shows the start and end dates of that week, and lists all weeks in the year. Useful for project planning, sprint scheduling, and any reporting that uses week-based periods.
Week numbers are the invisible backbone of project management tools, payroll systems, and manufacturing schedules. Knowing which week you’re in prevents scheduling errors across teams.
How ISO Weeks Work
Weeks Start Monday
ISO 8601 weeks always run Monday to Sunday. Week 1 = the week containing the year's first Thursday.
W01 Rule
Week 1 is the first week with 4+ days in January. Consequence: W01 may start in late December of the prior year.
52 or 53 Weeks
Most years have 52 weeks. Long years (where Jan 1 or Dec 31 is Thursday) have 53.
ISO vs US Numbering
ISO weeks start Monday; US weeks start Sunday. W01 definitions differ. The finder shows both.
How to Use Week Number Finder
Pick a date
Select any date from the calendar picker or type it manually.
Read the week number
The ISO week number (e.g. W19) and the Monday–Sunday span appear instantly.
Browse the year's weeks
The full week calendar shows all 52/53 weeks with their Monday–Sunday spans.
Copy or share
Copy the week reference (e.g. '2025-W19') for use in reports or sprint labels.
Week Number Quick Reference (2025)
| Week | Start (Mon) | End (Sun) |
|---|---|---|
| W01 | 2024-12-30 | 2025-01-05 |
| W10 | 2025-03-03 | 2025-03-09 |
| W20 | 2025-05-12 | 2025-05-18 |
| W26 | 2025-06-23 | 2025-06-29 |
| W52 | 2025-12-22 | 2025-12-28 |
Tips & Common Mistakes
Reference week numbers in sprint names. Instead of “May Sprint,” name it “W19–W20.” This is unambiguous, sorts correctly, and lets anyone calculate the exact dates.
W01 can start in December. If your planning spreadsheet has a row for “Week 1” and you’re populating it in late December, double-check whether those December days are W01 of next year.
Align with your team’s convention. EU and ISO tools default to Monday-start / ISO numbering. US and Google Calendar default to Sunday-start. Confirm which system your project management tool uses.
Related Tools
- Date Difference — calculate how many weeks between two dates
- Date Calculator — find the date N weeks from today
- Business Days Calculator — count working days within a week range