Week Number Calculator Guide
How to find ISO week number, week-year, day of year, and quarter for calendar reporting and planning.
Quick answer
Pick a date to see its ISO week number, ISO week-year, day of year, and quarter. Review the week-year carefully near January and December.
What this tool does
The week number calculator labels a calendar date for reporting and planning. It is useful for release notes, recurring summaries, project calendars, and week-based dashboards.
Step-by-step use
- Select the date
- Review the ISO week number
- Check the ISO week-year
- Note the day of year and quarter if needed
- Confirm whether your team uses ISO or custom fiscal weeks
Data handling and processing behavior
Week calculations are handled in the browser for this tool. Avoid entering sensitive milestone dates unless you have reviewed the implementation.
Best inputs
- Reporting dates
- Release dates
- Planning milestones
- Dashboard labels
- Calendar review dates
Examples
Year boundary
An early January date can belong to the final ISO week of the previous ISO week-year.
Quarter label
A reporting date can be checked for quarter and day-of-year labels before publishing a summary.
Assumptions and limits
- ISO weeks start on Monday
- ISO week 1 is based on the first Thursday rule
- Company fiscal weeks may use different rules
- Week number alone can be ambiguous without week-year
Common mistakes
Assuming January 1 is always week 1
ISO week rules can place early January dates in the prior ISO week-year.
Ignoring custom reporting calendars
Some organizations use fiscal periods that do not match ISO weeks.
Next steps
- Business Days Calculator — count workdays in a reporting range
- Date Add Calculator — move forward or backward from a date
- Countdown Calculator — track remaining time to a milestone
- Time Zone Converter — convert deadlines when the reported date has a time