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Week numbers are used constantly in project planning and payroll schedules, but the ISO 8601 standard's exact rule for which week is "week 1" trips people up, especially near the start or end of a year. This calculator finds the ISO 8601 week number for any date, following the standard's exact week-1 definition.
How it works
Enter a date. The calculator applies the ISO 8601 rule: weeks run Monday to Sunday, and week 1 of a year is defined as the week containing that year's first Thursday (equivalently, the week containing January 4th).
- Enter date.
- Click Calculate to see your results.
Examples
January 1, 2026
Since January 1, 2026 falls on a Thursday, it lands in ISO week 1 of 2026 (2026-W01).
Who should use it
- Finding which week number a project deadline or milestone falls in.
- Cross-referencing dates against week-based payroll, shipping, or manufacturing schedules.
Industry applications
- Project management and scheduling
- Payroll, logistics, and manufacturing planning
Advantages
- Follows the exact ISO 8601 standard, not an approximation.
- Reports both the week number and the ISO week-year, which can differ from the calendar year.
Limitations
- ISO 8601 week numbering is one of several regional week-numbering conventions — results may differ from a Sunday-start convention used in some countries.
Common mistakes to avoid
- Assuming week numbers always start over cleanly at January 1st — the ISO week-year boundary doesn't always align exactly with the calendar year boundary.
- Using a Sunday-start week convention (common in the US) instead of the ISO 8601 Monday-start convention, which can shift the week number by one for some dates.
Best practices
- When coordinating schedules internationally, confirm whether "week number" refers to the ISO 8601 standard (Monday start) or a regional convention (some use Sunday-start weeks), since they can disagree near year boundaries.
Tips
- Dates in very late December or very early January are the most likely to have a week number and ISO week-year that surprise you — double check those edge cases specifically.