Week Number Calculator
Find the ISO 8601 week number for any date along with week-of-month, year-week notation, and week start/end dates.
✓ Runs in your browser · Updated 2026-03-31Enter values and click Find Week Number to see results
What Is an ISO Week Number?
ISO 8601 defines a week numbering system where weeks start on Monday and week 1 is the week containing the year's first Thursday. This standard is used worldwide for scheduling, accounting periods, and project management.
Year-Week: YYYY-Www (e.g., 2025-W42)
Week of Month: ⌈day of month / 7⌉
How ISO Weeks Are Calculated
To find the ISO week: adjust the date to the nearest Thursday, then count weeks from January 1 of that Thursday's year. A year has 52 weeks, except years where January 1 is a Thursday (or Dec 31 in leap years), which have 53 weeks.
Week Start and End Dates
ISO weeks always start on Monday and end on Sunday. The calculator shows the exact Monday and Sunday bounding the given date, which is useful for weekly reports and timesheet periods.
Related Tools
Find the day of the week for any date with the Date to Day Converter. Count business days between dates using the Work Days Calculator.
Frequently Asked Questions
What is ISO week numbering?
ISO 8601 defines week 1 as the week containing the first Thursday of the year. Weeks start on Monday. A year has 52 or 53 weeks.