Week Number Calculator
Find the official ISO week number for any calendar date, plus start and end boundaries.
ISO Week Number
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How to Find the ISO Week Number of Any Date
- Pick any date from the interactive calendar.
- The tool computes the official ISO-8601 calendar week number (W01 to W53).
- See the total weeks in that calendar year and start/end dates of the current week.
ISO-8601 Week Standard
- Rule: Week 1 of any year is the week containing the first Thursday of that year (the week with January 4th).
- Weeks begin on Monday and conclude on Sunday according to international standards.
Example
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January 4, 2025 falls within
Week 1 (W01). -
Mid-year dates like July 1 fall typically between
Week 26 and Week 27.
Common Applications
- Corporate production planning and manufacturing batch codes (e.g. W24).
- International trade delivery schedules and retail shipping windows.
- Academic semester syllabi and weekly assignment schedules.
Frequently Asked Questions
What is the ISO-8601 standard for week numbers?
ISO-8601 is the global international standard designating Monday as the first day of the week, with Week 1 defined as the week with the majority of its days in the new year.
Can a year have 53 weeks?
Yes, years that begin on a Thursday (or leap years starting on a Wednesday) contain 53 ISO calendar weeks.