What Is an ISO Week Number?
An ISO week number is the week of the year defined by the ISO 8601 standard. ISO weeks start on Monday, and Week 1 is the first week of the year that contains at least four days in January (or the first Thursday of the year).
This system is widely used in business planning, scheduling, reporting, and international logistics.
How the ISO Week Number Calculator Works
This calculator determines the ISO week number by applying ISO 8601 rules to the selected date. It accounts for edge cases around the start and end of the year, where a date in early January may belong to the last ISO week of the previous year, or a date in late December may belong to Week 1 of the next year.
Why Week Numbers Matter
- Project management: Track deliverables using Week 32, Week 45, etc.
- Business reporting: Compare performance by week across months and quarters.
- Scheduling: Coordinate tasks and meetings using a shared week numbering system.
- Logistics: Plan shipments and production cycles by ISO weeks.
Examples of ISO Week Numbers
- Early January: Some dates may fall in the last ISO week of the previous year.
- Late December: Some dates may count as ISO Week 1 of the following year.
- Mid-year dates: Typically map directly to a week number between 1 and 52 (sometimes 53).
Because of these edge cases, using an ISO week number calculator is more reliable than guessing.
Frequently Asked Questions
What day does an ISO week start on?
ISO weeks start on Monday.
Why does the ISO week year differ from the calendar year?
ISO week numbering is based on weeks, not months. Dates at the start or end of a calendar year can belong to the previous or next ISO week year depending on the ISO 8601 rules.
Can a year have 53 ISO weeks?
Yes. Some years contain Week 53, depending on how the days align across the calendar.
Is this ISO week number calculator free?
Yes. The tool is completely free and runs directly in your browser.