Week Number Calculator - Find ISO Week Number for Any Date
Calculate the ISO week number for any date. Find which week of the year a date falls in, with week start and end dates. Essential for project planning and scheduling.
Week Number Calculator
Calculate the ISO week number for any date. Week numbers are essential for project planning, business scheduling, and organizing recurring events. This calculator uses the ISO 8601 standard for consistent week numbering.
What is ISO Week Numbering?
ISO 8601 is an international standard for week numbering:
- Week 1: The first week with a Thursday in it
- Week Start: Monday (week begins on Monday)
- Week End: Sunday (week ends on Sunday)
- Year: The year that contains the Thursday of week 1
How Week Numbers Work
Week 1 Definition
Week 1 is the week containing the first Thursday of the calendar year. This ensures:
- Most years have 52 weeks
- Some years have 53 weeks (when year starts on Thursday or is a leap year starting on Wednesday)
- Consistent numbering across years
Week Boundaries
Each week runs from Monday (00:00) to Sunday (23:59:59). This is different from some systems that use Sunday as the week start.
Common Uses
Business Planning
Week numbers help organize:
- Project timelines and milestones
- Recurring meetings and events
- Reporting periods
- Deadline tracking
Event Planning
Useful for:
- Scheduling recurring events
- Planning annual activities
- Organizing calendars
Week Number Examples
2025 Week Numbers
- Week 1: December 30, 2024 - January 5, 2025
- Week 26: June 23 - June 29, 2025 (mid-year)
- Week 52: December 22 - December 28, 2025
Special Cases
53-Week Years
Some years have 53 weeks instead of 52. This happens when:
- Year starts on Thursday (common year)
- Year starts on Wednesday (leap year)
- Year starts on Tuesday (leap year following a leap year)
Year Boundaries
Week 1 can start in the previous year. For example, if January 1st is a Friday, Saturday, or Sunday, week 1 actually starts in December of the previous year.
Week Number vs. Calendar Week
ISO week numbers differ from simple "week of year" calculations:
- ISO: Week 1 contains first Thursday
- Simple: Week 1 starts January 1st
ISO is more consistent and internationally recognized.
Tips for Using Week Numbers
- Use for planning: Week numbers are great for recurring schedules
- Consistent reference: "Week 15" is clearer than "mid-April"
- International standard: Works across countries and systems
- Project management: Essential for Gantt charts and timelines
Planning Tip: Week numbers are especially useful for project management and business planning. Instead of saying "the third week of March," you can reference "Week 12" which is consistent year-to-year. Many project management tools and business calendars use week numbers for this reason!