Week Number Calculator
Find the week number for any date.
Week Number Calculator
Find week numbers and detailed calendar information for any date
Week Numbering Standards:
- • ISO 8601: International standard where weeks start on Monday and week 1 contains January 4th
- • US Standard: American convention where weeks start on Sunday
- • Sunday-based: Alternative Sunday-first numbering system
- • Useful for project planning, payroll, and scheduling systems
How to Use the Week Number Calculator
Select Date: Choose the date for which you want to find the week number.
Choose Standard: Select from ISO 8601, US Standard, or Sunday-based numbering.
Get Results: View week numbers for all standards plus detailed calendar information.
Calculation Examples
January 15, 2024 (Monday)
ISO 8601:
Week 3
US Standard:
Week 3
Sunday-based:
Week 3
June 30, 2024 (Sunday)
ISO 8601:
Week 26
US Standard:
Week 27
Sunday-based:
Week 27
December 31, 2024 (Tuesday)
ISO 8601:
Week 1 (2025)
US Standard:
Week 53
Sunday-based:
Week 53
Frequently Asked Questions
What is ISO 8601 week numbering?
ISO 8601 is the international standard where weeks start on Monday and Week 1 is the first week containing January 4th. This is used by most international organizations and in Europe and Asia.
How does US Standard differ?
US Standard starts weeks on Sunday and Week 1 is the first week of the year (partial week allowed). This aligns with American calendar conventions.
Why might week numbers differ at year end?
Week numbers may belong to the following year in ISO 8601 standard near year-end. For example, December 31, 2024 is Week 1 of 2025 in ISO 8601 but Week 53 in US Standard.
Which standard should I use?
Use ISO 8601 for international projects, US Standard for American contexts. Most modern calendar applications support ISO 8601 week numbering. Stick to one standard throughout your project or organization for clarity.
Week Numbering Standards
ISO 8601 (International Standard)
- Week starts: Monday
- Week 1: The first week that contains January 4th
- Used by: Most international organizations, Europe, Asia
- Benefits: Consistent across years, widely recognized globally
US Standard
- Week starts: Sunday
- Week 1: The first week of the year (partial week allowed)
- Used by: United States, some American organizations
- Benefits: Aligns with American calendar conventions
Sunday-based Numbering
- Week starts: Sunday
- Week 1: Alternative Sunday-first calculation method
- Used by: Some organizations and scheduling systems
- Benefits: Flexible for specific business needs
Practical Applications
Project Management
Track project milestones and deliverables using standardized week numbers for consistent reporting.
Payroll Systems
Organize payroll periods and calculate pay dates using consistent week numbering across the year.
Academic Planning
Plan academic terms, assignments, and examination schedules using week-based organization.
Manufacturing
Schedule production runs and maintenance cycles using standardized week numbers for efficiency.
Retail Planning
Plan seasonal campaigns, inventory cycles, and promotional activities by week numbers.
Financial Reporting
Create consistent weekly financial reports and analyze business performance trends.
Week Number Tips
Choose the Right Standard
Use ISO 8601 for international projects, US Standard for American contexts.
Year-End Considerations
Week numbers may belong to the following year in ISO 8601 standard near year-end.
Consistency is Key
Stick to one standard throughout your project or organization for clarity.
Calendar Integration
Most modern calendar applications support ISO 8601 week numbering.
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