Week Number Calculator
Calculate week numbers from dates or find date ranges from week numbers
Select Date
Week Number Results
Quick Reference
ISO 8601 Standard
- • Week starts on Monday
- • First week contains January 4th
- • Used internationally in business
- • Format: YYYY-WW (e.g., 2025-W36)
- • Most common in Europe and Asia
US Standard
- • Week starts on Sunday
- • First week contains January 1st
- • Common in North America
- • Used by many US businesses
- • Calendar year aligned
When to Use Week Number Calculator
Project Management
Track project timelines and milestones using week numbers for clear scheduling and progress reporting across international teams.
Business Planning
Plan quarterly reviews, budget cycles, and business operations using standardized week numbering for consistent reporting.
Payroll Systems
Calculate weekly pay periods and manage employee scheduling with precise week number calculations for payroll processing.
Compliance Reporting
Meet regulatory requirements that specify week-based reporting periods using standardized week numbering systems.
Manufacturing
Coordinate production schedules, inventory cycles, and supply chain management using week-based planning systems.
Academic Planning
Plan academic calendars, course schedules, and semester timelines using standardized week numbering for educational institutions.
Frequently Asked Questions
What is a week number calculator?
A week number calculator is a tool that determines which week of the year a specific date falls into, or finds the date range for a specific week number. It supports different standards like ISO 8601 and US week numbering systems, making it essential for project management, payroll, and international business coordination.
What's the difference between ISO 8601 and US week numbering?
ISO 8601 weeks start on Monday and the first week contains January 4th (or the first Thursday), while US standard weeks typically start on Sunday and the first week contains January 1st. This can result in different week numbers for the same date, especially at the beginning and end of the year.
How do I calculate the current week number?
Simply enter today's date in the calculator or use the 'Use Today' button. The tool will automatically calculate the current week number according to your chosen standard (ISO 8601 or US). You can also switch between standards to compare the differences.
Why do some dates show different week numbers in different systems?
Different week numbering systems use different rules for determining the first week of the year and which day starts the week. ISO 8601 uses Monday as the first day and requires the first week to contain January 4th, while US standard uses Sunday and includes January 1st. These differences can cause the same date to have different week numbers.
Can I find the date range for a specific week number?
Yes, use the 'Week Number to Date Range' tab to enter a year and week number to get the complete date range for that week. This is particularly useful for project planning, scheduling meetings, and calendar management where you need to know the exact start and end dates of a specific week.
Is this week number calculator free to use?
Yes, this week number calculator is completely free to use with no registration required. You can perform unlimited calculations, download results, and access all features without any cost or subscription fees.
How accurate are the week number calculations?
Our calculator follows official ISO 8601 and US week numbering standards precisely. The calculations are validated against international standards and are suitable for business, academic, and professional use requiring exact week number accuracy.
Can I download or copy the calculated results?
Yes, you can copy the results to your clipboard or download them as a text file. This makes it easy to integrate the week number information into your projects, reports, or documentation.
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