Week Number Calculator: What Week of the Year Is It?
Determine the ISO week number for any date, or find out which dates fall within a specific week number. Essential for project management and international scheduling.
Week Number Calculator
Find the week number of any date (ISO 8601 or US system). Convert week numbers back to date ranges.
Date to Week Number
Week Number to Date Range
Current Week Information
Current Week Number
13
Week Start
Mon, Mar 23, 2026
Week End
Sun, Mar 29, 2026
Browse Weeks of 2026
Week Number Systems
ISO 8601 (International Standard)
- Week starts on Monday
- Week 1 is the week containing January 4th
- Year can have 52 or 53 weeks
- Used in Europe and most international contexts
US System
- Week starts on Sunday
- Week 1 starts on January 1st
- Year can have 52 or 53 weeks
- Commonly used in North America
How it works
Enter any date to find its week number, or enter a week number to see its date range. Choose between ISO 8601 (Monday start) or US system (Sunday start) based on your needs.
The calculator shows the exact start and end dates for any week. Week 1 definitions differ between systems - ISO uses the week containing January 4th, while US starts counting from January 1st.
ISO 8601 example:
Date: January 15, 2024 Week: 3 of 2024 Range: Mon Jan 8 - Sun Jan 14
Browse all weeks of a year with the interactive grid. Click any week to see its date range. Current week highlighting helps with quick reference for planning and reporting.
When You'd Actually Use This
Manufacturing production schedules
Factories plan by week numbers. Production runs reference week 23, not June 5th. Supply chains coordinate using standard week identifiers.
Pregnancy tracking
Doctors track pregnancy in weeks. Week 20 ultrasound, week 40 due date. Parents understand timeline in weekly increments.
Academic semester planning
Universities schedule by week. Week 1 orientation, week 15 finals. Students track progress through the term.
European business reporting
EU companies report by ISO week. Financial quarters break into weeks. Cross-border teams use standard week numbers.
Project sprint planning
Agile teams plan in week sprints. Sprint 23 runs week 23-24. Retrospectives reference specific week numbers.
Payroll processing
Weekly payroll uses week numbers. Timesheets reference specific weeks. HR systems track pay periods by week.
What to Know Before Using
ISO week 1 can start in previous year.If January 1st is Friday-Sunday, week 1 starts next Monday. Late December dates may belong to week 1 of next year. This is correct ISO behavior.
Some years have 53 weeks.ISO years with 53 weeks occur every 5-6 years. 2020, 2026, and 2032 have week 53. Most years have 52 weeks.
US system is simpler but less standard.Week 1 always starts January 1st. No cross-year complications. Common in North America but not internationally recognized.
Week numbers reset every year.Week 52 of 2024 differs from week 52 of 2025. Always include the year when referencing week numbers. Prevents scheduling confusion.
Pro tip: For international projects, always specify ISO 8601. It's the global standard. US system confuses European partners.
Common Questions
What's the difference between ISO and US?
ISO starts weeks on Monday, US on Sunday. ISO week 1 contains January 4th. US week 1 starts January 1st. ISO is international standard.
Why does week 1 sometimes start in December?
ISO rule: week 1 contains the first Thursday. If January 1st is late in the week, week 1 starts the previous Monday. This ensures consistent week definitions.
How do I find week numbers in Excel?
Use =WEEKNUM(date,2) for ISO weeks. The ,2 parameter sets Monday as week start. US system uses =WEEKNUM(date) or =WEEKNUM(date,1).
What week number is today?
The calculator shows current week information automatically. Check the "Current Week" section for this week's number and date range.
Can week numbers be negative?
No, week numbers range from 1 to 52 or 53. Invalid dates return no result. The calculator validates all inputs before computing.
Do week numbers affect payroll?
Weekly payroll often uses week numbers. Some years have 53 pay periods instead of 52. Budget for the extra pay period in 53-week years.
Which countries use ISO week numbers?
Most European countries use ISO 8601. UK, Germany, France, and Scandinavia all use it. US and Canada typically use Sunday-start weeks.
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