Week Number Calculator
Free online week number calculator. No sign-up, no installation. Runs entirely in your browser.
Find Week Number
Enter a date to find its ISO 8601 week number and year position.
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All Weeks in Year
Select a year to view all ISO weeks with their date ranges.
Jan 1 falls on: – ·
Dec 31 falls on: –
| Week | Start Date | End Date |
|---|---|---|
| Enter a year to view weeks | ||
What is ISO 8601 Week Numbering?
ISO 8601 is an international standard for date and time representation. The week numbering system divides each year into weeks, where:
- Week 1 is the first week containing a Thursday (or the week with 4 or more days in the new year)
- Weeks run from Monday to Sunday
- There are 52 or 53 weeks in each year
- This system is widely used in business, project management, and international contexts
How to Use This Calculator
- Find a Week Number: Select any date in the first section to instantly see its ISO week number, year, day of year, and remaining days
- View Year Overview: Enter a year in the second section to see all 52 or 53 weeks with their complete date ranges
- Copy Week Numbers: Use the “Copy Week Number” button to quickly copy the week number to your clipboard
- Quick Navigation: Click “Today” to instantly load today’s date and see current week information
Common Use Cases
- Project Planning: Use ISO weeks to organize sprints, milestones, and deadlines
- International Business: Align schedules across regions using the global standard
- Reporting: Create consistent weekly reports and performance metrics
- Manufacturing: Track production cycles and inventory by ISO week
- Academic Calendar: Organize semester schedules and assignment deadlines
Frequently Asked Questions
Why does week 1 sometimes start in the previous year?
ISO 8601 defines week 1 as the week containing the first Thursday of the year. If January 1–3 falls on a Monday–Wednesday and there’s no Thursday in those days, they belong to the last week of the previous year (week 52 or 53). This ensures proper week boundaries.
Why does my year have 53 weeks instead of 52?
Years with 53 ISO weeks occur roughly every 5–6 years. This happens when January 1 is a Thursday, or January 1 is a Wednesday in a leap year. The extra week captures the extended calendar period.
What’s the difference between week numbers and calendar weeks?
ISO weeks start on Monday, while some regions use Sunday or other days as the week start. ISO weeks also strictly follow the Thursday rule for week 1, which means dates in early January or late December might belong to a different year’s week count.
Can I use this for historical dates?
Yes, this calculator works for any date from 1900 to 2100. However, ISO 8601 was officially published in 1988, so technically it’s a retroactive application for older dates. The calculation itself is mathematically consistent across all dates.
How is the day of year calculated?
Day of year (ordinal day) counts from January 1 as day 1 through December 31 as day 365 (or 366 in leap years). It represents the calendar position within the year, independent of ISO week numbering.
Is this calculator accurate?
Yes, this tool uses JavaScript’s built-in Date object and the ISO 8601 algorithm to calculate week numbers accurately. Results match international standards and major platforms like Google, Excel, and programming libraries.