Week Number Calculator
Find the week number for any date online free — ISO 8601 week numbering for project planning and scheduling.
Last updated: 10 October 2026
Find the ISO 8601 week number for any date, including the week-year when they differ. 12 September 2026 returned week 37 of 2026; 31 December 2025 correctly returned week 1 of 2026 despite its calendar year.
Week Number
ISO Week Number
43
Week Year
2026
Day of Week
Friday
Week 43 of 53
2026 has 53 weeks
Worked Examples
Verified dates
- 12 September 2026 → ISO week 37 of 2026
- 31 December 2025 → ISO week 1 of 2026 (week-year differs from calendar year)
- ISO weeks start on Monday; week 1 contains the year's first Thursday
Where ISO weeks matter
- Financial and fiscal reporting often keys periods to ISO weeks.
- Sprint planning: a two-week sprint aligned to ISO weeks never splits a week.
- The week-year mismatch at year boundaries is the classic bug — always store the week-year alongside the number.
About Week Number Calculator
Find the ISO week number for any date using the ISO 8601 standard. Enter a date and see which week of the year it falls in, along with the year that week belongs to. Test data: September 12, 2026 returned ISO week 37 of 2026 — matching the calendar reference. December 31, 2025 returned ISO week 1 of 2026, correctly handling the year-boundary edge case where a date's ISO week year differs from its calendar year.
Limits: Only provides ISO 8601 week numbering — does not support US-style weeks (Sunday start) or other locale-based systems; Does not show the date range (Monday-Sunday) for the given week; Week year may differ from calendar year at year boundaries — requires understanding of ISO 8601; Does not output the day of the week alongside the week number
Who Uses This Tool
- A project manager planning sprint schedules by checking which ISO week a deadline falls in
- An accountant organizing financial reporting periods that follow ISO week numbering
- A logistics coordinator scheduling weekly shipments using ISO week references in purchase orders
Key Features
- ISO 8601 standard compliance
- Shows week number and year
- Handles year-boundary edge cases
- Instant calculation
- Works with any date
How to use Week Number Calculator
- Enter any date using the date picker.
- See the ISO week number and the year it belongs to.
- The result updates instantly as you change the date.
How It Compares
Unlike timeanddate.com's week number tool which requires page navigation and shows ads, this tool provides instant calculation with no page navigation. It also clearly displays the week year — timeanddate.com sometimes confuses users by not distinguishing between calendar year and ISO week year.
Privacy & Security
Because processing happens directly in your browser, your files and input are not uploaded to our servers.
Written and checked by Mohammad Sohail
Week numbers re-checked on 10 October 2026 against the ISO 8601 calendar.
About FreeDigitalToolkit →Frequently Asked Questions
What is ISO week numbering?
ISO 8601 defines weeks starting on Monday. Week 1 of a year is the week containing the first Thursday. This means some dates in late December belong to week 1 of the next year, and some dates in early January belong to the last week of the previous year.
Why does my date show a different year for the week number?
This is correct ISO 8601 behavior. For example, January 1, 2026 falls on a Thursday, so it's in week 1 of 2026. But January 1, 2027 falls on a Friday, so it belongs to week 52 of 2026. The week year and calendar year can differ at year boundaries.
What's the difference between ISO week numbering and regular weeks?
Regular (locale-based) weeks might start on Sunday or Monday and define week 1 differently. ISO 8601 always starts on Monday and defines week 1 based on the first Thursday. Many European and international businesses use ISO weeks.
How do I find the date range of a specific ISO week?
ISO week N always contains 7 days from Monday to Sunday. To find the Monday of week W in year Y, you can calculate: January 4 is always in week 1, so subtract days to find the Monday of your target week.
Does this work for dates before 1970?
Yes. The ISO 8601 standard applies to any date. The tool handles dates across all years supported by JavaScript's Date object.
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