Calendar Week Number Calculator
Find the ISO week number of any date.
What is Week Number?
The ISO week-numbering year differs from the calendar year around January and December. This tool computes the correct ISO week for any date.
How it works
ISO weeks start on Monday; week 1 is the week containing the first Thursday. The calculation follows that definition exactly.
- Enter your input. Type or paste your input into the field above — the tool responds instantly.
- Review the result. Output updates live as you type. Everything runs entirely in your browser.
- Copy or download. Use the Copy button to grab the result, or Download to save it as a file.
Examples
ISO week of a date
August 26, 2026 falls in ISO week 35 — useful for sprint planning, reporting, and week-based analytics.
Common mistakes
ISO vs US week numbering
ISO 8601 weeks start on Monday and week 1 contains the year's first Thursday. US systems often start weeks on Sunday and number differently around year-end.
Frequently asked questions
How is ISO week 1 defined?
Week 1 is the week containing the first Thursday of the year. That keeps every week fully inside a single year.
Can I convert a week number back to a date range?
Yes — enter a year and week number and get the Monday-through-Sunday range it covers.
Which week-numbering system should I use?
ISO 8601 (Monday-first) is the standard for software and international reporting; use US Sunday-first only if your tools require it.