Week Number Calculator
Find the ISO week number for any date.
Pick a date.
Uses the ISO 8601 standard: weeks start on Monday and week 1 contains the year's first Thursday.
Use on any page (bookmarklet)
Want Week Number Calculator without leaving the page you're on? Drag the button below to your bookmarks bar, then click it on any website to open Week Number Calculator right there — it runs entirely in your browser.
Use responsibly: a bookmarklet runs on whatever page you click it on. Avoid sensitive sites such as online banking, payment, or healthcare pages — you run it at your own risk. Everything is processed locally and no data is sent anywhere. See our Terms.
- Show your bookmarks bar if it's hidden — Ctrl+Shift+B (⌘+Shift+B on Mac).
- Drag the button above onto the bookmarks bar.
- Open any website and click the bookmark — the Week Number Calculator panel appears in the top-right corner. Use ✛ to move it between corners, or ›/‹ to tuck it against the edge and pull it back out.
- Click the bookmark again (or the ✕) to close it.
Note: a few sites with strict security policies may block bookmarklets.
Examples
Mid-year date
Input
2025-06-15
Output
ISO Week 24 of 2025 (2025-W24)
Year boundary
Input
2026-01-01
Output
ISO Week 1 of 2026
About this tool
This free online week number calculator tells you the ISO 8601 week number for any date — and defaults to today, so it answers "what week is it?" at a glance. It also shows the ISO week-numbering year, the weekday, and the day of the year. Everything runs in your browser with no upload.
How to use
- The tool opens on today's week number.
- Pick any other date to see its week number.
- Read the week label (e.g. 2025-W24), weekday, and day of year.
Common uses
Planning with week-based schedules, referencing fiscal or sprint weeks, coordinating with teams that use ISO week numbers, and labeling reports or releases by week.
Frequently asked questions
What week number is it?
Open the tool — it defaults to today and shows the current ISO week number. Pick any other date to see its week number.
Which week-numbering standard is used?
ISO 8601, the international standard. Weeks start on Monday and week 1 is the week that contains the year's first Thursday (equivalently, January 4th).
Why does early January sometimes show week 52 or 53?
Under ISO 8601, the first days of January can belong to the last week of the previous year if that week has more days in December. The tool shows the correct ISO week-numbering year alongside the week.
Is this week number calculator free?
Yes — it's a completely free online ISO week number calculator with no sign-up.
Does it also show the day of the year?
Yes. Alongside the week number, it shows the weekday and the day-of-year (1–366) for the selected date.
Is my data uploaded anywhere?
No. The calculation runs entirely in your browser with no upload.
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