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.
The ISO 8601 rule, and why it looks odd
ISO week numbering has exactly two rules: weeks start on Monday, and week 1 is the week containing the year's first Thursday. An equivalent phrasing that is easier to check by hand: week 1 is the week containing 4 January.
The Thursday rule exists so that every week belongs wholly to the year holding most of its days, which keeps weeks from being split across a year boundary. The visible consequence is that the first days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next one. That is correct ISO behaviour, not a bug — which is why this tool shows the ISO week-numbering year alongside the number, since “week 1” on its own is ambiguous around New Year.
Why your calendar app disagrees
Because it may not be using ISO. North American convention starts weeks on Sunday and treats the week containing 1 January as week 1, which yields a number one higher than ISO for much of the year. Spreadsheet and calendar software often defaults by locale, so two colleagues can genuinely see different numbers for the same day. When a schedule spans organisations, say “ISO week 14” rather than “week 14”.
The 53-week years that break things
Most years hold 52 ISO weeks, but some hold 53 — those starting on a Thursday, plus leap years starting on a Wednesday. Recent and upcoming examples are 2015, 2020, 2026, and 2032. Any system that assumes a fixed 52 weeks will misreport in those years, which is a recurring source of off-by-one errors in payroll runs, retail year-on-year comparisons, and sprint numbering. If you build week-based reports, they are worth testing against 2026 before it arrives.
Common uses
Planning against week-based schedules, referencing fiscal or sprint weeks, coordinating with European teams and manufacturing partners who quote delivery in week numbers, labelling reports and releases, and decoding a date code printed on a product. To move a date by a number of weeks, use date add & subtract, and to measure a span in weeks use date difference.
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.
Why does my calendar app show a different week number?
Because it is probably not using ISO 8601. North American convention starts weeks on Sunday and calls the week containing 1 January week 1, which produces a number one higher than ISO for much of the year. Microsoft systems offer several settings and the default varies by locale. If two teams disagree about which week it is, this is almost always why — agree on ISO and the ambiguity disappears.
Which years have 53 weeks?
A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. That works out to roughly 71 years per 400-year cycle — 2020, 2026, and 2032 are examples. Systems that assume 52 weeks break in those years, which is a recurring source of off-by-one bugs in reporting and payroll.
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 — the latter being the ordinal date format also defined by ISO 8601.
Is my data uploaded anywhere?
No. The calculation runs entirely in your browser with no upload.
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