Week Number (ISO 8601) Widget
Show today's week number on your site, or let visitors look one up for any date. The widget gives the ISO 8601 week and week-year used across Europe and in industry, the US Sunday-start week as a second answer, and the day of the year with the days still left.
Live preview
Under the widget on your page: Powered by A2Z Tools
Embed code
<iframe src="https://a2z.tools/embed/w/week-number" title="Week Number (ISO 8601) by A2Z Tools" width="100%" height="740" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>
A plain iframe. Works everywhere, including site builders that strip scripts. Adjust height if your content needs more room.
<div data-a2z-widget="week-number" data-height="740"></div> <script async src="https://a2z.tools/embed.js"></script>
Adds a small script (what it does) that sizes the widget to fit its content, loads it lazily and keeps it isolated from your page's CSS.
Works with
How it works
ISO 8601 weeks begin on Monday, and week 01 of a year is the week that contains that year's first Thursday - equivalently the week containing 4 January. The widget finds the Thursday of the chosen date's week; the calendar year of that Thursday is the ISO week-year, and counting Mondays from week 01 gives the number. That is why 31 December 2020 is 2020-W53, why 3 January 2021, a Sunday, still belongs to 2020-W53, and why 30 December 2024 is already 2025-W01. A year has 53 ISO weeks when it starts on a Thursday, or on a Wednesday in a leap year, as 2020 and 2026 do. The US count is different: weeks start on Sunday and week 1 is simply the week holding 1 January, so the two numbers can disagree by one. Day of the year counts 1 January as day 1. Dates are handled as whole calendar days, so the visitor's time zone cannot move them.
Calculation method
- ISO weekday: Monday = 1 ... Sunday = 7; Thursday of the week = date - weekday + 4
- ISO week-year = calendar year of that Thursday; week = floor((Thursday - Monday of the week holding 4 January) / 7) + 1
- ISO weeks in a year = week number of 28 December (52 or 53)
- US week = floor((day of year - 1 + weekday of 1 January, Sunday = 0) / 7) + 1
- Days left = days in the year (365 or 366) - day of the year
Worked examples
Turn of the year
Inputs: 3 January 2021 (Sunday)
Result: ISO 2020-W53; US week 2 of 2021
The week of 28 December 2020 - 3 January 2021 has its Thursday (31 December) in 2020.
A 53-week year
Inputs: 31 December 2026
Result: ISO 2026-W53, day 365 of 365
2026 begins on a Thursday, so it has 53 ISO weeks.
Limitations
- Only the Gregorian calendar from 1583 to 9998 is accepted.
- Middle Eastern Saturday-start week numbering is not offered.
- Fiscal weeks (4-4-5 retail calendars or a year starting in April) are not supported.
Where publishers use it
- Intranets and team wikis that plan sprints and rotas by week number
- Factory and warehouse sites where production batches and date codes use ISO weeks (2026W40)
- School and university pages listing term weeks
- Logistics portals where container bookings and delivery windows are quoted as calendar weeks
- A blog sidebar showing the current week for European readers who say KW 40 or semaine 40
Questions
Why does 1 January sometimes have week 52 or 53?
Because ISO week 1 is the week with the year's first Thursday. If 1 January falls on a Friday, Saturday or Sunday, those days still belong to the last week of the previous year: 1 January 2027 is a Friday, so it is 2026-W53.
How many weeks does a year have?
52 or 53 in ISO 8601. A year has 53 weeks when 1 January is a Thursday, or a Wednesday in a leap year. 2020 and 2026 have 53; 2025 and 2027 have 52. The widget shows the count for the ISO year of your date.
Why is the US week number different?
US calendars and Excel's WEEKNUM default start weeks on Sunday and call the week containing 1 January week 1, even if it holds only one day. On 4 January 2026 (a Sunday) the US count is week 2 while ISO is still week 1.
What does 2026-W40-4 mean?
It is the ISO week-date notation: week-year 2026, week 40, day 4 (Thursday). That is 1 October 2026.
Is Kalenderwoche (KW) the same as the ISO week?
Yes. Germany, Austria, Scandinavia and most of Europe number calendar weeks by ISO 8601, so KW 40 in a German diary is ISO week 40. Retail fiscal calendars and the US WEEKNUM convention can differ.
Can I fix the date for my readers?
Yes. Add v_date=YYYY-MM-DD to show a particular date; without it the widget always opens on the visitor's today.
Sources
- ISO 8601 - Date and time format - International Organization for Standardization . The standard's overview page (the week-date rules are in ISO 8601-1:2019, which is sold by ISO). Checked 2026-10-01.
- International standard date and time notation - Markus Kuhn, University of Cambridge . Summarises the week rules: weeks start Monday, week 01 holds the first Thursday / 4 January; 1996-12-31 is 1997-W01-2. Checked 2026-10-01.
- WEEKNUM function - Microsoft Support . System 1: the week containing 1 January is week 1 - the US convention shown as the second answer. Checked 2026-10-01.
Cite or recommend this tool
If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.
A2Z Tools Week Number (ISO 8601) https://a2z.tools/week-number-calculator
<a href="https://a2z.tools/week-number-calculator">A2Z Tools Week Number (ISO 8601)</a>
[A2Z Tools Week Number (ISO 8601)](https://a2z.tools/week-number-calculator)
Week Number (ISO 8601) by A2Z Tools - https://a2z.tools/week-number-calculator
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