Find the ISO week number for any date, convert a week number back to its exact date range, or browse a full year calendar — all computed locally, with no data sent to a server.
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Features
Live current week
Displays today's ISO week with ISO year, day of week and full date range. Navigate with prev/next buttons.
Date → week
Pick any date between 1900 and 2200 and get its ISO week number, ISO year, and the week's Monday–Sunday range.
Week → date range
Enter an ISO year and week number to get the exact Monday and Sunday of that week — with one-click copy.
Year calendar
Browse all ISO weeks for any year. 53-week years are flagged. The current week is highlighted automatically.
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All calculations happen in your browser. No date is ever sent to a server.
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Frequently asked questions
What is an ISO week number?
An ISO week number is defined by the ISO 8601 international standard. Week 1 is always the week containing the first Thursday of the year. All weeks start on Monday and end on Sunday. The ISO year and the calendar year can differ at the turn of the year — for example, December 31 may belong to ISO week 1 of the following year.
How is ISO week 1 determined?
ISO week 1 is the week that contains the year's first Thursday. Equivalently, it is the week containing January 4 (since January 4 is always in week 1), or the first week with a majority (at least four) of its days in the new year.
Can a year have 53 ISO weeks?
Yes. A long year has 53 ISO weeks when January 1 falls on a Thursday, or when it is a leap year and January 1 falls on a Wednesday. This happens roughly every 5 to 6 years. Recent examples: 2015, 2020, 2026.
Why can the ISO year differ from the calendar year?
Because ISO weeks must be complete seven-day weeks. December 29, 30 or 31 can belong to ISO week 1 of the following year if they are part of a week whose Thursday falls in January. Similarly, January 1, 2 or 3 can belong to ISO week 52 or 53 of the previous year.
How do I use the ISO week number in Excel?
Use the ISOWEEKNUM(date) function, available since Excel 2013. It returns the ISO 8601 week number directly. For the ISO year, you need a helper formula because Excel's YEAR() can differ from the ISO year at year boundaries.
How do I get the ISO week number in JavaScript?
Use a UTC-based calculation to avoid timezone shifts: convert the date to UTC, find the nearest Thursday, compute the difference in days from January 1, and divide by 7. The calculator on this page uses exactly this method.