Find the ISO 8601 week number for any date — plus the weekday and day of year.
Enter the date
The ISO week starts on Monday. The year’s first week contains the first Thursday.
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Under ISO 8601, the week starts on Monday, and the year’s first week is the one containing the first Thursday. The week number is computed from that reference:
9 July 2026 is in ISO week 28. At year-end, 31 December 2026 is in week 53.
It is a year’s week number under the ISO 8601 standard, used in business and across Europe. The week starts on Monday, and the year’s first week is the one containing the first Thursday.
Enter today’s date and you instantly get the week number. For example, 9 July 2026 is in ISO week 28.
If 1 January falls Thursday–Sunday, it belongs to the last week of the previous year under ISO 8601. That is why early January is sometimes “week 52” of the prior year.
Most years have 52 ISO weeks, but some have 53 (when 1 January or 31 December falls on a Thursday). For example, 2026 ends in week 53.
In ISO 8601 (used in Romania and Europe) the week starts on Monday and ends on Sunday. In some countries, like the US, the week starts on Sunday.
It is the year the ISO week belongs to, which can differ from the calendar year in early January or late December.
Planning, business reports, production, logistics and school schedules that number weeks instead of dates.
Every ISO week starts on Monday. By entering a date you also see the weekday, so you can identify the matching Monday.