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ISO week number calculator

ISO 8601 week number of the year

ISO week 32 ISO year 2026
Day of the year
220

Under the ISO 8601 standard, the week starts on Monday and week 1 is the one containing the year's first Thursday. It's the standard used by businesses and European calendars.

This ISO week number calculator works out the ISO 8601 week number for any date, along with its ISO year and day of the year.

The week businesses use

The ISO week is the standard in planning, logistics and European work calendars. Useful for reports, sprints and referring to dates like “week 23”.

FAQ

What is the ISO week?
The week numbering defined by the ISO 8601 standard: weeks start on Monday, and week 1 is the one containing the year's first Thursday. A year can have 52 or 53 weeks.
What week are we in today?
Open the tool: by default it shows today's ISO week. You can change the date to check any day.
Why doesn't the ISO week always match the calendar week?
Because the first days of January can belong to the last ISO week of the previous year, and the last days of December can belong to week 1 of the next year, depending on which day of the week they fall on.