This tool uses ISO 8601 only, not US Sunday-start weeks.

Calendar date parts from the picker are used; time of day is ignored.

Some years have 53 ISO weeks when January 1 falls on a Thursday (leap years: Wednesday).

Week Number Calculator – ISO 8601 Calendar Week

Pick a date to see its ISO week number and the week-year it belongs to—useful for reporting, payroll, or project planning.

Date

ISO week

Choose a date.

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The date is normalized to UTC midnight, then the ISO week algorithm finds the Monday-based week containing that day.

Week 1 is the week with the year’s first Thursday (equivalently, the week containing January 4).

ISO week numbering is common in Europe and business systems; it can differ from US week-of-year conventions.

Late December or early January dates may belong to a week-year different from the calendar year.

US week numbering?: This tool uses ISO 8601 only, not US Sunday-start weeks.
Time zone?: Calendar date parts from the picker are used; time of day is ignored.
Week 53?: Some years have 53 ISO weeks when January 1 falls on a Thursday (leap years: Wednesday).
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