Choose a date to find its ISO 8601 week number and the Monday–Sunday range for that week.
How ISO week numbers work
ISO 8601 numbers weeks from Monday through Sunday. Week 1 is the week containing January 4. Because of that rule, the first or last few days of a calendar year can belong to the neighboring ISO week-year.
Why the week-year can differA date near January 1 can still belong to the final ISO week of the previous year, and a late-December date can belong to week 1 of the next ISO year.
Week numbers for planning
Week numbers are useful for production schedules, project planning, reporting cycles and international calendars where ISO week notation is used.
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Common questions
Does an ISO week start on Sunday or Monday?
Monday. ISO 8601 defines Monday as day one of the week.
Can January 1 belong to the previous week-year?
Yes. The ISO week-year is determined by the week containing January 4, not simply by the calendar year printed on the date.
How many ISO weeks can a year have?
An ISO week-year has either 52 or 53 weeks.