Use this week number calculator to determine the ISO week number for any date. It also shows the Monday start date and Sunday end date of the same week, which is useful for planning, reporting, payroll calendars, shipping schedules and administrative work.
ISO week numbering is an international standard where weeks start on Monday and week 1 is the week containing the first Thursday of the ISO year.
Because ISO weeks follow whole Monday-to-Sunday ranges. Some early January dates still belong to the last ISO week that began in December.
Method: ISO weeks start on Monday. The week containing the first Thursday of the ISO year is week 1.
Example: January 1, 2026 falls in ISO week 1 of 2026, while some early-January dates can belong to the last ISO week of the previous ISO year.
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