Week Number Calculator

ISO 8601 defines week numbering: weeks start on Monday, and week 1 is the week containing the first Thursday of the year (or equivalently, the week containing January 4th). A year has 52 or 53 weeks. This standard is used internationally for business, logistics, and scheduling. The calculator uses ISO 8601 rules.

Yes — December 29-31 may belong to week 1 of the next year, and January 1-3 may belong to week 52 or 53 of the previous year. For example, January 1, 2024 (Monday) is in ISO week 1, but January 1, 2023 (Sunday) is in ISO week 52 of 2022. The calculator handles these edge cases correctly.

The calculator automatically shows today's ISO week number when you load the page. It also highlights the current week in the calendar view for quick reference. You can then select any other date to find its week number.

Most years have 52 weeks. A year has 53 weeks if January 1 falls on a Thursday, or on a Wednesday in a leap year. Long years (53 weeks) occur roughly every 5-6 years. The calculator indicates whether the selected year has 52 or 53 ISO weeks.

Enter a year and week number, and the calculator shows all seven dates (Monday through Sunday) in that week. This is useful for planning, scheduling, and converting between "Week 14" style references and actual calendar dates.