Date & Time
ISO Week Number Calculator
Find the ISO week number, day of year, quarter, and more for any date.
Enter any date to find its ISO 8601 week number. The ISO week system is widely used in business, manufacturing, and project management. Also shows the week's start (Monday) and end (Sunday) dates and handles year-boundary weeks correctly.
Features
ISO Week Number
Official ISO 8601 week number for any date.
Week Boundaries
Shows the Monday start and Sunday end of the week.
Current Week
See today's week number instantly.
Year Context
Handles week 52/53 and week 1 year-boundary cases.
How to Find the Week Number
- Select a date (defaults to today).
- The ISO week number and week boundaries appear instantly.
- Copy the week number for use in reports or planning tools.
FAQ
What is an ISO week?▾
ISO 8601 defines week 1 as the week containing the year's first Thursday. Weeks run Monday to Sunday. Some years have 53 weeks.
Why does January 1 sometimes fall in week 52 or 53?▾
If January 1 falls on a Friday, Saturday, or Sunday, it's considered part of the previous year's last week in the ISO system.
ISO Week Facts
- ISO weeks start on Monday and end on Sunday.
- A year has either 52 or 53 ISO weeks.
- Week 1 is the week containing the first Thursday of the year.
- Used by: SAP, ERP systems, European business calendars.
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