Date Tools
Week Number Calculator
Find the ISO week number for any date, or reverse-lookup: enter a week number and year to get the Monday and Sunday dates of that week. ISO 8601 compliant.
How it works
Week numbers follow the ISO 8601 standard. Week 1 is the week containing the year's first Thursday. Weeks always run Monday through Sunday. The reverse lookup calculates the Monday of the given week number by finding the ISO week 1 Monday and stepping forward.
Frequently asked questions
What is an ISO week number?
ISO 8601 defines weeks as starting on Monday. Week 1 of a year is the week containing the first Thursday, or equivalently, January 4th.
Why can week 1 start in December?
Because ISO week 1 must contain the year's first Thursday. If January 1 is a Friday, Saturday, or Sunday, the preceding Monday is in week 1 of the next year.
What does the reverse lookup do?
Enter a week number and year to find the exact Monday and Sunday dates of that week.
What does 'current week' mean?
The ISO week containing today's date as calculated in your browser's local time.