Summary
Find the ISO week number for a date.
Calculation details
The ISO week-year can differ from the calendar year near New Year.
Formula
ISO weeks start Monday; week 1 contains January 4.
Worked example
Worked example
January 1, 2021 is in ISO week 53 of 2020.
How it works
Enter a calendar date. The result uses ISO week rules (Monday start; week 1 contains January 4).
Frequently asked questions
Which week numbering system is used?
ISO 8601 week numbering: weeks start on Monday, and week 1 contains the year's first Thursday.
Why can week-year differ from calendar year?
Early January or late December dates can belong to the adjacent ISO week-year.
Is weekday 1 Sunday?
No. ISO weekday 1 is Monday and 7 is Sunday.