Overview
Week numbers are used at work, in schools and in schedules. This tool shows the internationally used ISO 8601 week number along with the week's start and end.
Formula
Under ISO 8601, weeks start on Monday and week 1 contains January 4. Weeks are numbered consecutively through the end of the week year.
How it works
Pick the date
Any day you want to place in a week.
Read the week number
The number and week year appear.
Check the boundaries
The week start and end are shown.
Worked examples
Early January
January 1 can fall in week 52 or 53 of the previous week year if the first week is incomplete.
Mid-year
July 1 usually falls in week 27.
Frequently asked questions
Why is the week number unexpected?
ISO 8601 uses full Monday-based weeks, so a December day can belong to the next week year.
How many weeks are in a year?
Usually 52, and some years have 53.
Can I start the week on Sunday?
This tool shows ISO 8601 only, where weeks start on Monday.
This calculator is for educational and informational purposes only and is not a substitute for professional advice.