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Date & Time

Free Week Number Calculator — Find ISO Week Number

Find the ISO week number for any date, or find what date a specific week number falls on. Standardized week numbering for any year.

Week Number
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Week Start
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Week End
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Day of Year
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ISO Week Numbering

ISO 8601 defines week 1 as the week containing the first Thursday of the year (or equivalently, the week containing January 4th). Weeks start on Monday and end on Sunday. Some years have 53 weeks.

Frequently Asked Questions

No. ISO week 1 is the week containing the first Thursday of the year. January 1 can fall in week 52 or 53 of the previous year if it is a Friday, Saturday, or Sunday.