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Week Number Calculator
Find the ISO week number for any date — or look up the dates for any week number.
How It Works
ISO 8601 week numbering defines weeks that start on Monday and end on Sunday. Week 1 of any year is the week that contains the first Thursday of that year. Most years have 52 weeks; some have 53.
- Choose a mode — find a week number from a date, or find dates from a week number.
- Enter your date or week number and year.
- Instantly see the ISO week number and the full Monday–Sunday date range.
Frequently asked questions
ISO 8601 defines a standard week numbering system where weeks run Monday to Sunday. Week 1 is the week that contains the first Thursday of the year. Most years have 52 weeks; years where Jan 1 or Dec 31 falls on a Thursday have 53.
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