Week Number Calculator
Find the ISO 8601 week number for any date. Useful for project planning, scheduling, and matching European/business-week conventions.
Inputs
ISO week
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Week starts (Monday)
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Week ends (Sunday)
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ISO format (YYYY-Www)
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About Week Number Calculator
ISO 8601 convention
Weeks start on Monday. Week 1 of a year is the week containing the first Thursday — equivalent to the week containing January 4th. So a year has 52 or 53 ISO weeks.
Why not just count from Jan 1?
Different countries and industries disagree. ISO 8601 is the international standard for business, project management, and date interchange. The US sometimes uses a different convention (week starts Sunday, week 1 is the week of Jan 1).
Edge case: late December
A date like December 31 can be in week 1 of the next year under ISO rules. For example, Dec 31, 2024 is in ISO week 1 of 2025. This calculator reports the ISO year correctly.