Week Number Calculator
Calculate a date's ISO 8601 week number, ISO year, and Monday-to-Sunday range.
Result
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The calculation runs locally in your browser.
ISO 8601 week number
ISO weeks begin on Monday. Week 1 contains the year's first Thursday, so some January dates belong to the previous ISO year and some December dates to the next one.
Complete week range
The result includes ISO week and year plus its opening Monday and closing Sunday.
Frequently asked questions
Does the week start on Sunday?
No. ISO 8601 weeks begin on Monday.
Does ISO year always match calendar year?
No. They can differ near year boundaries.
How many ISO weeks can a year have?
An ISO year can contain 52 or 53 weeks.