ISO Week Number Calculator: Week Number From Date
Find the ISO 8601 week number and week-numbering year for any date using Monday as the first day of the week.
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Result
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Calculation method
How to use ISO Week Number Calculator: Week Number From Date
Calculate the ISO week year and number using Monday as the first day of the week.
ExampleISO week 1 is the week containing the year’s first Thursday.
Displayed values follow the definitions stated above.
Calculation guide
Find the correct ISO week number for a date
ISO 8601 weeks start on Monday, and January 1 is not always in week one. Dates near New Year can belong to the previous or following week-numbering year.
Formula usedISO week 1 = the week containing the year’s first Thursday
Each week runs Monday through Sunday, and the result includes both the ISO week year and week number.
Recommended steps
- Enter the calendar date.
- Read the ISO week-numbering year and week together.
- Use the weekday calculator when you also need the day name.
How to interpret the result
- Early January can belong to week 52 or 53 of the previous ISO year.
- Late December can belong to week one of the next ISO year.
- A workplace using Sunday-first or another local rule may label weeks differently.
FAQ
Frequently asked questions
Is January 1 always in week one?
No. Under ISO rules, week one is the week containing the first Thursday, so January 1 can belong to the previous ISO year.
Which day starts an ISO week?
Monday starts the week and Sunday ends it.
Can an ISO year have week 53?
Yes. Some calendar arrangements produce 53 ISO weeks.
Useful together