Date & Time
Week Number Calculator
Calculate ISO week numbers and week-years.
August 30, 2026 is Sunday in ISO week 35.
ISO week number
35
ISO week-year
2026
ISO weekday
7 (Sunday)
Local weekday
Sunday
About This Tool
Find the ISO 8601 week label for a date when a normal month-and-day calendar is not enough. ISO weeks always start on Monday, and week 1 is defined by the year's first Thursday. That consistent rule is useful for sprint planning, manufacturing schedules, shipment tracking, timesheets, weekly reports, filenames, and cross-country workflows where different local week conventions could otherwise cause confusion.
How To Use It
- Enter the calendar date you want to classify.
- Read the ISO week number and ISO week-year together; the week-year can differ from the date's calendar year near New Year.
- Check the ISO weekday number, where Monday is 1 and Sunday is 7.
- Record the week-year and week number together, such as 2026-W35, when using the result in reports, schedules, or filenames.
Examples
Label a weekly report
Enter the report date and use its ISO week-year plus week number as a consistent weekly label. Keeping the week-year avoids ambiguity around late December and early January.
Plan a sprint or production week
Use the ISO week label to align work that runs Monday through Sunday, especially when teams in different regions may use different ideas of the first day of the week.
Check January 1, 2021
January 1, 2021 falls in ISO week 53 of ISO week-year 2020. This is a useful example of why the ISO week-year should be read separately from the calendar year.
Useful Notes
How ISO week 1 is chosen
ISO week 1 is the week containing January 4, equivalently the week containing the year's first Thursday. Because ISO weeks start on Monday, the first few January dates can belong to the previous ISO week-year.
Week-year can cross a calendar boundary
Late December can belong to week 1 of the next ISO week-year, while early January can belong to week 52 or 53 of the previous ISO week-year. Store the ISO week-year with the week number when the distinction matters.
ISO weekday numbering
ISO 8601 numbers weekdays from 1 through 7: Monday is 1, Tuesday is 2, and Sunday is 7. This differs from systems that treat Sunday as day 0 or the first day of the week.
Not every year has week 53
Most ISO week-years contain 52 weeks, while some contain 53. Do not assume that a week 53 label is invalid when processing schedules or imported data.
FAQ
Why can the ISO week-year differ from the calendar year?
ISO weeks are assigned as complete Monday-to-Sunday weeks. Week 1 contains the year's first Thursday, so dates near January 1 can belong to the previous ISO week-year and dates near December 31 can belong to the next one.
Does the calculator use Sunday or Monday as the first day of the week?
It follows ISO 8601, so every week starts on Monday and ends on Sunday.
What does a label such as 2026-W35 mean?
It means week 35 of ISO week-year 2026. The year in an ISO week label is the week-year, which is not always the same as the calendar year printed on the date.
Can an ISO year have 53 weeks?
Yes. Some ISO week-years have a week 53 because of how the Monday-start week system aligns with the calendar.
Is an ISO week number the same as every calendar app's week number?
Not necessarily. Some calendars use Sunday-start weeks or other locale-specific rules. This calculator specifically uses ISO 8601 week numbering.
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