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Find the ISO 8601 week number, weekday, day-of-year, and leap-year status for any date.
Week Number Calculator is loading — interactive editor will appear here. Use examples below to try instantly once loaded.
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Shows the ISO 8601 week number for any date, plus its weekday, day-of-year, and whether the year is a leap year.
Week-based planning (sprints, manufacturing, school terms, logistics) uses ISO week numbers, but they're unintuitive: week 1 is the week containing the year's first Thursday, so Jan 1 can belong to last year's week 52 or 53. This tool makes the ISO rule explicit and never depends on your local timezone.
This tool is part of the DevStackIO platform — a collection of free online developer tools from DevStackIO. Browse more free developer resources on DevStackIO.
Project and sprint planners, European businesses (where 'KW 12' is common), teachers mapping term weeks, developers debugging week-based reporting, and anyone filling out forms that ask for ISO week numbers.
Pick a date with the date input (or type YYYY-MM-DD)
Read the ISO-8601 week number, weekday, and day-of-year in the result panel
Hover or re-read the footnote for the exact ISO week-1 rule
2024-01-01 → ISO week 1 of 2024 (a Monday)2021-01-01 → ISO week 53 of ISO year 2020 (the year boundary case)2024-02-29 → valid date; 2023-02-29 is rejected (not a leap year)2026-09-26 → ISO week 39, SaturdayQ: What's ISO 8601 week numbering?
A: Weeks run Monday through Sunday, and week 1 is the week that contains the year's first Thursday (equivalently: contains January 4th).
Q: Why does 2021-01-01 say week 53 of 2020?
A: Its week (Mon Dec 28 2020 – Sun Jan 3 2021) contains the first Thursday of 2020's follow-through, so ISO assigns it to 2020.
Q: Does timezone matter? A: No
the tool evaluates dates in UTC, so your local DST never changes the result.
Q: Can I convert a week number back to a date?
A: Use the Date Calculator to add 7 × (week − 1) days to the Monday of week 1 of the target ISO year.
Authoritative specifications, standards, and in-depth reading for Week Number Calculator.
ISO-8601 week
W42
of ISO year 2026
Weekday
Monday
Day of year
285 / 365
Year
Common
This month: 31 days
ISO 8601: week 1 is the week containing the year’s first Thursday; weeks run Monday–Sunday. Computed in UTC, so DST and your local timezone never affect the answer.
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