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Find the ISO week number for any date with a full year calendar view.
The ISO 8601 week number is a standardized numbering system where weeks start on Monday and week 1 contains the first Thursday of January, giving each year 52 or 53 weeks.
| Quarter | Weeks | Months |
|---|---|---|
| Q1 | 1–13 | Jan–Mar |
| Q2 | 14–26 | Apr–Jun |
| Q3 | 27–39 | Jul–Sep |
| Q4 | 40–52/53 | Oct–Dec |
Formula
ISO Week = ⌈(Day of Year + (Thursday offset)) / 7⌉Day of Year = Sequential day count from January 1 (ordinal date)
Thursday offset = Adjustment so that week 1 contains the first Thursday of January
Worked Example
March 19, 2026 (Wednesday)
Did you know? ISO 8601 week numbering is the official standard in over 150 countries and is required in EU business reporting. The system ensures every week has exactly 7 days and belongs to exactly one year.
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