ISO Week Number
ISO 8601 week-of-year, day-of-year, and Monday-start week range.
Overview
The ISO Week Number tool reports the ISO 8601 week-of-year, the day-of-year ordinal, and the Monday-to-Sunday date range of the week containing any date you enter. The result follows the international standard used across Europe, in banking and aviation systems, and increasingly in project management tools that group dates by week.
Useful for European business teams that invoice and report by week number, retailers that plan sales cycles in week buckets, freelancers tracking timesheets in ISO weeks, and developers building reporting dashboards where rows must align with the same week labels used by their finance department.
How it works
ISO 8601 defines week 1 as the week containing the year's first Thursday — equivalently, the week containing January 4. Weeks run Monday through Sunday. That rule means a date in early January can belong to week 52 or 53 of the previous year, and a date in late December can belong to week 1 of the next.
A long year (one with 53 ISO weeks) occurs when January 1 falls on a Thursday, or on a Wednesday in a leap year. There are exactly 71 long years in each 400-year Gregorian cycle, working out to roughly one in every five and a half years.
Examples
2026-05-18 → ISO week 2026-W21 (Mon May 18 – Sun May 24), day 138
2025-12-31 → ISO week 2026-W01 (Mon Dec 29 – Sun Jan 4)
2027-01-01 → ISO week 2026-W53 (Mon Dec 28 – Sun Jan 3)
2024-02-29 → ISO week 2024-W09 (Mon Feb 26 – Sun Mar 3)
FAQ
Why does my January 1 belong to week 52 or 53?
When January 1 is a Friday, Saturday, or Sunday, it falls before the first Thursday of the new year, so it belongs to the last week of the prior year. Week 1 begins the following Monday.
Are there always 52 weeks in a year?
No — long years have 53 ISO weeks. The next long years from 2020 are 2020, 2026, 2032, 2037, and 2043.
Does this match the week numbers Excel shows?
Only if Excel is configured for ISO 8601 mode (WEEKNUM(date, 21) or ISOWEEKNUM(date)). Excel's default WEEKNUM uses a different convention where week 1 contains January 1.
Is ISO week the same as US "week 1"?
No — US conventions usually call the week containing January 1 "week 1," and the week may start on Sunday. The ISO definition is Monday-start and Thursday-anchored, which can disagree by a week.
What is the day-of-year figure?
A simple 1-based ordinal: January 1 is day 1, December 31 is day 365 (or 366 in a leap year). It is independent of the ISO week numbering.