Which numbered week of the year a date falls in, using the international ISO 8601 standard most calendar apps follow.
How it works
Week 1 is defined as the week containing the year’s first Thursday; weeks run Monday to Sunday, and the week number counts forward from there.
What this does not include
This does not include the weekday name for a date — for that, use this site’s day-of-week calculator instead. This tool reports the numbered week, not the day name.
How to use this calculator
- Enter a date.
A worked example
January 1, 2026: ISO week 1 of ISO year 2026.
December 31, 2025: also ISO week 1 of ISO year 2026 — a date in one calendar year can belong to the following ISO year’s first week.
What the variables mean
| Variable | Meaning |
|---|---|
| Date | Any calendar date |
| Week number | ISO 8601 week number, where week 1 contains the year’s first Thursday |
Edge cases worth knowing
ISO week numbering doesn’t always align with the calendar year. The second example above shows a late-December date belonging to ISO week 1 of the following year — a common source of confusion around New Year’s.
ISO weeks always run Monday through Sunday, unlike some other week-numbering conventions that start on Sunday.
Frequently asked questions
Why does week 1 depend on the year’s first Thursday?
The ISO 8601 standard defines it this way so that week 1 always contains at least 4 days of the new year, keeping week numbering consistent regardless of which weekday January 1st falls on.
Can a date near the end of December belong to next year’s week 1?
Yes — if December 31st falls early in the week (Monday through Wednesday), it can belong to week 1 of the following year under ISO rules.
Where is week numbering commonly used?
Business scheduling, payroll cycles, and project planning tools, which often reference “week 23” rather than a specific date range.
