Week Number Calculator
Find the ISO 8601 week number for any date.
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- Works on all devices
- Last updated
- 12-09-2026
Enter any date to find its ISO 8601 week number.
Your result
37
Week number
Week year2,026
Day of the weekSaturday
AI explanation
Formula
ISO 8601 week numbering: weeks start Monday, and week 1 is the week containing the year's first ThursdayWorked example
2026-09-12
| Field | Value |
|---|---|
| Date | 12-09-2026 |
| Week number | 37 |
| Week year | 2026 |
| Day of the week | Saturday |
Assumptions
- Uses the ISO 8601 week-numbering standard, where a late-December or early-January date can belong to a "week year" that differs from its calendar year.
Frequently asked questions
Why is week 1 defined by the first Thursday of the year?
The ISO 8601 standard defines week 1 as the week containing the year's first Thursday (equivalently, the week containing January 4) — this ensures each week is assigned to the year that contains most of its days.
Can the week year differ from the calendar year?
Yes — a date in late December can fall in "week 1" of the following year, and a date in early January can fall in the last week of the previous year, under ISO 8601 rules.
Do all countries use this week-numbering system?
ISO 8601 (Monday-start weeks, first-Thursday rule) is widely used internationally, including in most of Europe and in many business/scheduling contexts in India, though the US commonly uses a different (Sunday-start) week convention.