Week number
Find the ISO week number and its Monday-to-Sunday date range.
7 September 2026 — 13 September 2026
How to read this result
ISO 8601 weeks begin on Monday. Week 1 contains 4 January, or equivalently the first Thursday of the year. An ISO week year has either 52 or 53 weeks.
Late December can belong to week 1 of the following ISO year. Early January can still belong to the final week of the preceding ISO year. The calculator displays the ISO year separately from the calendar year to make this distinction visible.
Useful answers
Do these weeks start on Sunday?
No. This tool uses ISO weeks starting on Monday. Other numbering conventions may return different values.
Which years have 53 weeks?
Some ISO years do; 2026 is one example. Each week’s date range is calculated even when it crosses a calendar-year boundary.
Data checked on 8 September 2026 · Sources and methods