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Week Number Calculator
Find the week number for any date under ISO 8601, the standard used across Europe and in most business software, together with the Monday and Sunday of that week. The US-style week number is shown as well.
- Week starts (Monday)
- Mon, Oct 5, 2026
- Week ends (Sunday)
- Sun, Oct 11, 2026
- Weeks in this ISO year
- 53
- US week number (weeks start Sunday)
- 42
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Formula
How to use it
- Choose a date, or leave it as today.
- Read the ISO week number and the dates the week covers.
Worked examples
October 11, 2026
- ISO week number
- 41
- Week starts (Monday)
- Mon, Oct 5, 2026
- Week ends (Sunday)
- Sun, Oct 11, 2026
- Weeks in this ISO year
- 53
December 30, 2024 — already week 1 of 2025
- ISO week number
- 1
- Week starts (Monday)
- Mon, Dec 30, 2024
- Weeks in this ISO year
- 52
Why the year can “start” in December
Because ISO weeks always run Monday to Sunday and week 1 must contain a Thursday, the last days of December can belong to week 1 of the next year, and the first days of January can belong to week 52 or 53 of the previous year. December 30, 2024 is in week 1 of 2025.
Most years have 52 ISO weeks. Years in which January 1 is a Thursday — or a Wednesday in a leap year — have 53. 2026 is a 53-week year.
The US convention
In the United States, weeks are often counted from Sunday with week 1 being the week containing January 1. The two systems can differ by one, so say which you mean when a week number matters.
Questions people ask
What week of the year is it?
Leave the date as today and read the ISO week number. October 11, 2026 is in week 41.
How many weeks are in 2026?
53 ISO weeks, because January 1, 2026 is a Thursday.