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Week Number Calculator

Find the ISO 8601 and US week number of any date, with the weekday, day of the year and quarter, and see the dates of a given week. Free and private.

ISO week 1 of 2026

  • Monday2025-12-29
  • Sunday2026-01-04

All ISO weeks of 2026

WeekMondaySunday
W012025-12-292026-01-04
W022026-01-052026-01-11
W032026-01-122026-01-18
W042026-01-192026-01-25
W052026-01-262026-02-01
W062026-02-022026-02-08
W072026-02-092026-02-15
W082026-02-162026-02-22
W092026-02-232026-03-01
W102026-03-022026-03-08
W112026-03-092026-03-15
W122026-03-162026-03-22
W132026-03-232026-03-29
W142026-03-302026-04-05
W152026-04-062026-04-12
W162026-04-132026-04-19
W172026-04-202026-04-26
W182026-04-272026-05-03
W192026-05-042026-05-10
W202026-05-112026-05-17
W212026-05-182026-05-24
W222026-05-252026-05-31
W232026-06-012026-06-07
W242026-06-082026-06-14
W252026-06-152026-06-21
W262026-06-222026-06-28
W272026-06-292026-07-05
W282026-07-062026-07-12
W292026-07-132026-07-19
W302026-07-202026-07-26
W312026-07-272026-08-02
W322026-08-032026-08-09
W332026-08-102026-08-16
W342026-08-172026-08-23
W352026-08-242026-08-30
W362026-08-312026-09-06
W372026-09-072026-09-13
W382026-09-142026-09-20
W392026-09-212026-09-27
W402026-09-282026-10-04
W412026-10-052026-10-11
W422026-10-122026-10-18
W432026-10-192026-10-25
W442026-10-262026-11-01
W452026-11-022026-11-08
W462026-11-092026-11-15
W472026-11-162026-11-22
W482026-11-232026-11-29
W492026-11-302026-12-06
W502026-12-072026-12-13
W512026-12-142026-12-20
W522026-12-212026-12-27
W532026-12-282027-01-03

Shortcuts: Ctrl+Enter run · Ctrl+Shift+C copy result · / search (use ⌘ on Mac)

Free Week Number Calculator

Pick a date to see its week number in the ISO system (weeks start on Monday) and the US system (weeks start on Sunday). It gives developers a quick browser-based way to solve a focused week number calculator task before moving the result into a project.

How to use Week Number Calculator

Enter a date. To go the other way, enter a year and a week number to get the dates of that week.

Common mistakes

  • Using the ISO year with a calendar date: late December can belong to week 1 of the next year.
  • Assuming every year has 52 weeks: some have 53.

Frequently asked questions

What is an ISO week?
ISO 8601 weeks start on Monday, and week 1 is the week that contains the first Thursday of the year. A year has 52 or 53 weeks.
Why is January 1 sometimes in week 52 or 53?
If January 1 falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous ISO year.
What is the US week?
In the US system weeks start on Sunday and week 1 is the week that contains January 1.
Is anything uploaded?
No. The calculation runs in your browser and nothing you enter is sent to a server.
How do I use this week number calculator tool?
Use the interactive controls above; processing stays in your browser and the result is intended as a practical starting point for development work.

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