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Week Number Calculator

ISO-8601 week number for a date, and the dates a given week covers.

Enter values

Results update as you type.

Result

Week number

39

ISO weeks start on Monday, and week 1 is the week containing the first Thursday of the year.

ISO week year
2,026
Week starts (Monday)
Monday, September 21, 2026
Week ends (Sunday)
Sunday, September 27, 2026
Weeks left in the year
13
Day of the week
Tuesday

About the Week Number Calculator

ISO-8601 week number for a date, and the dates a given week covers.

Also known as: iso week calculator, what week is it, calendar week number, week of the year.

How to use this calculator

  1. Enter calculate.
  2. The result updates instantly as you type — press Calculate (or Enter) at any time.
  3. Review the breakdown shown under the main result.
  4. Use Copy result or Share to save the answer or send a link with your inputs.

Formula

ISO-8601: week 1 contains the first Thursday of the year

Worked example

Using the values pre-filled in the calculator above:

  • Calculate: Week number from a date
  • Date: 2026-09-22

the calculator returns:

  • Week number: 39
  • ISO week year: 2,026
  • Week starts (Monday): Monday, September 21, 2026
  • Week ends (Sunday): Sunday, September 27, 2026