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Date & Time Tools

Week Number Calculator

Find the ISO 8601 week number for any date, plus its week-year and weeks in the year.

  • 24ISO week number
  • 2026Week-numbering year
  • 53Weeks this year

ISO 8601 weeks start on Monday; week 1 contains the first Thursday of the year.

How to use it

  1. Pick a date Any date you want the week number for.
  2. Read the week The ISO week number appears instantly.
  3. Check the week-year See the week-numbering year and total weeks.

Examples

1 Jan 2024 week 1
1 Jan 2021 week 53 (2020)

About this tool

The ISO 8601 week number is the standard used across business, manufacturing and much of Europe to refer to weeks of the year. This calculator returns the week number for any date, along with the week-numbering year it belongs to.

Because ISO weeks don’t align neatly with calendar years, dates in early January or late December can fall in a different week-year — and this tool shows that clearly, plus whether the year has 52 or 53 weeks. It all runs in your browser.

Frequently asked questions

Why is the week-numbering year sometimes different?

Under ISO 8601, early January can belong to the last week of the previous year, and late December to week 1 of the next, so the week-year can differ from the calendar year.

When does the week start?

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

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Updated June 13, 2026