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Week Number Calculator

Enter a date to find its ISO 8601 week number.

Week number

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Weeks in this year

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What this calculator does

This online tool finds the ISO 8601 week number for any date — the standard used in business planning, payroll, and project scheduling across most of the world. It also shows the total number of weeks in that calendar year.

Worked example

September 2, 2026 falls in ISO week 36 of 2026 — enter any date above to see its week number and how many weeks that particular year contains.

Common mistakes

  • Assuming week 1 always starts on January 1. Under ISO 8601, week 1 is defined by the first Thursday of the year, which can push January 1 into the final week of the previous year.
  • Mixing ISO and US week numbering. The US convention starts weeks on Sunday and doesn't use the "first Thursday" rule, so results can differ by one week from ISO 8601.
  • Assuming every year has exactly 52 weeks. Some years have 53 ISO weeks depending on which day of the week January 1 falls on.

Frequently asked questions

What week numbering system does this use?+

ISO 8601, the international standard used across Europe and by most business calendars. Week 1 is the week containing the year's first Thursday, and weeks run Monday to Sunday.

Why does January 1 sometimes fall in week 52 or 53 of the previous year?+

Under ISO 8601, if January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last week of the previous year rather than week 1 of the new year, since ISO weeks always start on a Monday.

Do all countries use ISO week numbers?+

No — the United States commonly uses a different system where week 1 simply contains January 1, and weeks run Sunday to Saturday. If you need the US convention, expect results to sometimes differ by one week from ISO 8601.

How many weeks are in a year?+

Most years have 52 ISO weeks, but years where January 1 falls on a Thursday (or in a leap year, on a Wednesday) have 53 weeks. This calculator shows the total for whichever year you check.

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