Week Number Calculator

Find the ISO week number for any date

Find the ISO week number for any date. ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday.

Find the ISO 8601 week number for any date. ISO weeks start on Monday; week 1 contains the year's first Thursday.

ISO Week Number

43

ISO week-year 2026

Date

Oct 22, 2026

Thursday, October 22, 2026

How to Use This Calculator

  1. Enter a date.
  2. Review the ISO week number and the ISO week-year.

Why Use This Calculator?

  • Matching a week number used in business or project planning.
  • Finding which ISO week a date falls in.
  • Avoiding confusion between different week-numbering systems.

Real-World Examples

September 4, 2026

September 4, 2026 falls in ISO week 36 of 2026.

Frequently Asked Questions

What makes ISO weeks different?

ISO weeks always start on Monday, and week 1 is the week with the year's first Thursday. This means January 1 sometimes lands in week 52 or 53 of the previous year.

Is this the same week number my calendar uses?

Mostly, if your calendar follows ISO 8601. Some regions use different systems (like one that starts weeks on Sunday), which can differ by a week near year boundaries.

What's the ISO week-year?

It's the year the ISO week belongs to, which can differ from the calendar year for dates near January 1.

Important Disclaimer

Uses ISO 8601 week numbering. Other week-numbering systems exist and may differ near year boundaries.

Privacy

Your date entry stays in your browser and isn't stored or transmitted.

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