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Week Number Calculator : ISO 8601 Week of the Year (1–53)

The Week Number Calculator identifies the international standard ISO 8601 week number (1 to 53) for any date. It displays the Monday start date and Sunday end date for that specific calendar week.

Interactive Calculator

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ISO 8601 Week of the Year
Week 39 of 2026
Sep 21, 2026 (Monday) – Sep 27, 2026 (Sunday)
Total Weeks in Year
53 weeks
Weeks Remaining
14 weeks
Week Standard
ISO 8601 (Mon-Sun)

Step-by-Step Examples

Early January ISO Week

Finding the ISO week number of January 8, 2025.

Inputs: Date: 2025-01-08
Result: Week 2 of 2025 (Monday, Jan 6 – Sunday, Jan 12, 2025)

Falls in the second full week of 2025.

How to Use This Tool

  1. Choose any calendar date.
  2. Click Calculate to see the ISO week number, ISO year, Monday start date, and Sunday end date.

Frequently Asked Questions

According to ISO 8601, if January 1 falls on a Friday, Saturday, or Sunday, those days belong to Week 52 or 53 of the previous year because the majority of the week belongs to the prior year.

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