Date Calculator

Week Number Calculator

Find the official ISO 8601 week number for any date in the year.

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What is the Week Number Calculator?

The Week Number Calculator identifies which ISO 8601 week number (from 1 to 53) a specified date belongs to.

How to Use This Calculator

  1. Select any date.
  2. Click Calculate to get the official ISO week number and ISO year.

Calculation Logic & Formula

Under ISO 8601, weeks start on Monday. Week 1 is the week containing the first Thursday of the calendar year.

Worked Example: Week number for September 16, 2026

Scenario: Reporting quarterly manufacturing schedule in ISO weeks.

Calculation: Resolved to ISO Week 38.

Result: Week 38 of 2026 (Wednesday, Day 3 of ISO Week 38).

Frequently Asked Questions

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