Career & Time

Week Number Calculator

Find the ISO week number for any date.

Enter a date to find its ISO 8601 week number — commonly used in business, manufacturing, and project planning.

Select a valid date.

ISO Week Number

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How to Use This Tool

  1. Enter a date.
  2. Click Calculate to see its ISO 8601 week number.

What Is the ISO 8601 Week Number?

The ISO 8601 standard defines Week 1 of a year as the week containing that year's first Thursday (equivalently, the week containing January 4th) — weeks run Monday through Sunday. This standardized system is widely used internationally in business, manufacturing, payroll, and project planning.

Why Not Just Count from January 1st?

Simply counting 7-day blocks from January 1st would create inconsistent, non-standardized week boundaries that shift depending on what day of the week January 1st falls on. ISO 8601 solves this by anchoring week numbering to a consistent rule (Monday start, majority-of-days rule) so week numbers are consistent and comparable across systems and countries.

Week 53

Most years have 52 ISO weeks, but years where January 1st falls on a Thursday (or in leap years, a Wednesday) have 53 ISO weeks — this calculator correctly accounts for this.

Frequently Asked Questions

Why does ISO week numbering start on Monday?

The ISO 8601 international standard defines the week as running Monday through Sunday specifically to create a consistent, unambiguous standard usable across different countries and business systems, unlike other conventions that might start the week on Sunday.

Can a year have 53 weeks?

Yes — years where January 1st falls on a Thursday (or, in leap years, a Wednesday) have 53 ISO weeks instead of the usual 52, due to how the ISO 8601 standard's rules align with the calendar.

This calculator is provided for general informational and estimation purposes only. Results should not be treated as professional financial, tax, legal, medical, or engineering advice. Always verify critical calculations with a qualified professional or official source before making decisions.