Week Number Calculator

How ISO Week Numbers Work

The ISO 8601 standard defines weeks consistently across years. Week numbers run from 1 to 52 (or 53 in some years).

ISO week rules

• Weeks start on Monday.
• The first week of the year is the one that contains the first Thursday of January.
• Equivalently, it is the week containing January 4th.
• This means January 1st may belong to week 52 or 53 of the previous year.

Step-by-step

1. Select any date.
2. Click Calculate to get its ISO week number.

Worked example

June 25, 2026 falls in ISO week 26. January 1, 2026 (a Thursday) is in week 1. December 31, 2026 (a Thursday) is in week 53.

When to use

Week numbers are common in project management, manufacturing schedules, and European business planning. They provide a clean way to refer to a specific week without specifying exact dates.