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Week Number Calculator

Find the ISO 8601 week number for any date.

ISO Week
27
of 2026
Week Range
Jun 29 – Jul 5, 2026

About This Tool

Free week number calculator online — find the ISO 8601 week number for any date, along with the exact start (Monday) and end (Sunday) dates of that week and the total number of weeks in that year. ISO week numbering starts weeks on Monday and defines week 1 as the week containing the year's first Thursday, which is the standard used in Europe, ISO date formats, and many project management and payroll systems. Useful for filling in timesheets that track "week number," aligning sprint or billing cycles to calendar weeks, and cross-referencing dates against week-based schedules. Pick any date and the current or corresponding week number displays instantly.

How to Use

  1. 1

    Pick a date

    Select any date — defaults to today.

  2. 2

    Read the week number

    See the ISO week number, the year it belongs to, and the week's Monday-to-Sunday date range.

Frequently Asked Questions

What is ISO week numbering?+
ISO 8601 defines weeks as starting on Monday, with week 1 being the week that contains the first Thursday of the year (equivalently, the week containing January 4th). This differs from the US convention where weeks often start on Sunday.
Can a week number belong to a different year than the date's calendar year?+
Yes — dates in late December can fall in week 1 of the following year, and dates in early January can fall in the last week (52 or 53) of the previous year, under ISO week rules.

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