What is the ISO week date system?

ISO 8601 defines week 1 as the week containing the first Thursday of the year. Weeks start on Monday. This is the standard used in most of Europe and in business planning worldwide.

How is the Hijri date calculated?

We use your browser's built-in Intl.DateTimeFormat('ar-SA-u-ca-islamic', ...) which converts Gregorian dates to the Hijri (Islamic lunar) calendar. No external API calls or lookup tables are needed.

Why does the week number change when I switch week start?

Week numbering depends on which day is considered the start of the week. ISO uses Monday; many US calendars use Sunday. Switching the start day recalculates the week boundaries and numbering accordingly.

Week Number Calculator

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Pick any date to see its ISO week number, week boundaries, Hijri (Islamic) equivalent, Gregorian date, day of year, and a mini calendar of that week.

Date Week starts on

Week Number: --
Week Start: --
Week End: --
Day of Year: --

Gregorian

Hijri (Islamic)

Week Calendar

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How to use Week Number Calculator

Pick a date using the date picker or click Today.
Choose your week start day: Monday (ISO), Sunday, or Saturday.
Read the week number, start/end dates, Gregorian and Hijri dates.
Review the mini calendar, then click Copy Week Info to save the details.

What is Week Number Calculator?

Week numbering follows ISO 8601 by default (weeks start Monday, week 1 contains the first Thursday of the year). You can switch to Sunday or Saturday week starts. The Hijri date is computed using the browser's built-in Intl.DateTimeFormat with the Islamic calendar — no external libraries needed.

The mini calendar shows all 7 days of the selected week with today and the selected date highlighted for quick visual reference. Copy the week info to your clipboard with one click.

FAQ

What is the ISO week date system?: ISO 8601 defines week 1 as the week containing the first Thursday of the year. Weeks start on Monday. This is the standard used in most of Europe and in business planning worldwide.
How is the Hijri date calculated?: We use your browser's built-in <code>Intl.DateTimeFormat('ar-SA-u-ca-islamic', ...)</code> which converts Gregorian dates to the Hijri (Islamic lunar) calendar. No external API calls or lookup tables are needed.
Why does the week number change when I switch week start?: Week numbering depends on which day is considered the start of the week. ISO uses Monday; many US calendars use Sunday. Switching the start day recalculates the week boundaries and numbering accordingly.

Last updated: 2026-06-04

Week Number Calculator

Gregorian

Hijri (Islamic)

Week Calendar

How to use Week Number Calculator

What is Week Number Calculator?

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