How Week Number Calculator works

Find the ISO 8601 week number, week-year and Monday-to-Sunday range, or reverse a week number into dates.

The formula

ISO weeks start Monday, and Week 1 is the week containing January 4 (equivalently, the year’s first Thursday).

A practical example

September 7, 2026 is Monday of ISO Week 37 in ISO week-year 2026.

Important limitations

This tool follows ISO 8601. Other calendar systems may number weeks differently or begin weeks on Sunday.

Frequently asked questions

Is January 1 always in Week 1?

No. It may belong to Week 52 or 53 of the previous ISO week-year.

Can a year have Week 53?

Yes. The reverse lookup validates whether Week 53 exists in the chosen year.