ISO Week Number Calculator

How does this calculator work?

Select a calendar date. In ISO 8601, weeks start on Monday. Week 1 is the week containing January 4, or equivalently the year’s first Thursday. The week-year can differ from the calendar year near New Year.

Quick example

December 31, 2020 belongs to ISO week 53 of 2020. January 1, 2021 is still in that same ISO week.

Frequently asked questions

Why is my week-year different?

ISO week-years sometimes start in late December or finish in early January.

Do weeks start on Sunday?

No. This calculator specifically follows ISO 8601, which starts each week on Monday.