Dates and calendar

ISO Week Number Calculator

Yes. The first ISO week can begin in the previous calendar year.

Yes, when those dates fall before the first ISO week.

Enter a calendar date to identify its ISO 8601 week.

Your resultWeek 33

ISO week-based year 2026.

The shared link contains the values you entered.

How it works

ISO weeks begin on Monday. Week 1 is the week containing the year’s first Thursday.

Dates near New Year can belong to the previous or following ISO week-based year.

Last verified: 29 September 2026

03FAQ

When does an ISO week start?

On Monday.

Can week 1 begin in December?

Can early January belong to the previous ISO year?