When does week 1 start?

ISO Week 1 is the week containing the first Thursday of the year. It always includes January 4. Week 1 can start as early as December 29 of the previous year.

Do all countries use ISO week numbers?

Not all. The US and some other countries count weeks starting Sunday, with week 1 being the week containing January 1. This gives different week numbers from the ISO standard. The ISO system (Monday-start, first-Thursday rule) is standard in Europe and most international contexts.

Week Number Calculator — What Week of the Year Is It?

The Week Number Calculator tells you the ISO week number for any date. ISO weeks run Monday to Sunday; week 1 is the week containing the first Thursday of the year. Week numbers are used in manufacturing, logistics, project planning, and international business — particularly in Europe where week numbers appear on calendars and in contracts.

Year

Month (1–12)

Day (1–31)

How to Use This Calculator

1
Enter the date (year, month, day) you want to look up.
2
Click Calculate to see the ISO week number for that date.
3
ISO week 1 is the week containing the first Thursday of the year.

Frequently Asked Questions

A year has 53 ISO weeks when January 1 falls on a Thursday, or December 31 falls on a Thursday. This occurs roughly every 5–6 years. The extra week is week 53.

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