What Is an ISO Week Number?

ficklestudio26 · Facts checked October 15, 2026

Many planners print week numbers. They usually follow an international standard, and it explains why the first days of January sometimes belong to the previous year's last week.

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The definition

The international standard ISO 8601 numbers weeks like this: a week starts on Monday, and week 1 of a year is the week that contains the year's first Thursday. That is the same as saying it is the week that contains January 4. Another way to put it: week 1 is the first week with most of its days in the new year.

How many weeks in a year

A week-numbering year has either 52 or 53 weeks. It has 53 weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. Every other year has 52.

An example: the start of 2027

January 1, 2027 is a Friday. January 4, 2027 is a Monday, so ISO week 1 of 2027 runs from Monday, January 4 to Sunday, January 10. The first three days of 2027 (Friday to Sunday) are not in week 1. They belong to the last week of 2026, which is week 53, because 2026 started on a Thursday and so has 53 weeks. We checked these dates by calculating them ourselves.

Practical notes

A calendar that starts weeks on Sunday, as is common in the US, will not line up with ISO week numbers.
Some organizations number weeks differently, so use the same system as the people you work with.