What is ISO week?

ISO week definition

An ISO week is a week number used in the ISO week-date system. Weeks start on Monday, and week 1 is the week containing the first Thursday of the year.

This means ISO week numbers do not always match a normal January-to-December calendar view. The first few days of January can sometimes belong to the final ISO week of the previous year, and the final days of December can sometimes belong to ISO week 1 of the next year.

How ISO weeks work

ISO weeks follow three simple rules:

The week starts on Monday.
The week ends on Sunday.
ISO week 1 is the week that contains the first Thursday of the calendar year.

ISO week year = the year containing that week’s Thursday
ISO week 1 = the week containing the first Thursday of January
Week start = Monday
Week end = Sunday

Why ISO weeks matter

ISO week numbers are useful when people need a consistent way to talk about weeks across dates, reports, schedules or international teams. Instead of saying “the week starting Monday 6 July”, someone might refer to “week 28”.

They are common in work planning, reporting, payroll exports, project schedules and systems that need week numbers to behave consistently every year.

Simple example

Imagine a week that starts on Monday 29 December and ends on Sunday 4 January. Even though part of the week is in December, the ISO week may belong to the next ISO week-year if the Thursday falls in January.

Example: If the Thursday of that week is in the new year, the ISO week belongs to the new ISO year. This is why ISO week-years can look slightly different from calendar years near New Year.

Common ISO week confusion

The main confusion is that an ISO week-year is not always the same as the calendar year shown in the date. A date at the start of January can sometimes still be counted as part of the previous ISO year.

That is why week-number reporting should be checked carefully when deadlines, payroll periods, rotas or project reports fall close to the start or end of the year.

ISO week FAQs

Does an ISO week always start on Monday?

Yes. In the ISO week-date system, weeks start on Monday and end on Sunday.

Can an ISO week belong to a different year?

Yes. Dates near the start or end of a calendar year can belong to the previous or next ISO week-year, depending on where the Thursday of that week falls.

Is ISO week the same as a working week?

No. An ISO week is a week-number system. A working day is usually a day when work is normally carried out, often excluding weekends and sometimes bank holidays depending on the context.