What Is an ISO Week Number?

Weeks need rules

A week has seven days, but a year does not divide into whole weeks: it has 52 weeks and one or two extra days. Any system that numbers the weeks of the year therefore needs rules for the first week and the last. The international standard, ISO 8601, gives one such set of rules, and it is used widely in Europe, in business planning and in software.

The result is a week number from 1 to 52 or 53, combined with a week-based year that can differ from the calendar year around the turn of the year.

The ISO rules

ISO weeks start on Monday and end on Sunday. Week 1 of a year is the week that contains the first Thursday of the year. Another way to say this is that week 1 is the first week that has at least four days in the new year, and it always contains 4 January.

Each week belongs to the year that contains its Thursday. That single rule explains everything else about week numbers at the turn of the year.

For example, 22 October 2026 is a Thursday in week 43 of 2026. The full written form of the date is 2026-W43-4, where 4 stands for Thursday, the fourth day of the week counted from Monday.

When January belongs to the old year

Because the week follows its Thursday, the first days of January can fall in the last week of the previous year. In 2021, 1 January was a Friday, and the days from 1 January to 3 January belonged to week 53 of 2020. Week 1 of 2021 began on Monday 4 January.

The same happens at the other end. 31 December 2024 was a Tuesday and belonged to week 1 of 2025, and Monday 30 December 2024 was the first day of that week.

If your software shows a week number for 1 January that looks too large, or for 31 December that looks too small, this is the reason. The date is not wrong. It belongs to the week-based year, not the calendar year.

Years with 53 weeks

Most years have 52 ISO weeks, but some have 53. A year has 53 weeks if 1 January is a Thursday, or if it is a leap year that begins on a Wednesday. Over the 400-year Gregorian cycle, 71 years have 53 weeks, which is roughly one year in every five or six.

2026 is such a year, because it began on a Thursday. The years 2015, 2020, 2026, 2032 and 2037 each have 53 weeks. In 2026, 31 December is a Thursday and falls in week 53, and the following week, beginning on Monday 4 January 2027, is week 1 of 2027.

Planning calendars that use ISO weeks have to allow for the extra week, which affects weekly reporting, payroll and production schedules.

ISO weeks and US weeks

The United States and some other countries number weeks differently. A common US scheme starts the week on Sunday and makes the week that contains 1 January week 1. Under that scheme, week numbers and the first day of the week differ from ISO.

In some years the two systems give the same number for a date, as on 22 October 2026, which is week 43 in both. In others they differ by one, particularly around the turn of the year.

When you exchange week numbers with another team, say which system you use. A phrase such as ISO week 43 prevents a mismatch.

Where week numbers are used

Week numbers are common in Europe, in school timetables, project plans, delivery schedules and manufacturing. Many calendars and diaries print them, and suppliers often quote a delivery in week 43 instead of a date.

They are convenient because they are short and unambiguous, and they make it easy to compare the same week in different years. They are less convenient for dates that cross a year, which is where the rules above matter.

A date such as 2026-W43-4 is the ISO week date format. It is rarely used in everyday life but is common in data exchange and in logistics.

Reading a week-based schedule

A supplier who promises delivery in week 45 of 2026 means a period of seven days that starts on Monday 2 November 2026 and ends on Sunday 8 November 2026. If you plan around week numbers, convert the key ones to dates once, write them down, and use the dates for anything that has to be exact.

Weeks are also handy for repeating events. A meeting that takes place every second week can be fixed to the even weeks, and a payroll that runs in a four-week cycle can be tracked by week number. Because week 53 appears only in some years, such schedules need a rule for the years that have one.

Using the calculator

Enter any date on the Day of the week tab and the calculator gives its ISO week number, together with the week-based year if it differs from the calendar year. It shows the week in brackets as, for example, 43 (2026).

The result of an Add or subtract calculation also shows the ISO week of the new date, which is useful for planning in weeks.