ISO Week Numbers

The three ISO rules

ISO 8601 defines a week with three rules. Weeks run from Monday to Sunday. Week 1 of a year is the week containing that year's first Thursday, which is the same as the week containing 4 January. The week-numbering year is the calendar year of the week's Thursday.

Because the year is taken from the Thursday rather than from the Monday or the date itself, an ISO week can belong to a different calendar year from the days it contains. That is deliberate: it keeps every week exactly seven days long and every label unique, at the cost of a boundary effect at both ends of the year.

Notation

ISO writes a week as the week-numbering year, the letter W, and a two-digit week number, so 2026-W01 is week 1 of the week-numbering year 2026. Adding a third element gives the day within the week, counted from Monday as 1, so 2026-W01-4 is the Thursday of that week.

Because 1 January 2026 is a Thursday, it is the fourth day of 2026-W01. That week runs from Monday 29 December 2025 to Sunday 4 January 2026, which is why a week labelled 2026 begins in December 2025. The same pattern appears in 2020-W53-4, which is Thursday 31 December 2020.

When a date belongs to another year

1 January 2021 is a Friday. The ISO week containing it runs from Monday 28 December 2020 to Sunday 3 January 2021, and its Thursday is 31 December 2020. The week-numbering year is therefore 2020, and 1 January 2021 is in 2020-W53, not in week 1 of 2021.

The boundary at the end of the year works the same way. 29 December 2025 is a Monday, and its week runs to Sunday 4 January 2026 with a Thursday of 1 January 2026, so it is 2026-W01. 1 January 2026 is that Thursday and so is also 2026-W01, while 4 January 2026 closes the same week.

The practical consequence is that a calendar date alone does not identify an ISO week. Any record that stores a week number should store the week-numbering year with it, and should be careful about records created in late December or early January.

Years with 53 weeks

A common year is 52 weeks and 1 day; a leap year is 52 weeks and 2 days. Those spare days mean that every five or six years the year's first Thursday comes early enough, and its last Thursday late enough, for the year to own 53 Thursdays, and so 53 ISO weeks. That happens when the year starts on a Thursday, or when it is a leap year that starts on a Wednesday.

By that rule, 2015, 2020, 2026 and 2032 have 53 weeks, while 2021, 2025 and 2027 have 52. In 2026 the extra week, 2026-W53, runs from Monday 28 December 2026 to Sunday 3 January 2027, so 1 January 2027 belongs to week 53 of 2026 and 2027-W01 begins on Monday 4 January 2027. Anything divided into weekly periods, such as payroll or weekly sales targets, needs a 53rd period in those years.

How ISO differs from Sunday-first numbering

The main alternative, common in the United States, starts weeks on Sunday and makes week 1 the week that contains 1 January, however few days of it fall in January. Weeks are numbered within the date's own calendar year, so the last days of December are in week 52 or 53 of that year even when they share a row with 1 January.

1 January 2022 was a Saturday. Under Sunday-first numbering it is week 1 of 2022, a week that began on Sunday 26 December 2021. ISO places it in 2021-W52, because the Thursday of its Monday-to-Sunday week was 30 December 2021.

Calendars that start the week on Sunday or Saturday still need a week label for each row. Rather than invent a separate numbering for each, the DateUtils calendar view labels every row with the ISO week of that row's Thursday, as described in the methodology. A row contains exactly one Thursday, so the label is never ambiguous.

ISOWEEKNUM and WEEKNUM in spreadsheets

Excel and Google Sheets both provide ISOWEEKNUM, which returns the ISO week number. WEEKNUM with its default settings uses the Sunday-first scheme above. The two agree for most of the year and disagree near New Year: for Monday 29 December 2025, ISOWEEKNUM returns 1 and WEEKNUM returns 53. Neither function returns the year, so for ISO weeks the week-numbering year has to be worked out separately, from the date of the week's Thursday.

Mixing the two functions in one dataset produces errors that only show up in late December and early January. Where a week means an ISO week, use ISOWEEKNUM throughout, and check boundary dates with the week number calculator, which returns the week and the week-numbering year together.

Where ISO weeks are used

ISO week numbers are the standard wherever a week is the reporting unit and the calendar must be shared across countries: payroll periods, manufacturing and retail planning, weekly production counts, and European business calendars. In these settings a week is identified by the pair year and number, such as 2026-W01, and the week-numbering year is the part that is often left out.

To find the weekday of a date before working out its week, use the day of the week calculator.