ISO 8601 week numbering

The ISO 8601 standard defines week numbers with these rules:

Weeks start on Monday
Week 1 is the week containing the year's first Thursday
This means Dec 29–31 may fall in week 1 of the following year, and Jan 1–3 may fall in week 52/53 of the previous year

US vs. ISO week numbering

The US convention starts weeks on Sunday and defines week 1 as the week containing January 1st. This can produce different week numbers than ISO 8601 for the same date, especially at year boundaries.

How to find week number in common tools

Excel: =ISOWEEKNUM(A1) for ISO weeks; =WEEKNUM(A1,1) for US convention (1=Sunday start).
Python: from datetime import date; date.today().isocalendar()[1] returns the ISO week number.
JavaScript: use the date-fns library’s getISOWeek(date), or the upcoming Temporal API’s date.weekOfYear.
SQL (PostgreSQL): EXTRACT(WEEK FROM date_column) returns the ISO week number.

Business applications

Manufacturing: ISO week calendars are standard in production planning; parts are scheduled and tracked by week number, not calendar date.
Retail: many retailers use fiscal week reporting aligned to ISO or a 4-4-5 calendar for year-over-year comparison.
Project management: Agile sprints are often numbered by week; “Sprint W22” is immediately understood across time zones without date conversion.

Note: the ISO week year can differ from the calendar year for dates in late December or early January. Always use ISOWEEKNUM paired with ISOWEEK year to avoid off-by-one errors in year boundaries.

Long years (53-week years)

Most years have 52 ISO weeks. A year has 53 weeks when January 1 falls on a Thursday (or on a Wednesday in a leap year), because this causes week 1 to start in the previous December, pushing the last week of the year to number 53. Recent and upcoming 53-week years: 2020, 2026, 2032, 2037.