ISO vs US week numbers

Two rules, two answers

ISO 8601 is the international standard and the one Europe works to. Weeks run Monday to Sunday. Week 1 is the week containing the year's first Thursday, which is the same as saying the week that holds most of its days in the new year. A consequence: the first days of January can belong to the previous year's week 53, and late December can already be week 1 of the year to come.

The US convention starts weeks on Sunday, and week 1 is simply whichever week contains 1 January — even if only one day of it falls in the new year. Numbering restarts every calendar year, so a US week never belongs to a different year.

The same dates, side by side

The turn of the year is where the two part company hardest. 1 January 2027 is a Friday: ISO puts it in week 53 of 2026, because that week's Thursday is still in December. The US rule calls it week 1 of 2027, because the week contains 1 January.

Making a spreadsheet agree

In Excel and Google Sheets, WEEKNUM(date) follows the US rule by default. For the ISO number use ISOWEEKNUM(date), or WEEKNUM(date, 21), which selects the Monday-start ISO system explicitly. The difference is not cosmetic: a report that joins two tables on week number will mismatch rows if one side used each.

Outlook, Google Calendar and iOS can all display week numbers, and each has its own setting for which day starts the week. If a colleague's "week 32" is not yours, that setting is the first place to look.

Which one should you use

If anyone outside the United States will read it, use ISO. It is the standard, it is unambiguous, and it is what European manufacturing, logistics and project planning run on. Inside the US, follow whatever the rest of the organisation already uses, and write the convention down somewhere — most week-number arguments turn out to be two people who are both right under different rules.