ISO week

Why week numbers differ

Calendars disagree on when a week starts and which week is first. US calendars often start on Sunday and count the week containing January 1 as week 1. ISO weeks start on Monday and use the first Thursday rule, so the first days of January can belong to the last week of the previous year, and the last days of December to week 1 of the next.

Where ISO weeks are used

Business planning, payroll and logistics in Europe, sprint planning, reporting, and weekly rankings that need one unambiguous week label. Aback Launch names every launch week by its ISO week, for example 2026-W43, and archives each closed week at /week/{year}/{week}.

Writing an ISO week

The standard format is 2026-W43 for the week, and 2026-W43-5 for a day within it, where 1 is Monday and 7 is Sunday. In spreadsheets, ISOWEEKNUM returns the ISO week of a date.

Common pitfalls

Mixing week systems in one report is the classic mistake. A spreadsheet that uses WEEKNUM in one tab and ISOWEEKNUM in another will disagree around New Year and in some years all the way through. Pick ISO weeks for anything shared across countries and say so in the column header.

The year matters too. 29 December 2025 belongs to 2026-W01, so a label that pairs the calendar year with the ISO week, such as 2025-W01, points at the wrong week entirely. Always take the year from the ISO week itself, which programming languages call the ISO week year.

Example

Friday 23 October 2026 is in ISO week 43 of 2026, written 2026-W43-5.

Questions people ask

What ISO week is it today?

Use our free ISO week calculator, which shows today's week and converts any date to its ISO week and back.