ISO Week Number

A supplier schedule shows delivery in CW 34, a report says 2026-W12, or the factory system only speaks in week numbers. Converting that in your head is asking for trouble, because ISO weeks follow their own rules. Pick a date here (the page opens with today already filled in) and you get the ISO 8601 week number, the matching ISO year and the full formatted code, ready to paste into a spreadsheet or system.

Two rules explain nearly everything: ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday (equivalently, the week containing January 4). That produces the odd cases at year boundaries: January 1, 2021, a Friday, belongs to week 53 of 2020, while December 31, 2024, a Tuesday, is already week 1 of 2025. This is why the tool also shows the ISO year, which does not always match the calendar year of your date.

When you record weeks in reports, always keep the ISO year and week together (2020-W53, not 2021-W53): mixing them up is the classic year-boundary mistake. Watch out for Excel too: the default WEEKNUM function numbers weeks from January 1 and does not follow the ISO standard; to match this page, use ISOWEEKNUM instead. The whole computation runs in your browser, and the date you enter never leaves it.