How to Get the ISO Week Number in Excel

Use ISOWEEKNUM for European-style week numbers, find the ISO year at the turn of the year, and build labels like 2026-W53.

You will learn to

• Return the ISO 8601 week number
• Work out which year an ISO week belongs to
• Build a YYYY-Www week label

How it works

ISOWEEKNUM(date) returns the ISO 8601 week number used across Europe and in many planning systems. ISO weeks start on Monday, and week 1 is the week that contains the first Thursday of the year — so December 29 can be week 1 and January 1 can be week 53.

Because of that, the ISO week’s year is not always YEAR(date). Use the Thursday of the same week instead: =YEAR(B2-WEEKDAY(B2,2)+4). Combine it with TEXT(ISOWEEKNUM(B2),"00") to build sortable labels like 2026-W53.

=ISOWEEKNUM(B2)