ISOWEEKNUM

Category: MS Excel compatibility function · Native since: Excel 2013

Returns the ISO 8601 week number of the year for a date.

Syntax

=ISOWEEKNUM(date)

Arguments

Argument	Required	Description
date	Required	the date

Returns

Returns the ISO 8601 week number (1-53) of the date as a number. An array argument is computed element-wise and spills as an array of the same shape on dynamic-array hosts. Returns #VALUE! if the date is non-numeric or negative, and #NUM! if the serial cannot be converted to a date.

Examples

Formula	Result	Description
=ISOWEEKNUM(DATE(2026,1,1))	1	2026-01-01 is week 1
=ISOWEEKNUM(DATE(2023,1,1))	52	Last week of prior ISO year

Notes

• Array input is computed element-wise and returned as an array of the same shape.
• Supported: Excel 2010+. Registered as ISOWEEKNUM (drop-in) on hosts without the native function, and as EG.ISOWEEKNUM on modern Excel that has it.