ISO_WEEKNUM() : ISO week number (week starting Monday)

The ISO_WEEKNUM function returns the week number of a given date according to the ISO calendar, as a number. The week starts on Monday, and week 1 is the one that contains the first Thursday of the year.

The output format of the result can be configured in the output field options of the formula editor. For more details, see Output format options in the Formula column article.

Syntax

ISO_WEEKNUM(date)

Calling function parameters in TimeTonic:

$field_name = field (link, linked table column, number, selection, formula, text...).
"free text" = free text to add between quotation marks.
Do not use quotation marks for numbers.

Parameters

# Parameter Description

1 date Date for which to calculate the ISO week number.

ISO_WEEKNUM() or WEEKNUM()?

Function	Start of week	Definition of week 1
WEEKNUM(date)	Sunday	Week containing January 1st
ISO_WEEKNUM(date)	Monday	Week containing the first Thursday of the year

The choice between WEEKNUM() and ISO_WEEKNUM() depends on the business context (internal planning, ISO standards, delivery rounds, reporting).

Examples

Find the ISO week number of a date

ISO_WEEKNUM(2026-1-1)

Result: 1. January 1, 2026 falls in the 1st ISO week of the year 2026.

Go further

WEEKNUM

The classic week number: week starting Sunday.

Read more →

Or continue with

Formula glossary

All available operators and functions, sorted by category.