The ISOWEEKNUM function accepts a date and returns the ISO week number (1 through 54) of that date. The date must be in a valid format , so you have to use the DATE function , wrap it in quotes, or use a cell reference to a cell containing a date.

An ISO week starts on Monday. The first week of the year is the first week containing a Thursday in that year.

If your days, months, and years are in different cells, use the DATE function to combine them.



  • date - The date for which you want the ISO week number. This is required.

Related functions

DATE - Takes separate year, month, and day values and returns them as a date

DATEVALUE - Returns the serial value of a date

WEEKNUM - Returns the number of a given week


#NUM - The inputs are numbers but are not valid dates for the date input or the type input is out of range. This could happen if you use a negative number for the date.

#VALUE! - The inputs are something that doesn't convert to a number such as "The other day" or "Yester-yester-day".


Example 1 - Plain and Simple

Formula Result
=ISOWEEKNUM("March 1,2017") 9

Example 2 - ISOWEEKNUM with DATE function

Formula Result
=ISOWEEKNUM( DATE (2019,11,7)) 45


 Live examples in Sheets 

Go to this spreadsheet for the examples of the ISOWEEKNUM function shown above that you can study and use anywhere you would like.