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.
Syntax
=ISOWEEKNUM(date)
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
Errors
#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”.
Examples
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.