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 must 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. Use the WEEKNUM function for similar functionality for non-ISO week numbers.
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 date is required.
See the video for a narrated walkthrough of this function with examples and explanations.
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 (non-ISO)
#NUM – The inputs are numbers but are not valid dates for the date input, or the input is out of range.
#VALUE! – The inputs don’t convert to a number such as “The other day” or “Yester-yester-day”.
Example 1 – Standard Usage of ISOWEEKNUM
A straightforward usage of the function is typing the
date directly into the function and surrounding it with quotes.
Example 2 – ISOWEEKNUM with the DATE Function
The ISOWEEKNUM function can accept another function as its input. In this case, we are using the DATE function to ensure the input is valid.
Live Examples of ISOWEEKNUM in Sheets
Go to this linked spreadsheet for the preceding examples of the ISOWEEKNUM function shown above that you can study and use anywhere you would like.