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The ISOWEEKNUM formula in Google Sheets is used to return the ISO week number of a given date. The ISO week numbering system is a standard week numbering system used by countries around the world. It assigns a number to each week of the year starting from Monday as week 1 and ending on Sunday as week 52 (or 53 if the year has 53 weeks). This formula is commonly used in financial and accounting spreadsheets to track weekly data.


Use the ISOWEEKNUM formula with the syntax shown below, it has 1 required parameter:

  1. date (required):
    The date for which you want to find the ISO week number. This can be in any valid date format or a reference to a cell containing a date.


Here are a few example use cases that explain how to use the ISOWEEKNUM formula in Google Sheets.

Track weekly sales

If you have a spreadsheet that tracks sales data, you can use the ISOWEEKNUM formula to group sales by week. This makes it easy to see how sales are trending week over week.

Calculate overtime

If you need to calculate overtime for employees based on a weekly schedule, you can use the ISOWEEKNUM formula to determine which week each day falls in. This can help you determine how many hours an employee worked in a given week and whether they are eligible for overtime pay.

Track project milestones

If you are managing a project with multiple milestones, you can use the ISOWEEKNUM formula to track progress by week. This can help you ensure that you are on track to meet your deadlines and identify potential issues early on.

Common Mistakes

ISOWEEKNUM not working? Here are some common mistakes people make when using the ISOWEEKNUM Google Sheets Formula:

Incorrect date format

If the date provided is not in a recognized date format, the ISOWEEKNUM formula will return an error. Make sure the date is properly formatted as a date in the cell or use the DATE function to create a valid date.

Incorrect function name

Make sure the function name is spelled correctly and capitalized correctly (all caps).

Missing input

Make sure the formula includes a valid input for the date. If the input is missing or blank, the ISOWEEKNUM formula will return an error.

The following functions are similar to ISOWEEKNUM or are often used with it in a formula:


    The WEEKNUM formula returns the week number of a specific date. It is most commonly used to group data by week or to calculate the number of weeks between two dates.

  • YEAR

    The YEAR formula in Google Sheets returns the year of a specified date. This function is useful when you need to extract the year from a date for analysis or calculations. The YEAR formula takes one argument, which is the date from which to extract the year. The output is a four-digit number representing the year.


    The MONTH formula is used to extract the month from a given date, returning it as a number between 1 and 12. This function is commonly used to summarize and analyze data that includes dates.

  • DAY

    The DAY function is used to extract the day of the month from a given date. It takes in a single argument, a date in a valid format, and returns the day of the month as a number between 1 and 31. This function can be useful for tasks such as sorting or filtering data based on the day of the month.

Learn More

You can learn more about the ISOWEEKNUM Google Sheets function on Google Support.