Please Note: This article is written for users of the following Microsoft Excel versions: 2007, 2010, 2013, 2016, 2019, 2021, and Excel in Microsoft 365. If you are using an earlier version (Excel 2003 or earlier), this tip may not work for you. For a version of this tip written specifically for earlier versions of Excel, click here: ISO Week Numbers in Excel.

ISO Week Numbers in Excel

Written by Allen Wyatt (last updated February 10, 2024)
This tip applies to Excel 2007, 2010, 2013, 2016, 2019, 2021, and Excel in Microsoft 365


Excel provides a function that can be used to calculate the week number, of the year, of any given date. How you can use the WEEKNUM function has been recounted in other ExcelTips. There are some methods of determining weeks where the WEEKNUM function won't do, however. For instance, you may need to find the week number based on the ISO method of calculating weeks.

In the ISO scheme of things, weeks are calculated based upon the start of the week being Monday. Not so tough, right? However, the first week of the year is defined as beginning on the Monday of the week in which the first Thursday of January occurs. Whoa! This means that the first week of the year can actually begin on Monday, December 29, of the preceding year and it is possible that the last week of the preceding year can end as late as Sunday, January 3, of the current year.

This is too much for WEEKNUM to handle. Instead, you will need to use a specialized function to determine the ISO week number. This example provides the ISO week number for whatever date is in cell A1:

=ISOWEEKNUM(A1)

The ISOWEEKNUM function was introduced in Excel 2013, so if you are using an earlier version of Excel (or your workbook will be used by others with earlier versions of Excel), then you cannot rely on it. Instead you'll need to use a formula to calculate the ISO week number for the year:

=INT((A1-DATE(YEAR(A1-WEEKDAY(A1-1)+4),1,3)+
WEEKDAY(DATE(YEAR(A1-WEEKDAY(A1-1)+4),1,3))+5)/7)

If you need more information on the how to handle ISO week numbers and dates in Excel, you'll find this page helpful:

http://www.cpearson.com/Excel/WeekNumbers.aspx

ExcelTips is your source for cost-effective Microsoft Excel training. This tip (7847) applies to Microsoft Excel 2007, 2010, 2013, 2016, 2019, 2021, and Excel in Microsoft 365. You can find a version of this tip for the older menu interface of Excel here: ISO Week Numbers in Excel.

Author Bio

Allen Wyatt

With more than 50 non-fiction books and numerous magazine articles to his credit, Allen Wyatt is an internationally recognized author. He is president of Sharon Parq Associates, a computer and publishing services company. ...

MORE FROM ALLEN

Saving in Document Format from a Macro

Saving a document in a different format is easy if you are manually using the Save As command. Saving a document in an ...

Discover More

Moving a Table Column

Want to move a column in a table very easily? You can do so by using the same editing techniques you are already using.

Discover More

Viewing Multiple Pages

If you have a large monitor, you can view more than one page at a time in Word. This is very handy when you want to ...

Discover More

Professional Development Guidance! Four world-class developers offer start-to-finish guidance for building powerful, robust, and secure applications with Excel. The authors show how to consistently make the right design decisions and make the most of Excel's powerful features. Check out Professional Excel Development today!

More ExcelTips (ribbon)

Calculating Months for Billing Purposes

Different businesses have different ways to calculate elapsed time for billing purposes. Figuring out a formula that ...

Discover More

Calculating Differences in Months using Non-Standard Date Values

Dates can be entered into a worksheet in any number of unique or novel ways. Working with those dates can be a challenge, ...

Discover More

Tombstone Date Math

Doing math with dates is easy in Excel. Doing math with old dates, such as those you routinely encounter in genealogy, is ...

Discover More
Subscribe

FREE SERVICE: Get tips like this every week in ExcelTips, a free productivity newsletter. Enter your address and click "Subscribe."

View most recent newsletter.

Comments

If you would like to add an image to your comment (not an avatar, but an image to help in making the point of your comment), include the characters [{fig}] (all 7 characters, in the sequence shown) in your comment text. You’ll be prompted to upload your image when you submit the comment. Maximum image size is 6Mpixels. Images larger than 600px wide or 1000px tall will be reduced. Up to three images may be included in a comment. All images are subject to review. Commenting privileges may be curtailed if inappropriate images are posted.

What is five less than 9?

There are currently no comments for this tip. (Be the first to leave your comment—just use the simple form above!)


This Site

Got a version of Excel that uses the ribbon interface (Excel 2007 or later)? This site is for you! If you use an earlier version of Excel, visit our ExcelTips site focusing on the menu interface.

Newest Tips
Subscribe

FREE SERVICE: Get tips like this every week in ExcelTips, a free productivity newsletter. Enter your address and click "Subscribe."

(Your e-mail address is not shared with anyone, ever.)

View the most recent newsletter.