Please Note: This article is written for users of the following Microsoft Excel versions: 2007, 2010, 2013, and 2016. 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

by Allen Wyatt
(last updated September 22, 2018)

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, refer to these sources:

https://www.rondebruin.nl/win/s8/win002.htm
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, and 2016. 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

Jumping to a Relative Section

Navigating through a long document can be challenging, at times. Here's a way you can move forward or backwards in your ...

Discover More

Copying Headers and Footers

Need to get headers and footers from one document to another? You can use the steps in this tip to help make quick work ...

Discover More

Locating the My Documents Folder

Need to do some macro processing of documents in the user's My Documents folder? First step is to figure out where the ...

Discover More

Excel Smarts for Beginners! Featuring the friendly and trusted For Dummies style, this popular guide shows beginners how to get up and running with Excel while also helping more experienced users get comfortable with the newest features. Check out Excel 2013 For Dummies today!

More ExcelTips (ribbon)

Finding the Dates for Minimums and Maximums

If you use Excel to maintain a collection of data, you may need to find information in one column based on information in ...

Discover More

Monthly Close-Out Dates

If your company closes out its accounting months at the end of each calendar quarter, figuring out the proper closing ...

Discover More

Ages in Years and Months

Calculating an age is a common task when working with dates. If you want to figure out the number of years and months ...

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}] 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 9 - 6?

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.