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.
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.
Comprehensive VBA Guide Visual Basic for Applications (VBA) is the language used for writing macros in all Office programs. This complete guide shows both professionals and novices how to master VBA in order to customize the entire Office suite for their needs. Check out Mastering VBA for Office 2010 today!
When doing data entry into a worksheet, you might want to enter dates without the need to type the separators that are ...
Discover MoreYou can use Excel to work with times and dates. Sometimes, however, figuring out the best way to do that can be tricky. ...
Discover MoreIf you store dates in your worksheets, you may want to update those dates at the end of the year. This tip explains ...
Discover MoreFREE SERVICE: Get tips like this every week in ExcelTips, a free productivity newsletter. Enter your address and click "Subscribe."
There are currently no comments for this tip. (Be the first to leave your comment—just use the simple form above!)
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.
FREE SERVICE: Get tips like this every week in ExcelTips, a free productivity newsletter. Enter your address and click "Subscribe."
Copyright © 2025 Sharon Parq Associates, Inc.
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:
Comments