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: Using the WEEKNUM Function.

Using the WEEKNUM Function

by Allen Wyatt
(last updated November 11, 2017)

2

One of the handy date and time functions provided by Excel is WEEKNUM. This function is used, oddly enough, to return the week number represented by a particular date. You use the function in this way:

=WEEKNUM(A5,1)

In this instance, A5 contains a date serial number, and the value 1 indicates that WEEKNUM should assume that all weeks start on a Sunday. If you prefer your weeks to begin on Mondays, then you can use the value 2 instead.

You should realize that WEEKNUM always considers the first day of any given year to be in the first week of the year. Thus, it is possible for the above formula to return up to 54 weeks in a year. How can this be? Let's use the year 2011 as an example. In 2011 January 1 falls on a Saturday. As far as WEEKNUM is concerned, this is in the first week of the year. Now, January 2 for that year falls on a Sunday. Since WEEKNUM believes that every Sunday starts a new week, the second day of the week is considered in the second week of the year.

This is fine, until you get to the end of the year. The fifty-second week of 2011 ends (according to WEEKNUM) on December 24, and the fifty-third week begins on December 25 (a Sunday).

An even more interesting scenario is when the year begins on a Saturday and the year is a leap year. This happened in the year 2000. In that instance, the fifty-third week began on December 24, and the fifty-fourth week began on December 31.

ExcelTips is your source for cost-effective Microsoft Excel training. This tip (7804) 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: Using the WEEKNUM Function.

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. ...

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What is 2 + 4?

2017-11-15 11:41:40

Willy Vanhaelen

@Gerard S
ISOWEEKNUM is indeed introduced in Excel 2013.
If you want to follow ISO 8601 definition in Excel 2010, you can use WEEKNUM.
WEEKNUM( serial_number, [return_type] )
setting [return_type] to 21.


2017-11-13 09:04:41

Gerard S

It should be noted that WEEKNUM does not adhere to the ISO 8601 definition of Week Numbers.
ISOWEEKNUM is the function that follows the ISO 8601 rules. This function was introduced with Excel 2013 (I believe).
One can google ISO 8601 and read all about date and time definitions and formats. Recommended reading if one deals internationally.
When it comes to ISO week numbers, 2 important things to know are:
* Week 1 is the first week in January with a Thursday in it. (Which is not necessarily the week with Jan 1st.)
* Weeks start on Monday (by definition).


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