# Excel function WEEKNUM and change week start day – Excel 2019

I have been using week number function from quite some time now, but I came across a situation where I need to change week start day in excel from Sunday to Thursday. This required use of WEEKNUM function and use of return type for WEEKNUM function.

### What does WEEKNUM function do

A typical excel year has weeks. Whenever excel is presented with a date, WEEKNUM function takes a date and returns a number between 1 and 54. Default, week starts on on Sunday i.e the WEEKNUM function initializes counting on the week that contains January 1, and increments week numbers on Sunday.

### How to use WEEKNUM function

Syntax for WEEKNUM

WEEKNUM(serial_number,[return_type])

WEEKNUM function uses two variables serial number and return type. Serial number is valid excel date in serial number format and Return_type defines day on which week will initialize or new week will start.

Till now I had never used return type argument as it was optional but this does the trick. By default if not provided its 1 i.e. Sunday. But it has other options as well

### Change week start day on Monday

So if you want week day to start on Monday we will use formula

So if your date is in cell A1

=WEEKNUM(A1, 11)

Now you will get your week number in the cell containing formula with week starting on Monday.

### Change week start day on Thursday

Now if you want your week to initialize on Thursday you will modify above formula as

=WEEKNUM(A1,14)

Now your week number will change from Thursday

### More ways to use excel WEEKNUM function

Other variations of above formula

=WEEKNUM(“mm-dd-yyyy”, return_type)

Usage

=WEEKNUM(“01-01-2021”,14)

### WEEKNUM function with return_type 21

Return type 21 is used when you want to use week as per ISO8601 standard

Syntax to use

=WEEKNUM(“01-01-2021”,21)

Standards followed by ISO 8601 can be viewed at below link

https://en.wikipedia.org/wiki/ISO_8601#Week_dates

Note: Above link will take you to an external link and information provided on it are not within our control.