Excel WEEKNUM() function

Last update on August 19 2022 21:50:34 (UTC/GMT +8 hours)

WEEKNUM() function

This function is used to return the week number of a specific date. For example, the week containing January 1 is the first week of the year and is numbered week 1.

Two systems have been used for this function:

The 1st one is System 1 The week containing January 1 is the first week of the year, and is numbered week 1.
and the 2nd one is System 2 The week containing the first Thursday of the year is the first week of the year and is numbered as week 1. This system is the methodology specified in ISO 8601, which is commonly known as the European week numbering system.

Version : Excel 2013

Syntax:

WEEKNUM(serial_number,[return_type])

Parameters

Name	Description	Data Type	Required/ Optional
Serial_number	A sequential number that represents the date of the day you are trying to find.	Date	Required
Return_type	A number that determines the type of return value.	Number	Optional

Here is a table below show that, a number that determines on which day the week begins. The default is 1.

Return_type	Week begins on	System
1 or omitted	Sunday	1
2	Monday	1
11	Monday	1
12	Tuesday	1
13	Wednesday	1
14	Thursday	1
15	Friday	1
16	Saturday	1
17	Sunday	1
21	Monday	2

Remarks:

Excel stores date as sequential serial numbers so they can be used in calculations. By default, January 1, 1900, is serial number 1. January 1, 2010, is serial number 40179 because it is 40178 days after January 1, 1900.
when Serial_number is out of range for the current date base value, a #NUM! error is returned.
when Return_type is out of the range specified in the table above, a #NUM! error is returned.

Example - 1:

The example below returns the number of the week in the year that 4/20/2012 occurs, based on weeks beginning on Sunday (default).

Example - 2:

The example below returns the number of the week in the year that 4/20/2012 occurs, based on a week beginning on Monday ( 2).

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