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Excel WEEKNUM Function

Excel WEEKNUM function
Summary 

The Excel WEEKNUM function takes a date and returns a week number (1-54) that corresponds to the week of year. The WEEKNUM function starts counting on the week that contains January 1. By default, weeks begin on Sunday, but this can be changed.

Purpose 
Get the week number for a given date
Return value 
A number between 1 and 54.
Syntax 
=WEEKNUM (serial_num, [return_type])
Arguments 
  • serial_num - A valid Excel date in serial number format.
  • return_type - [optional] The day the week begins. Default is 1.
Version 
Usage notes 

The WEEKNUM function takes a date and returns a number between 1 and 54 that corresponds to the week of the year. By default, the WEEKNUM function starts counting on the week that contains January 1 and increments week numbers on Sunday.

The WEEKNUM function accepts two argumentsserial_num and return_type. The serial_num argument must have a valid Excel date. The return_type argument controls what day of the week begins a new week number. Return_type is optional and defaults to 1, which sets new week numbers to start on Sunday. When return_type is set to 2, week numbers begin on Monday. 

With a return_type of 1-17, week number 1 in a given year is assigned to the week that contains January 1. With return_type 21, week 1 is the week containing the first Thursday of the year, following ISO 8601. The table below summarizes return_type options.

Return_type Week begins
1 (default) Sunday
2 Monday
11 Monday
12 Tuesday
13 Wednesday
14 Thursday
15 Friday
16 Saturday
17 Sunday
21 Monday (see note above)

Example #1 - basic usage

The formulas below return the week number for the last day of 2020 and the first day of 2021:

=WEEKNUM("31-Dec-2020") // returns 53
=WEEKNUM("1-Jan-2021") // returns 1

Example #2 - return type

The return_type argument controls what day of week begins a new week number. By default, return_type is 1, and numbers increment on Sunday. When return_type is provided as 2, week numbers begin on Monday. For example, January 3, 2021 is a Sunday and, WEEKNUM will return 2, since new numbers start on Sundays:

=WEEKNUM("3-Jan-2021") // returns 2

However, when return_type is set to 2, WEEKNUM will return 1 and start week 2 on Monday:

=WEEKNUM("3-Jan-2021",2) // returns 1
=WEEKNUM("4-Jan-2021",2) // returns 2

Note: the examples above show dates as text values for readability, but working native Excel dates is more reliable. To create a date from scratch in a formula, use the DATE function.

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