Author: Oscar Cronquist Article last updated on November 02, 2018

The WEEKNUM function calculates a given date's week number based on a return_type parameter that determines which day the week begins.

Formula in cell D3:

=WEEKNUM(B3,C3)

You also have the option to choose which system to use, see the [return_type] argument.

 System 1 Week 1 is the week that contains January 1. System 2 Week 1 is the week that contains the first Thursday of the year. (European week numbering system).

### Excel Function Syntax

WEEKNUM(serial_number,[return_type])

### Arguments

 serial_number Required. A date. If you enter the date in the function then use the DATE function to calculate the correct serial_number. [return_type] Optional. A number determining which day the week begins. If omitted, 1 is used.

The following table shows what the LOGEST function returns if [stats] argument is TRUE.

 [return_type] Week starts on System 1 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