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:


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



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


The WEEKNUM function returns #NUM! error if:

  • serial_number is out of range.
  • [return_type] is out of range.