The WEEKNUM function accepts two arguments, serial_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.

Example #1 - basic usage

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

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

Dave Bruns

Hi - I’m Dave Bruns, and I run Exceljet with my wife, Lisa. Our goal is to help you work faster in Excel. We create short videos, and clear examples of formulas, functions, pivot tables, conditional formatting, and charts.