WEEKNUM

Returns the ISO week number of a given date.

Syntax:

WEEKNUM(date, mode)

returns the international standard ISO8601 week number, where week 1 is the week that contains January 4th. date is the date as a date-time serial number or as text. mode specifies which day is considered the first day of the week: 1 for Sunday, 2 for Monday.

Example:

WEEKNUM("2010-01-07", 2)

returns 1. 7 Jan 10 is a Thursday, and 4 Jan 10 the Monday of that week.

Application:

Imagine you're a retail manager who wants to see how sales are performing week-over-week. You have a table with daily sales data, but you need to aggregate it by week to get a clear picture. The WEEKNUM function allows you to assign a week number to each date, making it easy to group and analyze your data.

Here's an example of how you could use it in a spreadsheet:

Scenario: A retail store needs to analyze its sales data on a weekly basis.

Objective: Use the WEEKNUM function to determine the week number for each sale date and then use that information to summarize sales for each week.

Sales Data Table

	Date	Daily Sales	Week Number
	A	B	C
1	1/1/2025	$1,500.00	1
2	1/2/2025	$1,850.00	1
3	1/3/2025	$1,600.00	1
4	1/4/2025	$2,100.00	1
5	1/5/2025	$2,500.00	2
6	1/6/2025	$1,750.00	2
7	1/7/2025	$1,900.00	2
8	1/8/2025	$2,200.00	2
9	1/9/2025	$2,400.00	2
10	1/10/2025	$2,600.00	2

In this table:

Column A contains the dates of the sales.
Column B contains the amount of sales for that day.
Column C is where we use the WEEKNUM function. The formula WEEKNUM(A2) in cell C2 returns the week number for the date in cell A2.

The WEEKNUM function, by default, uses a system where the week containing January 1st is Week 1, and the week starts on Sunday.