When working with dates in Excel, one of the most useful functions for organizing and analyzing time-based data is WEEKNUM. This function allows me to determine the week number of a given date based on different week numbering systems.
What is the WEEKNUM function?
The WEEKNUM function in Excel returns the week number of a specified date. By default, weeks start on January 1st, but Excel provides flexibility to define the first day of the week according to different conventions.
WEEKNUM Syntax
The syntax of the WEEKNUM function is straightforward:
=WEEKNUM(serial_number, [return_type])- serial_number: A valid Excel date (required).
- return_type: Defines the starting day of the week (optional).
Return Type Options
The return_type argument allows me to select which numbering system to use. The two most common options are:
- 1 or omitted: Weeks start on Sunday (default).
- 2: Weeks start on Monday.
- 11: Monday as the first day using ISO 8601 standard (Europe-style week numbering).
| Return Type | Week Starts On | System Used |
|---|---|---|
| 1 | Sunday | US Standard |
| 2 | Monday | European Standard |
| 11 | Monday | ISO 8601 |
Practical Examples
Let’s explore a few examples to understand how WEEKNUM works:
Example 1: Basic Usage
=WEEKNUM("2024-06-05")This will return 23, meaning June 5, 2024, is part of the 23rd week of the year.
Example 2: Setting Monday as the First Day
=WEEKNUM("2024-06-05", 2)This will still return 23 but uses Monday as the start of the week.
Example 3: Using ISO 8601 Standard
=WEEKNUM("2024-01-01", 11)Since ISO 8601 weeks start on Monday and a full week is required to count as week 1, January 1 (a Monday) may still fall under the last week of the previous year.
Common Errors and How to Fix Them
Excel’s WEEKNUM function is generally reliable, but here are a few common mistakes I’ve encountered:
- Using non-date values: If I enter text instead of a valid date, Excel will return a
#VALUE!error. - Invalid return type: Choosing an incorrect
return_typewill return a#NUM!error. - Incorrect date format: Excel recognizes dates stored as serial numbers. If my date is stored as text, I need to convert it using
DATEVALUE.
WEEKNUM vs. ISOWEEKNUM
Excel also has a related function called ISOWEEKNUM. Unlike WEEKNUM, which allows different starting days, ISOWEEKNUM strictly follows the ISO 8601 standard:
=ISOWEEKNUM("2024-01-01")This method ensures compatibility with international business calendars, where a week always starts on Monday.
Final Thoughts
Understanding WEEKNUM in Excel is essential for managing weekly financial reports, sales forecasts, and scheduling. With its flexible options and compatibility with other date functions, it’s a powerful tool for any Excel user. Whether I need US-based week numbering or the internationally accepted ISO 8601 system, the WEEKNUM function always has me covered.
Other interesting article:
How WEEKDAY works in Excel? Best WEEKDAY examples