# WEEKNUM

In this comprehensive guide, we will explore the WEEKNUM formula in Excel, which is used to calculate the week number of a specific date. This formula is particularly useful when you need to organize and analyze data based on weeks. We will cover the syntax, examples, tips and tricks, common mistakes, troubleshooting, and related formulae for the WEEKNUM function.

## WEEKNUM Syntax

The WEEKNUM formula in Excel has the following syntax:

=WEEKNUM(serial_number, [return_type])

Where:

• serial_number (required) is the date for which you want to find the week number. This can be a date entered directly, a cell reference containing a date, or a formula that returns a date.
• return_type (optional) is a number that determines the day the week begins. If omitted, the default value is 1, which means the week begins on Sunday. The available return_type values are:
• 1 – Week begins on Sunday (default)
• 2 – Week begins on Monday
• 11 – Week begins on Monday (used in Excel 2010 and later)
• 12 – Week begins on Tuesday
• 13 – Week begins on Wednesday
• 14 – Week begins on Thursday
• 15 – Week begins on Friday
• 16 – Week begins on Saturday
• 17 – Week begins on Sunday

## WEEKNUM Examples

Here are some examples of how to use the WEEKNUM formula in Excel:

Example 1: Basic usage of WEEKNUM

Suppose you have a date in cell A1 (e.g., 01/01/2022) and you want to find the week number with the week starting on Sunday (default). You can use the following formula:

=WEEKNUM(A1)

This will return the week number of the date in cell A1, which is 1 in this case.

Example 2: Using WEEKNUM with a different return_type

If you want to find the week number of a date with the week starting on Monday, you can use the following formula:

=WEEKNUM(A1, 2)

This will return the week number of the date in cell A1 with the week starting on Monday, which is 52 in this case.

Example 3: Using WEEKNUM with a date formula

You can also use the WEEKNUM formula with a date formula, such as TODAY(). For example, to find the week number of today’s date with the week starting on Sunday, you can use the following formula:

=WEEKNUM(TODAY())

## WEEKNUM Tips & Tricks

Here are some tips and tricks to help you get the most out of the WEEKNUM formula in Excel:

1. Remember that the WEEKNUM function returns the week number as per the specified return_type. If you need to find the week number according to a specific calendar system (e.g., ISO week numbers), you may need to use a different formula or function, such as the ISOWEEKNUM function in Excel 2013 and later.
2. If you want to find the week number of a date in a specific year, you can use the YEAR function in combination with the WEEKNUM function. For example:

=WEEKNUM(DATE(YEAR(A1), MONTH(A1), DAY(A1)), 2)

This formula will return the week number of the date in cell A1 in the same year, with the week starting on Monday.

## Common Mistakes When Using WEEKNUM

Here are some common mistakes users make when using the WEEKNUM formula in Excel:

1. Using an incorrect return_type value: Make sure you use a valid return_type value (1-17) to specify the day the week begins. Using an invalid value will result in a #NUM! error.
2. Entering dates as text: Ensure that the date you enter or reference is in a valid date format recognized by Excel. If the date is entered as text, the WEEKNUM function will return a #VALUE! error.
3. Not considering the calendar system: The WEEKNUM function calculates the week number based on the specified return_type, which may not be suitable for all calendar systems. If you need to find the week number according to a specific calendar system, consider using a different function or formula.

## Why Isn’t My WEEKNUM Working?

If your WEEKNUM formula isn’t working as expected, consider the following troubleshooting steps:

1. Check for errors: If your formula returns an error (e.g., #NUM! or #VALUE!), review the common mistakes section above and ensure that you are using a valid return_type value and a valid date format.
2. Verify the date: Make sure the date you are using in the WEEKNUM formula is correct and in a valid date format recognized by Excel.
3. Consider the calendar system: If the week number returned by the WEEKNUM function doesn’t match your expectations, ensure that you are using the correct return_type value and consider whether you need to use a different function or formula for your specific calendar system.

## WEEKNUM: Related Formulae

Here are some related formulae that you may find useful when working with dates and week numbers in Excel:

1. ISOWEEKNUM: This function returns the ISO week number of a date (available in Excel 2013 and later). The ISO week number is based on the ISO 8601 calendar system, where the week begins on Monday and the first week of the year contains January 4th.
2. YEAR: This function returns the year of a specified date.
3. MONTH: This function returns the month of a specified date.
4. DAY: This function returns the day of a specified date.
5. TODAY: This function returns the current date.

By combining these related formulae with the WEEKNUM function, you can perform more advanced date and week number calculations in Excel.

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