# ISOWEEKNUM function: Description, Usage, Syntax, Examples and Explanation

## What is ISOWEEKNUM function in Excel?

ISOWEEKNUM function is one of Date and Time functions in Microsoft Excel that returns number of the ISO week number of the year for a given date.

## Syntax of ISOWEEKNUM function

ISOWEEKNUM(date)

The ISOWEEKNUM function syntax has the following arguments.

**Date:**Date is the date-time code used by Excel for date and time calculation.

## ISOWEEKNUM formula explanation

- Microsoft Excel stores dates as sequential numbers so they can be used in calculations. By default, January 1, 1900 is serial number 1, and January 1, 2008 is serial number 39448 because it is 39,448 days after January 1, 1900.
- If the date argument is not a valid number, ISOWEEKNUM returns the #NUM! error value.
- If the date argument is not a valid date type, ISOWEEKNUM returns the #VALUE! error value.

## Example of ISOWEEKNUM function

Steps to follow:

1. Open a new Excel worksheet.

2. Copy data in the following table below and paste it in cell A1

Note: For formulas to show results, select them, press F2 key on your keyboard and then press Enter.

You can adjust the column widths to see all the data, if need be.

Date | ||

3/9/2012 | ||

Formula | Description | Result |

=ISOWEEKNUM(A2) | Number of the week in the year that 3/9/2012 occurs, based on weeks beginning on the default, Monday (10). | 10 |