ISOWEEKNUM: Excel Formulas Explained

Excel users know there are countless formulas that allow you to manipulate data at your fingertips. From SUM to CONCATENATE, you can easily customize your spreadsheets to get the results you want. However, one formula that doesn't get enough attention is ISOWEEKNUM. This formula may sound intimidating, but trust me, it's a handy tool that can help you in many situations.


ISOWEEKNUM is an Excel formula that helps you determine the week number based on the ISO week numbering system. The ISO week numbering system varies from the standard Gregorian calendar as it assigns the year's first week to the week containing the first Thursday.

When using the ISOWEEKNUM formula, you can easily calculate the week number for any date, taking into account the ISO week numbering system.


ISOWEEKNUM is helpful in many situations where traditional week numbering can cause confusion. For example, if you're working on an international project or having meetings across different time zones, ISOWEEKNUM assigns a consistent calendar week number making the scheduling process less complicated.

This formula is also useful for analyzing data. If you want to compare sales data from different years, you can just use the ISOWEEKNUM formula to ensure you're comparing the same weeks. By assigning consistent week numbers to your data, you can avoid any discrepancies that might arise if traditional week numbering is used.

How to use ISOWEEKNUM?

Using the ISOWEEKNUM formula is easy. Start by selecting the cell where you want the formula's result to appear, and then follow these steps:

  1. Begin the formula with "=ISOWEEKNUM("
  2. Use a cell reference or a date enclosed within quotation marks to represent the date you want to calculate.
  3. End the formula with a right parenthesis.
  4. Press enter to calculate the week number.

For example, suppose cell A2 contains the date 01/25/2022. To calculate its corresponding week number, enter "=ISOWEEKNUM(A2)" in your chosen cell, and the result will be "4" since week 4 of 2022 started on the first Thursday of that year.

ISOWEEKNUM and Conditional Formatting

Conditional formatting is a great tool that helps you visualize trends and patterns in your data. With ISOWEEKNUM, you can easily use conditional formatting to highlight specific weeks and analyze your data.

For instance, let's say you're working with a sales sheet that contains data from the beginning of the year. You want to compare sales figures for weeks 3 and 4 in January. Simply apply conditional formatting to those two weeks, and you'll be able to see their data side by side.

To use conditional formatting with the ISOWEEKNUM formula:

  1. Select the range of cells you want to format.
  2. Click the "Conditional Formatting" button on the Home tab.
  3. Select "New Rule" from the drop-down menu.
  4. Choose "Use a formula to determine which cells to format."
  5. Type the formula "=ISOWEEKNUM(A1)=4" (if you want to highlight the cells for week 4, for example.)
  6. Select the formatting style you want to apply to the cells (e.g., fill color, font color, etc.)
  7. Click "OK."

Voila! Your data for your chosen week(s) is now highlighted.

Final Thoughts

Understanding ISOWEEKNUM may seem complicated, but it's a useful tool that simplifies the process of calculating weeks. When working with dates and analyzing data, using the ISO week numbering system can make things much easier. Try using the ISOWEEKNUM formula in your spreadsheets and see how it can help you streamline your work.

Integrating ISOWEEKNUM is a valuable tool that can help you in so many different ways. From scheduling international conferences to analyzing sales data, the applications for this formula are vast. So, the next time you want to compare data from different weeks or take into account ISO week numbering, be sure to incorporate ISOWEEKNUM into your Excel sheet. Your team will thank you for it!

By clicking “Accept”, you agree to the storing of cookies on your device to enhance site navigation, analyze site usage, and assist in our marketing efforts. View our Privacy Policy for more information.