# How to Use ISOWEEKMUM Function in Excel – Examples

Excel includes a plethora of built-in functions that are intended to simplify data analysis and manipulation. The **ISOWEEKNUM** function in Excel is one example. This function computes the **ISO **week number for a given date.Â

In this article, we’ll go over the **ISOWEEKNUM** function in Excel in-depth and show you how to use it.

## Syntax of ISOWEEKNUM Function

`=ISOWEEKNUM(date)`

## The argument of ISOWEEKNUM Function

**date**– the date for which you want to calculate the ISO week number. This argument can be typed directly into the function or as a date value cell reference.

## Return of the ISOWEEKNUM Function

The **ISOWEEKNUM **function returns a number between **1** and **53**, representing the year’s week number. The ISO week numbering system starts on **Mondays**.

**Example #1: Simple application of ISOWEEKNUM Function in Excel**

Suppose you are given some dates and want to return the serial number of the week for each date in a particular year.

Apply the following formula and AutoFill for the remaining cells

`=ISOWEEKNUM(B2)`

The result is

**Example #2: Get the serial number of the week for the current date**

Let’s merge the two functions like this:

`=ISOWEEKNUM(TODAY())`

Now, you’re instructing Excel to first calculate the current date using the **TODAY() **function, and then determine the ISO week number for that date using the **ISOWEEKNUM** function. The ISO week number for the current date is returned as the output.

In Excel, the **TODAY()** function returns the current date as a serial number. For example, on **March 10, 2023**, the **TODAY()** function would return the serial number **44858**, which represents the date March 10, 2023.

The **ISOWEEKNUM **function would then determine that March 10, 2023, corresponds to ISO week number 10 of the year.

**Formula Evaluation**

```
=ISOWEEKNUM(TODAY())
=ISOWEEKNUM(44858)
=10
```

Similarly, you can count the remaining week of any year using the formula below

`=52 - ISOWEEKNUM(TODAY())`

As previously discussed, if today’s date is March 10, 2023, the ISO week number for that date is 10. As a result, the formula’s output would be 52 – 10 = 42, which means that there are 42 weeks left in the year.

**Example #3: ISOWEEKNUM Function with other Function**

Using the following formula, you can count the number of weeks with the same serial number in a given year from a data set.

`=SUMPRODUCT(--(ISOWEEKNUM(B3:B7)=F4))`

**Explanation:**

- This formula combines three parts:
**ISOWEEKNUM, SUMPRODUCT,**and the unary**double operator (–)**. - The
**ISOWEEKNUM**function is being used to calculate the ISO week numbers for a range of dates in cells**B3**through**B7**. - If
**ISOWEEKNUM(B3:B7)=F4**the condition is satisfied, and the formula returns a**1**; otherwise, the formula returns a**0**. - The
**SUMPRODUCT**multiplies respective elements in the defined arrays (in this case, an array of Boolean values) and needs to return the sum of those products. - The
**double unary operator (–)**, also known as the double negative, is used to convert the comparison operator’s Boolean values to numeric values. This is required because**SUMPRODUCT**only works with numbers.

**Formula Evaluation:**

```
=SUMPRODUCT(--(ISOWEEKNUM(B3:B7)=F4))
=SUMPRODUCT(--({2;5;9;13;13}=F4))
=SUMPRODUCT(--{FALSE;FALSE;FALSE;TRUE;TRUE})
=2
```

**Things to Keep in Mind about ISOWEEKNUM Function**

- The
**ISOWEEKNUM**function is only applicable to dates, not times. To calculate the ISO week number for a date and time value, first, extract the date portion of the value using a formula such as**=INT(date value).** - The function will return a
**#VALUE!**error if the date is not recognized as a valid Excel serial number or text string. - Based on the Gregorian calendar, the ISO week counting system has
**52**or**53**weeks in a year.

**ISOWEEKNUM vs WEEKNUM** Function

- The
**ISOWEEKNUM**function uses the ISO 8601 week numbering standard. The week containing January 4th is the first week of the year, according to this standard, and a week always begins on a Monday and ends on a Sunday. The**WEEKNUM**function, on the other hand, uses your computer’s system settings to determine the first week of the year. - The
**ISOWEEKNUM**function returns week numbers between**1**and**53**. This is because some years have**53**weeks instead of**52**due to an extra day at the start or end of the year. The**WEEKNUM**function, on the other hand, returns week numbers ranging from 1 to 54, depending on the first day of the year. - The
**ISOWEEKNUM**function works only in Excel 2013 and later versions, whereas the**WEEKNUM**function is available in earlier Excel versions.