In this article, you will learn to calculate week numbers in Excel quickly and easily. However, it's crucial to know that there is a difference between North America and other countries when calculating week numbers to avoid mistakes.
Why there are 2 WEEKNUM functions in Excel
- In Europe, week numbers are calculated according to the ISO 8601 standard. This standard considers that the first week of the year must have at least four days. Therefore, if January 1 falls on Friday, Saturday, or Sunday, those days are counted as part of the previous year’s final week.
- However, North America counts week 1 from January 1, even if that day is a Sunday. This explains why there can be a one-week difference.
In the following picture, you can see this difference in action.
- With the WEEKNUM function, the 1st January is always the week #1
- The ISOWEEKNUM function takes into consideration the day of the week. If it's Friday, Saturday or Sunday, the function will return the last week number of the previous year.
So, depending on the region of the world you are in, the formula applied to calculate the day based on the week number will not be the same.
Formula to calculate the week number (WEEKNUM method)
The formula for returning the first Monday of a chosen week following the USA and Canadian rules is this one.
=DATE(Year,1,1)+(Week-1)*7-WEEKDAY(DATE(Year,1,1),2)+1
CAUTION! Week #1 may not have a Monday with this formula. If that happens, add an IF statement to handle the formula. If the year is not the chosen year, leave the result blank. If your Excel version has the LET function, you can write your formula like this
If the week starts on a Sunday, remove the +1 at the end of the formula
=DATE(Year,1,1)+(Week-1)*7-WEEKNUM(DATE(Year,1,1),2)
Formula to calculate the week number (ISOWEEKNUM method)
If you want to calculate Monday according to the ISO week number rules, the formula is:
=DATE(Year,1,1)+IF(WEEKDAY(DATE(Year,1,1),2)<5,(WeekNum-1)*7,WeekNum*7)-WEEKDAY(DATE(Year,1,1),2)+1
This formula correctly shifts the first week into January when January 1 falls on Friday, Saturday, or Sunday.
