Formula for week number in excel iso#
The ISO system is commonly known as the European week numbering system, and since week numbers are used a lot in Europe, the old and confusing WEEKNUM function, where you have to choose return type 21, has caused countless errors in a large number of companies and governments across Europe. – Return type 21 was introduced in Excel 2010, and this is the official ISO Week Date System where the week starts on Monday and the week containing the first Thursday of the year is considered week 1. With option 1, the week begins on a Sunday, with option 2 the week begins on Monday, option 11 is Tuesday, 12 is Wednesday, etc.
– The return types 1, 2, 11, 12, 13, 14, 15, 16 and 17 are all based on the rule that the week of January 1st is week 1.
The old WEEKNUM function has 8 options for return type in its second argument: The week numbers on the picture above are calculated according to the official ISO standard, where the week starts on a Monday and the week containing the first Thursday of the year is considered week 1. This is the default definition if you use the WEEKNUM function in Excel In that case, week number 1 would only have one day in 2022, because January 1st is a Saturday, and the week number changes on Sunday, January 2nd.
According to one definition, January 1st would always be in Week number 1, and then the week number changes every Sunday. There are different ways to define the week number of a specific date.
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