# Excel WEEKNUM function for Power Query

If you are looking for the Excel equivalent of the WEEKNUM function for Power Query this article is for you.

As this is also a function that has many regional options, I was lucky to find an algorithm that I could use for its main part here: M functions to convert between ISO 8601 Week & Year ⇄ dates (e.g., `2014-12-29``"2015-W01-1"`) (github.com)

### Excel WEEKNUM function for Power Query

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 let func = (Date as date, optional Return_type as number) => let // For a detailled description about the options of the Return_types see the official documentation: // https://support.microsoft.com/en-us/office/weeknum-function-e5c43a03-b4ab-426c-b411-b18c13c75340 // PQ native Date.WeekFromYear starts to count from 0(Sunday) to 6(Saturday) as opposed to Excel from 1(Sunday) to 7(Saturday) ConvertedNumber = if Return_type = null then 0 else Record.Field( [1 = 0, 2 = 1, 11 = 1, 12 = 2, 13 = 3, 14 = 4, 15 = 5, 16 = 6, 17 = 0, 21 = 21], Text.From(Return_type) ), IsoWeek = // this function comes from r-k-b on Github: https://gist.github.com/r-k-b/18d898e5eed786c9240e3804b167a5ca let getDayOfWeek = (d as date) => let result = 1 + Date.DayOfWeek(d, Day.Monday) in result, getNaiveWeek = (inDate as date) => let // monday = 1, sunday = 7 weekday = getDayOfWeek(inDate), weekdayOfJan4th = getDayOfWeek(#date(Date.Year(inDate), 1, 4)), ordinal = Date.DayOfYear(inDate), naiveWeek = Number.RoundDown((ordinal – weekday + 10) / 7) in naiveWeek, thisYear = Date.Year(Date), priorYear = thisYear – 1, nwn = getNaiveWeek(Date), lastWeekOfPriorYear = getNaiveWeek(#date(priorYear, 12, 28)), // http://stackoverflow.com/a/34092382/2014893 lastWeekOfThisYear = getNaiveWeek(#date(thisYear, 12, 28)), weekYear = if nwn < 1 then priorYear else if nwn > lastWeekOfThisYear then thisYear + 1 else thisYear, weekNumber = if nwn < 1 then lastWeekOfPriorYear else if nwn > lastWeekOfThisYear then 1 else nwn in Number.RoundDown(weekNumber), Default = Date.WeekOfYear(Date, ConvertedNumber), Result = if Return_type = 21 then IsoWeek else Default in Result , documentation = [ Documentation.Name = " Xls.WEEKNUM.pq ", Documentation.Description = " Returns the week number of a specific date. For example, the week containing January 1 is the first week of the year, and is numbered week 1. Equivalent of the YEARFRAC-Function in Excel. ", Documentation.LongDescription = " Returns the week number of a specific date. For example, the week containing January 1 is the first week of the year, and is numbered week 1. There are two systems used for this function: System 1 The week containing January 1 is the first week of the year, and is numbered week 1. System 2 The week containing the first Thursday of the year is the first week of the year, and is numbered as week 1. This system is the methodology specified in ISO 8601, which is commonly known as the European week numbering system. Equivalent of the YEARFRAC-Function in Excel. ", Documentation.Category = " Xls.Date ", Documentation.Source = " www.TheBIcountant.com – https://wp.me/p6lgsG-2ta . ", Documentation.Version = " 1.0 ", Documentation.Author = " Imke Feldmann ", Documentation.Examples = {[Description = " ", Code = " let Serial_number = #date(2012, 3, 9) , Return_type = 2, FunctionCall = Xls_WEEKNUM(Serial_number, Return_type) in FunctionCall ", Result = " 11 "]}] in Value.ReplaceType(func, Value.ReplaceMetadata(Value.Type(func), documentation))

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Xls.WEEKNUM.pq

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### Function parameters

The function takes the same parameters as its Excel equivalent. So this is just a copy for your convenient reference:

WEEKNUM(serial_number,[return_type])

The WEEKNUM function syntax has the following arguments:

• Serial_number     Required. A date within the week. Dates should be entered by using the DATE function, or as results of other formulas or functions. For example, use DATE(2008,5,23) for the 23rd day of May, 2008. Problems can occur if dates are entered as text.
• Return_type     Optional. A number that determines on which day the week begins. The default is 1.
Return_type Week begins on System
1 or omitted Sunday 1
2 Monday 1
11 Monday 1
12 Tuesday 1
13 Wednesday 1
14 Thursday 1
15 Friday 1
16 Saturday 1
17 Sunday 1
21 Monday 2

If you are interested in more Power Query functions that replicate Excel functions who haven’t made it into the M-language (yet?) please check out this collection.

Enjoy and stay queryious 😉

### Comments (2) Write a comment

1. Hello
I want to share with you my own ISO week number function

(d as date) =>
let
`Week_Year =Date.Year(Week_Thursday),`
``` Year_First_Thursday = Date.AddDays(#date(Week_Year,1,7),-Date.DayOfWeek(#date(Week_Year,1,1),Day.Friday)), ```
```Week_Num = Duration.Days(Week_Thursday - Year_First_Thursday)/7+1 ```