How to autofill formula for known number of columns but variable number of rows in excel macro - vba

Question

I writing a macro within which I need to autofill some rows with formulas, across multiple columns.
The number of columns is fixed, but each time the macro runs, the number of rows is variable. I use the "record macro" function and the current macro only ever fills my rows to row 16. Below is the code:
Range("D3:P3").Select
Selection.AutoFill Destination:=Range("D3:P16")
I obviously need to change the "P16" to something dynamic.
I have tried to use the following:
Dim LR As Long
LR = Range("D3:P3" & Rows.Count).End(xlUp).Row
Range("B3:P3").AutoFill Destination:=Range("B3:P" & LR)
I am unsure whether the "Dim LR as Long" has to be placed at the very beginning of my macro - or can it just be placed anywhere?
I am getting an error anyway with what i attempted above giving me an "autofill selectio error" (sorry i cant remember the exact error message.
Would someone be able to point me in the right direction?

Answer 1

LR can be declared anywhere before where you first use it, but it's best to do it at the beginning. Your range for LR is incorrect.
LR = Range("D3:P3" & Rows.Count).End(xlUp).Row
Should be
LR = Range("D3:P3").End(xlUp).Row
You should use xlDown if you are trying to find the end of a range BELOW D3:P3
LR = Range("D3:P3").End(xlDown).Row
Would give you the last row with data in all columns D:P in it below D3:P3

Answer 2

I think you're looking for this:
LR = Range("D3:P" & Rows.Count).End(xlUp).Row
but note that this finds the last row with any content in Column D - if there are later rows with content in Cols E-P but not in Col D then those rows will be ignored.

Answer 3

So I used the information provided to me and managed to get the following:
Dim LR As Long
LR = Range("C3:P" & Rows.Count).End(xlDown).Row
Range("D3:P3").AutoFill Destination:=Range("D3:P" & LR)
ActiveSheet.ListObjects.Add(xlSrcRange, Range("$D$2:P" & LR), , xlYes).Name = _
"Table10"
This allowed me to count the number of rows that had already been populated in column "C", and then take the formulas that already existed in cells D3:P3 and autofill them down through the range until the last populated row of column C.
I then used that structure to make the whole range a table, in this case named "Table10".
Great stuff guys - your help allowed me to get exactly what I wanted. Thanks