Use CountIf To Use As Variable To Determine How Far To Paste Down In VBA

I would like to be able to count the number of rows with data and use it in a macro. For instance, Columns A:C and Rows 1:50 or more have data (the number of rows are variable). In Cell D1 I have a simple formula

Code:
=A1&","&B1&","&C1

I am using a formula in VBA like:

Code:
Range("D2").Select
Selection.Copy
Range("D3:D10000").Select
ActiveSheet.Paste

I don't want to have an arbitrary number like D10000 in the range because it will paste in cells even if there is nothing there resulting in a ,, in the cell. I know this is avoidable by wrapping in a

Code:
=IF(ISBLANK(A1)=TRUE,"",D1)

but the "" is not truly blank and jacks up the sorting.

If there is a better way by setting the row count to a variable like

Code:
countx=COUNTIF(A:A)

and then using it in the selection such as:

Code:
Range("D2").Select
Selection.Copy
Range("D3:countx").Select
ActiveSheet.Paste

Am I going out about this wrong, maybe missing something fundamental about this?