Copying data

What is the code for copying data from a cell down to the point where the data in the cell to the left stops.

I have data in cells C12 to C50 I need to write a code to copy the formula i have in cell D12 to D50
The number of rows that has the data will change every time i run the macro so i need to copy the data down as far as required.(i.e not down to row 50 every time)

Thanks
Lee