The following subroutine works fine in a blank sheet but
only the first line works in my calculation sheet in the
same workbook. Any ideas please?

Sub ResetDefaults()

Cells(12, 4).Formula = "=20"
Cells(15, 4).Formula = "=250"
Cells(21, 4).Formula = "=PI()*D20^2/4"
Cells(16, 12).Formula = "=D13/D14"
Cells(17, 12).Formula = "=D13/D21"

End Sub