You will need to use the Windows API function GetDeviceCaps() to retrieve the "logical" number of pixels per inch for the display in both the X and y directions, and multiply your coordinates by the returned values. Screen measurements are based on a concept of "Logical Pixels" and may not always reflect a true measurement. Printers should give satisfactory results when measurement are compared against a ruler. When printing, delete the lines for GetDc() and ReleaseDc() and simply pass Printer.Canvas.Handle to the GetDeviceCaps() function. Example:

procedure TForm .Button Click(Sender: TObject);
var
  dc : hdc;
  InchX : integer;
  InchY : integer;
begin
  dc := GetDc(0);
  InchX := GetDeviceCaps(dc, LOGPIXELSX);
  InchY := GetDeviceCaps(dc, LOGPIXELSY);
  ReleaseDC(0, dc);
  Form .Canvas.Rectangle(Round(0 * InchX),
                         Round(0 * InchY),
                         Round( .5 * InchX),
                         Round( .5 * InchY));
end;