function ExtendedToString(zahl: extended; tausender: boolean): string; 
var 
  pe, pk, i, k, m, z: integer; 
  ex: boolean; 
  hlp: string; 
begin 
  result := floattostr(zahl); 
  pe := pos('E', result); 
  if pe > 0 then begin 
    ex := copy(result, pe + 1, 1) = '-'; 
    i := strtoint(copy(result, pe + 1 + ord(ex), maxint)); 
    result := copy(result, 1, pe - 1); 
    pk := pos(Decimalseparator, result); 
    if pk = 0 then z := 0 
    else begin 
      delete(result, pk, 1); 
      z := 2; 
    end; 
    if ex then begin 
      result := stringofchar('0', i - pk + z) + result; 
      k := pk - i; 
      if k < 2 then k := 2; 
      insert(Decimalseparator, result, k); 
    end else 
      result := result + stringofchar('0', i + ord(pk > 0) * (pk - pe + 1)); 
  end else begin 
    hlp := FloatToStrF(zahl, ffFixed, 18, 18); 
    if length(hlp) > length(result) then result := hlp; 
    while (pos(Decimalseparator, result) > 0) 
      and ((ansilastchar(result) = '0') 
      or (ansilastchar(result) = Decimalseparator)) 
      do delete(result, length(result), 1); 
  end; 
  if not tausender then exit; 
  pk := pos(',', result); 
  if pk = 0 then i := length(result) - 2 
  else i := pk - 3; 
  m := ord(copy(result, 1, 1) = '-') + 1; 
  while i > m do begin 
    insert(Thousandseparator, result, i); 
    dec(i, 3); 
  end; 
end;

// Beispielaufrufe  
  
procedure TForm1.Button4Click(Sender: TObject); 
var zahl: extended;  
begin  
  zahl := 1.23456789E-12;  
  Label1.caption := Floattostr(zahl);                   // 1,23456789E-12  
  Label2.caption := FloatToStrF(zahl, ffFixed, 18, 18); // 0,000000000001234568  
  Label3.caption := ExtendedToString(zahl, false);      // 0,00000000000123456789  
end;  
  
procedure TForm1.Button5Click(Sender: TObject);  
var zahl: extended;  
begin  
  zahl := 1.23456789E+4; 
  Label1.caption := ExtendedToString(zahl, false);  //  12345,6789  
  Label2.caption := ExtendedToString(zahl, true);   // 12.345,6789  
end;