Автор: http://www.swissdelphicenter.ch

function BaseConvert(NumIn: string; BaseIn: Byte; BaseOut: Byte): string;
var
  i: integer;
  currentCharacter: char;
  CharacterValue: Integer;
  PlaceValue: Integer;
  RunningTotal: Double;
  Remainder: Double;
  BaseOutDouble: Double;
  NumInCaps: string;
  s: string;
begin
  if (NumIn = '') or (BaseIn < 2) or (BaseIn > 36) or (BaseOut < 1) or (BaseOut
    > 36) then
  begin
    Result := 'Error';
    Exit;
  end;

  NumInCaps := UpperCase(NumIn);
  PlaceValue := Length(NumInCaps);
  RunningTotal := 0;

  for i := 1 to Length(NumInCaps) do
  begin
    PlaceValue := PlaceValue - 1;
    CurrentCharacter := NumInCaps[i];
    CharacterValue := 0;
    if (Ord(CurrentCharacter) > 64) and (Ord(CurrentCharacter) < 91) then
      CharacterValue := Ord(CurrentCharacter) - 55;

    if CharacterValue = 0 then
      if (Ord(CurrentCharacter) < 48) or (Ord(CurrentCharacter) > 57) then
      begin
        BaseConvert := 'Error';
        Exit;
      end
      else
        CharacterValue := Ord(CurrentCharacter);

    if (CharacterValue < 0) or (CharacterValue > BaseIn - 1) then
    begin
      BaseConvert := 'Error';
      Exit;
    end;
    RunningTotal := RunningTotal + CharacterValue * (Power(BaseIn, PlaceValue));
  end;

  while RunningTotal > 0 do
  begin
    BaseOutDouble := BaseOut;
    Remainder := RunningTotal - (int(RunningTotal / BaseOutDouble) *
      BaseOutDouble);
    RunningTotal := (RunningTotal - Remainder) / BaseOut;

    if Remainder >= 10 then
      CurrentCharacter := Chr(Trunc(Remainder + 55))
    else
    begin
      s := IntToStr(trunc(remainder));
      CurrentCharacter := s[Length(s)];
    end;
    Result := CurrentCharacter + Result;
  end;
end;

// Example, Beispiel

procedure TForm1.Button1Click(Sender: TObject);
begin
  BaseConvert('FFFF', 16, 10);
  // returns, ergibt '65535'.
end;