function MemoLines(const m: TMemo): Integer; 
var 
  dc: THandle; 
  tm: TTextMetric; 
begin 
  dc := GetDC(m.handle); 
  SelectObject(dc, m.Font.handle); 
  GetTextMetrics(dc, tm); 
  Result := (m.ClientRect.Bottom - m.ClientRect.Top) div 
    (tm.tmHeight + tm.tmExternalLeading); 
  DeleteDC(dc); 
end; 
 
// Beispielaufruf 
 
procedure TForm1.Button1Click(Sender: TObject); 
begin 
  showmessage(inttostr(MemoLines(Memo1))); 
end;