function startCalc(){
  interval = setInterval("calc()",1);
}
function calc(){
  one = document.megrendelesForm.elsomennyiseg.value;
  two = document.megrendelesForm.ertmenny.value; 
  document.megrendelesForm.mennyiseg.value = Math.round(one * two * 1000) /1000;
}
function stopCalc(){
  clearInterval(interval);
}