function calcolo(form)
   {
   if (form.l1.value == "" || form.l2.value == "" || form.h.value == "") {
	   alert("E' necessario inserire valori numerici (es. 3.5) in tutte le caselle rimaste vuote."); }
   else if (isNaN(form.l1.value) || isNaN(form.l2.value) || isNaN(form.h.value) || isNaN(form.kf.value)) {
       alert("E' necessario inserire valori numerici e non lettere."); }
   else {
       form.frig.value=Math.round(eval(form.l1.value+'*'+form.l2.value+'*'+form.h.value+'*'+form.kf.value),0);
       form.btuf.value=Math.round(eval(form.frig.value+'*'+3.965),0);
       form.wf.value=Math.round(eval(form.frig.value+'/'+0.86),0);
       form.cal.value=Math.round(eval(form.l1.value+'*'+form.l2.value+'*'+form.h.value+'*'+form.kc.value),0);
       form.btuc.value=Math.round(eval(form.cal.value+'*'+3.965),0);
       form.wc.value=Math.round(eval(form.cal.value+'/'+0.86),0); }
   }