Service tax calculation using javascript

  
  1)  SERVICE TAX REVERSE CALCULATION
  
   function Premiumcalc()
{
    var premium= document.Form1.premium.value;
    var stax= document.Form1.stax.value;
    var netprm= document.Form1.netprm.value;
    var currstax= document.Form1.currstax.value;
    var staxval="";
    var stvalue=100;

    if((netprm!="") && (!isPhone(netprm))){
          //document.Form1.premium.focus();
            document.Form1.premium.value="";
            document.Form1.netprm.value="";
            alert("Please enter valid Net Premium");
            return false;
        }
    else
    {
       
        staxval=eval(stvalue)+eval(currstax);
        //document.Form1.netprm.value=parseInt(premium)+(stax);
        document.Form1.premium.value=Math.round((netprm/staxval)*100);
        document.Form1.stax.value=Math.round(document.Form1.premium.value*(currstax/100));
        //document.Form1.premium.value=Math.round(eval(netprm)-eval(stax));
        document.Form1.premium.readOnly=true;
    }
   
}

FORMULA:

staxval=100+CURRENT SERVICE TAX %
Premium=(netpremium/staxval)*100
SERVICETAX VALUE=Premium*((CURRENT SERVICE TAX %) /100);



2) SERVICE TAX CALCULATION

function netPremiumcalc()
{
    var premium= document.Form1.premium.value;
    var stax= document.Form1.stax.value;
    var netprm= document.Form1.netprm.value;
    var currstax= document.Form1.currstax.value;
    var staxval="";

    if((premium!="") && (!isPhone(premium))){
         // document.Form1.premium.focus();
          document.Form1.netprm.value="";
          document.Form1.premium.value="";
            alert("Please enter valid Premium");
            return false;
        }
    else
    {
        staxval=premium*(currstax/100);
        //alert(staxval);
        document.Form1.stax.value=Math.round(staxval);
        document.Form1.netprm.value=Math.round(eval(premium)+eval(staxval));
        //document.Form1.netprm.value=parseInt(premium)+(stax);
        //document.Form1.netprm.value=Math.round(eval(premium)+eval(stax));
        document.Form1.netprm.readOnly=true;
    }
   
}


Sample HTML:



<form name="Form1" id="Form1" method="post"    enctype="multipart/form-data" onsubmit="">
<input type="hidden" name="currstax" id="currstax" value="<%=stper%>"/>
        <table width="63%"  align="center" cellpadding="2" cellspacing="2" class="tablebg">
          
         
      <tr class="text_heading">
           <td width="20%"   class="text_heading">Premium:<br>
        <td colspan="6"   align="left">
          <input name="premium" id="premium" type="text" value=""  size="45" onChange="netPremiumcalc();">
        </td>
      </tr>   
      <tr class="text_heading">
           <td width="20%"   class="text_heading">Service Tax @ 12.36 % :<br>
        <td colspan="6"   align="left">
          <input name="stax" id="stax" type="text" value=""  size="45" readonly="readonly">
        </td>
      </tr>   
      <tr class="text_heading">
           <td width="20%"   class="text_heading" >Net Premium <span class="alert"> *</span>:<br>
        <td colspan="6"   align="left">
          <input name="netprm" id="netprm" type="text" value=""  size="45" onChange="Premiumcalc();">
        </td>
      </tr>   
     
    
     
           
      <tr >
      <td colspan="7"   align="center"> <input type="submit" value="Submit" class="button" onClick="return formsubmit();"></td>
      </tr>
     
      <tr >
   
   </table>
     
   </form>