<form name="bandwidth">     

       <p><input type="text" name="original" size="20" value=1024> <select size="1" name="units">     
         <option value="Bytes">Bytes</option>     
         <option value="Kb">Kb</option>     
         <option value="Mb">Mb</option>     
         <option value="Gb">Gb</option>     
       </select> <input type="button" value="Calculate" name="B1" onClick="calculate()"></p>     
     </form>     

     <p>     

     <script>     

     var bytevalue=0     
     function calculate(){     
     var invalue=document.bandwidth.original.value     
     var selectunit=document.bandwidth.units.options[document.bandwidth.units.selectedIndex].value     
     if (selectunit=="Bytes")     
     bytevalue=invalue     
     else if (selectunit=="Kb")     
     bytevalue=invalue*1024     
     else if (selectunit=="Mb")     
     bytevalue=invalue*1024*1024     
     else if (selectunit=="Gb")     
     bytevalue=invalue*1024*1024*1024     

     alert (invalue+" "+selectunit+" is equal to:\n\n- "+bytevalue+" Bytes\n- "+Math.round(bytevalue/1024)+" Kb\n- "+Math.round(bytevalue/1024/1024)+" Mb\n- "+Math.round(bytevalue/1024/1024/1024)+" Gb\n")     
     }     

     </script>