Conversor de unidades informaticas

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

<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+" es igual a:\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>