function calculate(){

var width = Number(document.forms[0].width.value);
var length = Number(document.forms[0].length.value);
var rate = Number(document.forms[0].rate.value);

var area = width * length
document.forms[0].area.value = width*length;
var cost = area * rate
document.forms[0].cost.value = area*rate;
}

<!--

var message="Sorry this image is not to be downloaded";

function click(e) {
if (document.all) {
if (event.button == 2) {
alert(message);
return false;
}
}
if (document.layers) {
if (e.which == 3) {
alert(message);
return false;
}
}
}
if (document.layers) {
document.captureEvents(Event.MOUSEDOWN);
}
document.onmousedown=click;
// -->