очередной калькулятор
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<label for=""><input type="text">Ширина</label>
<label for=""><input type="text">Высота</label>
<select>
<option value="0" selected>-</option>
<option value="150">Да</option>
</select>
<span>0</span>
<script>
var items = document.querySelectorAll("input,select"),
span = document.querySelector("span");
[].forEach.call(items, function(a) {
a.addEventListener("change", function() {
span.innerHTML = [].reduce.call(items, function(a, b) {
return a * (+b.value||0)
}, 1)
})
});
</script>
</body>
</html>