function calculate() {
	var total = 0;
	for(var elemNo = 0; ; elemNo++) {
		var unitElem = document.getElementById("units[" + elemNo + "]");
		if(unitElem == undefined) {
			var totalElem = document.getElementById("totalbox");
			totalElem.value = total;
			return;
		}
		if(!(isNaN(unitElem.value))) {
			var volumeElem = document.getElementById("volume[" + elemNo + "]");
			total += (volumeElem.value * unitElem.value);
		}
		else {
			unitElem.value = 0;
		}
	}
}

$(function(){
	calculate();
});