pulling information from multiple calculated fields into a sum to display in a single field

Question

I am trying to get this calculated field to pull from multiple forms (15 to be exact) and display a grand total. As of now I can't get anything to show up in the field box. Any help is greatly appreciated!

<script type="text/javascript">
function update()
{
  var form = document.forms[0];
  var sum = eval(form.sum.value) + eval(form.sum.value);
  form.sum.value = isNaN(sum,'all') ? "" : sum;
}
</script>

<form name="form0" onsubmit="false">
<fieldset>
<legend>Pledge Total (total before tax & shipping)</legend>
<label for="f1">Total ($)</label>
<input id="f2" name="sum" readonly="readonly">
</fieldset>
</form>