a/20267650/1636522

HTML

<table width="400" border="1" cellspacing="0">
  <tr>
    <td colspan="2" align="center">
    SUM: 
    <input type="text" name="sum" id="sum" /></td>
  </tr>
  <tr>
    <td><input type="text" name="textfield" id="textfield" /></td>
    <td><input type="text" name="textfield" id="textfield" /></td>
  </tr>
  <tr>
    <td><input type="text" name="textfield" id="textfield" /></td>
    <td><input type="text" name="textfield" id="textfield" /></td>
  </tr>
</table>

JavaScript

var inputs = $('input:not(#sum)');
inputs.keyup(function () {
    var sum = 0;
    inputs.each(function () {
        sum += parseFloat($(this).val()) || 0;
    });
    $('#sum').val(sum);
});