One way to make automatic calculations at client side is to use serializeEditData.
Note that we post thea ready updated data to theserver

<?php 
require_once '../../../../php/demo/tabs.php';
?>
<!DOCTYPE html>
<html>
  <head>
    <title>jqGrid PHP Demo</title>
    <link rel="stylesheet" type="text/css" media="screen" href="../../../../css/jquery-ui.css" />
    <link rel="stylesheet" type="text/css" media="screen" href="../../../../css/trirand/ui.jqgrid.css" />
    <link rel="stylesheet" type="text/css" media="screen" href="../../../../css/ui.multiselect.css" />
    <style type="text">
        html, body {
        margin: 0;            /* Remove body margin/padding */
        padding: 0;
        overflow: hidden;    /* Remove scroll bars on browser window */
        font-size: 75%;
        }
        
    </style>
    <script src="../../../../js/jquery.min.js" type="text/javascript"></script>
    <script src="../../../../js/trirand/i18n/grid.locale-en.js" type="text/javascript"></script>
         <script src="../../../../js/trirand/jquery.jqGrid.min.js" type="text/javascript"></script> <script type="text/javascript">         
    $.jgrid.no_legacy_api = true;
    $.jgrid.useJSON = true;
    $.jgrid.defaults.width = "700";
    </script>
     
    <script src="../../../../js/jquery-ui.min.js" type="text/javascript"></script>
  </head>
  <body>
      <div>
          <?php include ("grid.php");?>
      </div>
      <br/>
      <?php tabs(array("grid.php"));?>
   </body>
</html>
grid.php.
<?php
require_once '../../jq-config.php';
// include the jqGrid Class
require_once ABSPATH."php/PHPSuito/jqGrid.php";
// include the driver class
require_once ABSPATH."php/PHPSuito/DBdrivers/jqGridPdo.php";
// Connection to the server
$conn = new PDO(DB_DSN,DB_USER,DB_PASSWORD);
// Tell the db that we use utf-8
$conn->query("SET NAMES utf8");

// Create the jqGrid instance
$grid = new jqGridRender($conn);
// Write the SQL Query
$grid->SelectCommand = 'SELECT account_id, name, debit, credit, balance from accounts';
// Set the table to where you update the data
$grid->table = 'accounts';
// Set output format to json
$grid->dataType = 'json';
$grid->setPrimaryKeyId('account_id');
// Let the grid create the model
$grid->setColModel();
// Set the url from where we obtain the data
$grid->setUrl('grid.php');
// Set some grid options
$grid->setGridOptions(array(
    "rowNum"=>10,
    "rowList"=>array(10,20,30),
    "sortname"=>"account_id"
));
$grid->setColProperty('account_id', array("editoptions"=>array("readonly"=>true)));
$grid->setColProperty('balance', array("editable"=>false));
// Enable navigator
$grid->navigator = true;
// Enable only editing
$grid->setNavOptions('navigator', array("excel"=>false,"add"=>false,"edit"=>true,"del"=>false,"view"=>false, "search"=>false));
// Close the dialog after editing
$grid->setNavOptions('edit',array("closeAfterEdit"=>true,"editCaption"=>"Update Account","bSubmit"=>"Update"));
// calculate the balance field
$calcs = <<< CALCS
function(postdata) {
    postdata['balance'] = parseFloat(postdata['debit']) - parseFloat(postdata['credit']);
    return postdata;
}
CALCS;
$grid->setNavEvent("edit", "serializeEditData", $calcs);
// Enjoy
$grid->renderGrid('#grid','#pager',true, null, null, true,true);
$conn = null;
?>