how to center pop up window on scereen?

Good day to every one
im trying to centre the pop up window and not sure how to make it.
below is the script

<script language="javascript">
function target_popup(form) {
    window.open('', 'formpopup', 'width=900,height=400, resizeable,scrollbars,');
    form.target = 'formpopup';
}
</script>

HTML:

 <form id="theform" name="order" method="post" action="confirm.php" onsubmit="target_popup(this)" enctype="multipart/form-data" style="position:absolute; left:0px; top:0px">

Code (markup):

Thank you

 

is my question celar? or difficult

Please if any one can help.
Thank you

 

why don't u use modal box?

 

You will need the width & height of the window:

function popupwindow(url, title, w, h) {
  var left = (screen.width/2)-(w/2);
  var top = (screen.height/2)-(h/2);
  return window.open(url, title, 'toolbar=no, location=no, directories=no, status=no, menubar=no, scrollbars=no, resizable=no, copyhistory=no, width='+w+', height='+h+', top='+top+', left='+left);
}

Code (markup):

Source