When constructing add to cart forms, it can be nice to only allow customers to enter whole numbers into a quantity field. To do this, simply paste the following script right before the closing </head> tag of your page templates, adding a class of atc_number to any input you want to validate to a whole number.

<script type="text/javascript">
jQuery(document).ready(function() {
  jQuery("input.atc_number").change(function() { 
    var n = parseInt(jQuery(this).val()); 
    var i = (n===+n && n===(n|0)) ? n : 0;
    jQuery(this).val(i);
  });
});
</script>

http://forum.foxycart.com/comments.php?DiscussionID=5120&page=1#Item_3