jQuery

<script type="text/javascript">function hasValue(variable){if(typeof(variable)=="undefined"){return false;}if(variable==''){return false;}if(variable==null){return false;}return true;}$(document).ready(function(){$('#pixels2ems-calc').click(function(){var pixels=$('#in-pixels').val();if(hasValue(pixels)){$('#out-emswidth').removeAttr('disabled').val('width: '+Math.round((pixels/13)*1000)/1000
+'em;\n*width: '+Math.round((pixels/13.3333)*1000)/1000
+'em;');}});});</script>

<form id="pixels2ems">
<div class="form-item">
  <label>Pixels:</label>
  <input id="in-pixels" type="text" size="4" maxlength="20" class="text-input" />px
</div>
<hr />
<div class="form-item">
  <label>CSS width in Ems:</label><br />
  <textarea id="out-emswidth" disabled="disabled" rows="2" cols="30"></textarea>
</div>
<div class="form-items form-buttons">
  <button type="button" id="pixels2ems-calc" value="Calculate">Calculate</button></div>
</form>