<html>
  <head>
    <style>
      #rectangle { 
        width: calc(100%/3 - 20px/3);
        height: calc(30%/3 + 10px/3);
        background: green;
      }
    </style>
  </head>
  <body>
  	<div id="rectangle"></div>
  </body>
</html>