Scale control for CartoDB maps (using Leaflet).

<!DOCTYPE html>
<html>
  <head>
    <title>Easy example | CartoDB.js</title>
    <meta name="viewport" content="initial-scale=1.0, user-scalable=no" />
    <meta http-equiv="content-type" content="text/html; charset=UTF-8"/>
    <link rel="shortcut icon" href="https://cartodb.com/assets/favicon.ico" />
    <style>
      html, body, #map {
        height: 100%;
        padding: 0;
        margin: 0;
      }
      .list {
        position: absolute;
        background: white;
        top: 180px;
        left: 20px;
        padding: 20px;
        z-index: 10;
      }
    </style>
 
    <link rel="stylesheet" href="https://libs.cartocdn.com/cartodb.js/v3/themes/css/cartodb.css" />
    
  </head>
  <body>
    <div id="map"></div>
 
    
    <script src="https://libs.cartocdn.com/cartodb.js/v3/cartodb.uncompressed.js"></script>
 
    <script>
 
      function main() {
 
        cartodb.createVis('map', 'https://documentation.cartodb.com/api/v2/viz/7eb2096a-51d9-11e3-89a7-5404a6a683d5/viz.json', {
            shareable: false,
            title: false,
            description: false,
            search: false,
            tiles_loader: true,
            center_lat: 42,
            center_lon: -3,
            zoom: 6,
            legends: false
        })
        .done(function(vis, layers) {
          var map = vis.getNativeMap();
          L.control.scale({ position: 'topright' }).addTo(map);
 
        })
        .error(function(err) {
          console.log(err);
        });
      }
 
      window.onload = main;
    </script>
  </body>
</html>

Modified http://libs.cartocdn.com/cartodb.js/v3/cartodb.uncompressed.js to a secure url

https://libs.cartocdn.com/cartodb.js/v3/cartodb.uncompressed.js