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HTML 

              
                <!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8" />
    <meta name="viewport" content="width=device-width" />
    <title>Facebook Dimensions</title>
    <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Montserrat:500|Open+Sans:400,600" />
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" />
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" />
    <style>
      body {font-family: 'Open Sans', 'Segoe UI'; padding-top: 75px;}
      .navbar-brand,
      h1, h2, h3, h4, h5, h6 {font-family: 'Montserrat'; font-weight: 500;}
      .dim-form .panel-body input,
      .dim-form .panel-body button {width: 50px; display: inline-block; vertical-align: middle;}
      .dim-form .panel-body button {width: auto; line-height: 1.5; position: relative; top: 1px; margin-left: 20px;}
      h2 {margin: 0 0 15px; font-size: 1.5em;}
    </style>
    <script src="https://code.jquery.com/jquery-2.2.4.js"></script>
    <script src="https://stackpath.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
    <script>
      $(function () {
        // Hide the CSS hidden elements by JavaScript.
        $(".hidden").hide().removeClass("hidden");
        // Reset the layout to initial one.
        var resetLayout = function () {
          $(".dim-need, .dim-cons").fadeOut(function () {
            $(".dim-need .list-group, .dim-cons .list-group").empty();
            $(".dim-form").addClass("col-sm-offset-4");
          });
        };
        // Display the error message and reset the layout if found an error.
        var error = function (msg) {
          if (msg.length !== 0) {
            $(".alert-danger").text(msg).fadeIn();
            resetLayout();
          }
          else
            $(".alert-danger").fadeOut(function () {
              $(this).text("");
            });
        }
        // Calculate the proportions.
        var prop = function (dim) {
          if (!!dim.x) {
            var y = 630 / 1200 * dim.x;
            return (dim.x.toFixed(2) + " &times; " + y.toFixed(2));
          }
          if (!!dim.y) {
            var x = 1200 / 630 * dim.y;
            return (x.toFixed(2) + " &times; " + dim.y.toFixed(2));
          }
        }
        // Display the list of proportional dimensions.
        var display = function (x, y) {
          var need = [];
          var cons = [];
          need.push(prop({x: x}));
          need.push(prop({y: y}));
          need = need.filter(function (value, index, self) { 
            return self.indexOf(value) === index;
          }).filter(function (v) { return v !== undefined });
          $(".dim-need .list-group, .dim-cons .list-group").empty();
          need.forEach(function (v) {
            $(".dim-need .list-group").append('<li class="list-group-item">' + v + "</li>");
          });
          cons.push(prop({x: getRound(x, 1)}));
          cons.push(prop({y: getRound(y, 1)}));
          cons.push(prop({x: getRound(x, 2)}));
          cons.push(prop({y: getRound(y, 2)}));
          cons = cons.filter(function (value, index, self) { 
            return self.indexOf(value) === index && need.indexOf(value) === -1;
          }).filter(function (v) { return v !== undefined });
          if (cons.length)
            cons.forEach(function (v) {
              $(".dim-cons .list-group").append('<li class="list-group-item">' + v + "</li>");
            });
          else
            $(".dim-cons .list-group").append('<li class="list-group-item">Sorry, nothing here.</li>');
        }
        // Get the rounded value.
        var getRound = function (n, p) {
          if (!p)
            p = 1;
          var t = Math.pow(10, p);
          if (n % t === 0)
            return n;
          else
            return parseInt(n / t) * t;
        }
        // Calculate the proportion.
        var calculate = function () {
          var strX = $("#dim-x").val().trim();
          var strY = $("#dim-y").val().trim();
          var origX = parseInt(strX, 10);
          var origY = parseInt(strY, 10);
          if (strX === "" && strY === "") {
            error("You need to provide at least one value!");
            return false;
          }
          if ((strX !== "" && isNaN(origX)) || (strY !== "" && isNaN(origY))) {
            error("Given values should be integers!");
            return false;
          }
          var finX = isNaN(origX) ? null : origX;
          var finY = isNaN(origY) ? null : origY;
          error("");
          $(".dim-form").removeClass("col-sm-offset-4");
          display(finX, finY);
          $(".dim-need, .dim-cons").fadeIn();
          return false;
        };
        // Event listeners.
        $("#dim-x, #dim-y").keyup(function (e) {
          if (e.keyCode === 13)
            calculate();
        });
        $("#btn-c").click(calculate);
      });
    </script>
  </head>
  <body>
    <nav class="navbar navbar-inverse navbar-fixed-top">
      <div class="container-fluid">
        <div class="navbar-header">
          <a class="navbar-brand" href="#">Facebook Dimensions</a>
        </div>
      </div>
    </nav>
    <div class="container-fluid">
      <div class="row">
        <div class="col-sm-12">
          <h2>What is this tool about?</h2>
          <p>This is a quick helper tool for getting the right dimensions for your post's image. Facebook requires the previews to be in proportion of 1200 &times; 630 for a full size share. This tool helps you to set one of the dimension with the other not modified with respect to your image.</p>
          <div class="alert alert-danger text-center hidden"></div>
        </div>
        <div class="col-sm-4 dim-form col-sm-offset-4">
          <div class="panel panel-primary">
            <div class="panel-heading">
              <h3 class="panel-title">Original Image Dimensions</h3>
            </div>
            <div class="panel-body text-center">
              <input type="text" class="form-control input-sm text-center" id="dim-x" placeholder="X" /> &times;
              <input type="text" class="form-control input-sm text-center" id="dim-y" placeholder="Y" />
              <button type="button" class="btn btn-primary btn-sm" id="btn-c">Calculate</button>
            </div>
          </div>
        </div>
        <div class="col-sm-4 dim-need hidden">
          <div class="panel panel-primary">
            <div class="panel-heading">
              <h3 class="panel-title">Dimensions you Need</h3>
            </div>
            <ul class="list-group text-center"></ul>
          </div>
        </div>
        <div class="col-sm-4 dim-cons hidden">
          <div class="panel panel-primary">
            <div class="panel-heading">
              <h3 class="panel-title">Considerable Dimensions</h3>
            </div>
            <ul class="list-group text-center"></ul>
          </div>
        </div>
      </div>
    </div>
  </body>
</html>
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