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HTML 

              
                <div class="box">
    <div class="screen">
      <div class="main-screen" id="output">0</div>
      <div class="sub-screen" id="output2"></div>
    </div>

    <input type="hidden" id="num1">
    <input type="hidden" id="operator">
    <input type="hidden" id="num2">
    <input type="hidden" id="temp">

    <!--a class="github" href="https://github.com/greyli/calculator" target="_blank"><img style="position: absolute; top: 0; right: 0; border: 0;" src="image/banner.png" alt="Fork me on GitHub"></a-->

    <div class="container-fluid">
      <div class="buttons">
        <button class="btn-clear btn btn-danger" id="clearButton">AC</button>
        <button class="btn btn-warning" id="deleteButton">CE</button>
        <button class="btn-operate btn btn-default" value="/">/</button>
        <button class="btn-operate btn btn-default" value="*">x</button>
      </div>

      <div class="buttons">
        <button class="nums btn btn-default" value="7">7</button>
        <button class="nums btn btn-default" value="8">8</button>
        <button class="nums btn btn-default" value="9">9</button>
        <button class="btn-operate btn btn-default" value="-">-</button>
      </div>

      <div class="buttons">
        <button class="nums btn btn-default" value="4">4</button>
        <button class="nums btn btn-default" value="5">5</button>
        <button class="nums btn btn-default" value="6">6</button>
        <button class="btn-operate btn btn-default" value="+">+</button>
      </div>

      <div class="buttons">
        <button class="nums btn btn-default" value="1">1</button>
        <button class="nums btn btn-default" value="2">2</button>
        <button class="nums btn btn-default" value="3">3</button>
        <button class="btn-equal btn btn-info" id="resultButton">=</button>
      </div>

      <div class="buttons">
        <button class="nums btn-zero btn btn-default" value="0">0</button>
        <button class="nums btn btn-default" value=".">.</button>
      </div>
    </div>
  </div>
  <br>
  <footer class="text-center">
      Made by <a href="http://greyli.com" target="_blank">Grey Li</a> /
        Fork me on <a href="https://github.com/greyli/calculator" target="_blank">GitHub</a>
  </footer>
!

CSS 

              
                .box {
  font-family: 'Orbitron', sans-serif;
  width: 285px;
  height: 350px;
  margin: 15% auto auto;
  border: 1px solid #9e9b97;
  box-shadow: 0 4px 8px 0 rgba(0, 0, 0, 0.2), inset -1px -6px 12px 0.1px #89847e;
  border-radius: 20px;
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

.screen {
  height: 60px;
  width: 240px;
  border: 1px solid #7c877f;
  background: #c7d3c5;
  margin: 25px 20px 5px 20px;
  border-radius: 6px;
}

.main-screen {
  width: 240px;
  height: 22px;
  padding: 10px 5px;
  font-size: 20px;
  text-align: right;
}

.sub-screen {
  max-width: 280px;
  height: 15px;
  padding: 10px 5px;
  font-size: 12px;
  text-align: right;
}

.buttons {
  margin: 10px auto;
}

button {
  margin: 5px;
  padding: 3px 10px;
  width: 50px;
  height: 30px;
  border: none;
  box-shadow: 1px 2px #666;
}

button:focus {outline:0 !important;}

button:active {
  box-shadow: none;
  transform: translateY(4px);
}

.btn-zero {
  width: 113px;
  margin-right: 7px;
}

.btn-equal {
  position: absolute;
  margin-left: 10px;
  height: 75px;
}
!

JS 

              
                // todo: calculate based on result

  $(document).ready(function() {
  var mainOutput = $('#output');
  var subOutput = $('#output2');
  var op = $('#operator');
  var num2 = $('#num2');
  var num1 = $('#num1');
  var temp = $('#temp');
  var clearData = function() {
    num1.val('');
    op.val('');
    num2.val('');
    temp.val('');
  };

  var clearOutput = function() {
    mainOutput.html('');
    subOutput.html('');
  };

  var digitError = function() {
    //mainOutput.html('0');
    subOutput.html('Reach Digit Limit');
    temp.val(0);
  }

  $('.nums').click(function() {
    if (('+-*/').indexOf(mainOutput.html()) != -1) {
      mainOutput.html('');
    }
    // avoid multiple dot
    if ($(this).val() == '.' && (mainOutput.html()).indexOf('.') != -1) return;

    if (mainOutput.html() == '0') {
      clearOutput()
    }

    // clear output for new input
    if (temp.val() !== '') {
      clearOutput()
      clearData();
    }

    if (mainOutput.html().length > 12) {
      subOutput.html('Reach Digit Limit');
    } else {
      mainOutput.append($(this).val());
      subOutput.append($(this).val());
    }

  });

  $('#clearButton').click(function() {
    mainOutput.html('0');
    subOutput.html('');
    clearData();
  });

  $('#deleteButton').click(function() {
    if (subOutput.html() == 'Reach Digit Limit') {
      subOutput.html('');
    }
    if (mainOutput.html() != '0') {
      mainOutput.html(mainOutput.html().substring(0, mainOutput.html().length - 1));
      subOutput.html(subOutput.html().substring(0, subOutput.html().length - 1));
      if (mainOutput.html() == '') {
        mainOutput.html('0');
      }
    }
  });

  $('.btn-operate').click(function() {
    if (subOutput.html() == 'Reach Digit Limit') {
      subOutput.html('');
    }
    var newOperator = $(this).val();
    if (num1.val() !== '' && ('+-*/').indexOf(num1.val()) == -1 && op.val() !== '') {
      num2.val(mainOutput.html());
      if (('+-*/').indexOf(num2.val()) != -1) return;
      var number1 = parseFloat(num1.val());
      var operator = op.val();
      var number2 = parseFloat(num2.val());
      var result;
      if (operator == '+') {
        result = number1 + number2;
      } else if (operator == '-') {
        result = number1 - number2;
      } else if (operator == '*') {
        result = number1 * number2;
      } else if (operator == '/') {
        result = parseFloat(number1 / number2);
      }

      if (result.toString().length > 12) {
        mainOutput.html('0');
        digitError();
      } else {
        mainOutput.html(newOperator);
        subOutput.html(result + newOperator);
        num1.val(result);
        op.val(newOperator);
      }
    } else {
      num1.val(mainOutput.html());
      op.val(newOperator);
      mainOutput.html(newOperator);
      subOutput.append(newOperator);
    }
  });

  $('#resultButton').click(function() {
    if (mainOutput.html() === '' || ('+-*/').indexOf(mainOutput.html()) != -1) return;
    num2.val(mainOutput.html());
    var number1 = parseFloat(num1.val());
    var operator = op.val();
    var number2 = parseFloat(num2.val());
    var result;
    if (operator == '+') {
      result = number1 + number2;
    } else if (operator == '-') {
      result = number1 - number2;
    } else if (operator == '*') {
      result = number1 * number2;
    } else if (operator == '/') {
      result = parseFloat(number1 / number2);
    }
    if (result.toString().length > 12) {
      mainOutput.html('0');
      digitError();
    } else {
      mainOutput.html(result);
      subOutput.html('');
      temp.val(result);
    }
  });
});
!
999px

Console