<div ng-app="demo"> <div ng-controller="MainCtrl"> <input type="number" ng-model="radius"/> <div>Area {{area}}</div> </div> </div> <script> angular.module('demo', []) .controller('MainCtrl', function($scope) { $scope.radius = 1; $scope.$watch('radius', function (radius) { var result = Math.PI * $scope.radius * $scope.radius; console.log("radius changed, area = " + result); $scope.area = result; }); }); </script>