title: JavaScript calculator calculates unchecked box
body: <p>I'm rather new to javascript and i'm trying to make a calculator that instantly calculates a total price of selected products.</p>
  
  <p>The calculator works fine but right now the checkbox gets calculated alongside the rest of the  products even when unchecked.
  I'm trying to make it so that when it is unchecked the value is 0 and when it is checked it is 0.50. </p>
  
  <p>This is what I have so far:</p>
  
  <pre><code>&lt;head&gt;        
     &lt;style type="text/css"&gt;                                                               
          .right{
              margin-top: 10px;
              width: 150px;
              height: 45px;
              background-color: #66CCFF;
          }
  
          p{
              margin: 0;
              padding: 0;
          }
      &lt;/style&gt;
  
      &lt;script type="text/javascript"&gt;
      &lt;!--
          function calculate() {
  
              var A1 = document.getElementById('Amount1').value
              var A2 = document.getElementById('Amount2').value
              var A3 = document.getElementById('Amount3').value
  
              var PStart  = 0;                
              var P1      = A1*1.50;
              var P2      = A2*1.00;
              var P3      = A3*1.00;
              var PTotal  = P1+P2+P3;
  
              var Insert  = document.getElementById("TotalPrice");
              TotalPrice.innerHTML= PTotal;
          }
     --&gt;
     &lt;/script&gt;
  &lt;/head&gt;
  
  &lt;body onload="calculate()"&gt;
      &lt;form&gt;
          &lt;fieldset&gt;
              &lt;legend&gt;Price&lt;/legend&gt;
              &lt;div class="FormRow"&gt;
                  &lt;label for="Amount2"&gt;Console $1,50&lt;/label&gt;
                  &lt;input type="text" min=0 id="Amount1" name="Amount1" oninput="calculate()" /&gt;
              &lt;/div&gt;
              &lt;div class="FormRow"&gt;
                  &lt;label for="Amount2"&gt;Controller $1,00&lt;/label&gt;
                  &lt;input type="text" min=0 id="Amount2" name="Amount2" oninput="calculate()" /&gt;
              &lt;/div&gt;
               &lt;div class="FormRow"&gt;
                  &lt;label for="Amount3"&gt;Batteries? $0,50&lt;/label&gt;
                  &lt;input type="checkbox" value="0.50" id="Amount3" name="Amount3" onchange="calculate()"/&gt;
              &lt;/div&gt;
              &lt;div class="right" &gt;
                  &lt;p&gt;&lt;b&gt;Total price:&lt;/b&gt;&lt;/p&gt;
                  &lt;p id="TotalPrice"&gt;
  
                  &lt;/p&gt;
              &lt;/div&gt;
          &lt;/fieldset&gt;
      &lt;/form&gt;
  &lt;/body&gt;
  </code></pre>