JavaScript - Why Doesn't My Javascript Calculator Work?

<form name="Calc" style="position: fixed; top: 25px; right: 5px; background: #5F4A29;" action="">
  <table border="6">
    <tr>
      <td colspan="4"><input type="text" name="Input" size="25" /></td>
    </tr>
    <tr>
      <td><input class="button" type="button" value="  c  " onclick="Calc.Input.value = ''" /></td>
      <td></td>
      <td></td>
      <td><input class="button" type="button" value="  =  " onclick="Calc.Input.value = eval(Calc.Input.value)" /></td>
    <tr>
    </tr>
      <td></td>
      <td><input class="button" type="button" value="  (  " onclick="Calc.Input.value += '('" /></td>
      <td><input class="button" type="button" value="  )  " onclick="Calc.Input.value += ')'" /></td>
      <td><input class="button" type="button" value="  ^  " onclick="Calc.Input.value += '^(x,y)'" /></td>
    </tr>
    <tr>
      <td><input class="button" type="button" value="  1  " onclick="Calc.Input.value += '1'" /></td>
      <td><input class="button" type="button" value="  2  " onclick="Calc.Input.value += '2'" /></td>
      <td><input class="button" type="button" value="  3  " onclick="Calc.Input.value += '3'" /></td>
      <td><input class="button" type="button" value="  +  " onclick="Calc.Input.value += '+'" /></td>
    </tr>
    <tr>
      <td><input class="button" type="button" value="  4  " onclick="Calc.Input.value += '4'" /></td>
      <td><input class="button" type="button" value="  5  " onclick="Calc.Input.value += '5'" /></td>
      <td><input class="button" type="button" value="  6  " onclick="Calc.Input.value += '6'" /></td>
      <td><input class="button" type="button" value="  -  " onclick="Calc.Input.value += '-'" /></td>
    </tr>
    <tr>
      <td><input class="button" type="button" value="  7  " onclick="Calc.Input.value += '7'" /></td>
      <td><input class="button" type="button" value="  8  " onclick="Calc.Input.value += '8'" /></td>
      <td><input class="button" type="button" value="  9  " onclick="Calc.Input.value += '9'" /></td>
      <td><input class="button" type="button" value="  *  " onclick="Calc.Input.value += '*'" /></td>
    </tr>
    <tr>
      <td></td>
      <td><input class="button" type="button" value="  0  " onclick="Calc.Input.value += '0'" /></td>
      <td><input class="button" type="button" value="  .  " onclick="Calc.Input.value += '.'" /></td>
      <td><input class="button" type="button" value="  /  " onclick="Calc.Input.value += '/'" /></td>
    </tr>
  </table>
</form>

<script>
function packCalculation(){
	
	if ( document.getElementById('coverage').value == '' )
     {
             alert('You must enter a number')
             return false;
     }

	var coverage = parseFloat(document.getElementById('coverage').value); //declare coverage variable-->
	  
	  if (isNaN(coverage)) 
		{ 
         alert('Please only enter numbers')
			return false;
		};
	var coverage = parseFloat(document.getElementById('coverage').value);

	var packSize = parseFloat(document.getElementById('pack-size').innerHTML);

	var price = parseFloat(document.getElementById('price').value);

	var packNumFinal = coverage/packSize; <!--work out how many packs are needed-->

	var finalPrice = packNumFinal*price;  <!--work out final price -->


	document.getElementById('packsNeeded').innerHTML = packNumFinal.ceil(); <!--round up to the next full pack-->

	document.getElementById('calcPrice').innerHTML = parseFloat(finalPrice).toFixed(2); <!--fix to 2 decimal points-->

	document.getElementById('results').className='show';

	return false;
}
</script>

<form id="packC">
<label>Enter Area to cover ( sq m ):</label> <input type="text" name="coverage" id="coverage" />

<p><span>Pack Coverage (sq m):</span> <span id="pack-size">1.982</span></p>

<input type="hidden" name="price" id="price" value="<?php echo $priceFormat; ?>" />

<input type="button" value="Calculate" id="calculate"  onclick="packCalculation()"/>

<!-- results -->

  <span class="packs">Packs:</span> <div id="packsNeeded"></div><br/>
  <span class="cost">Total:</span> <div id="calcPrice"></div>
 </form>

<form name="loginform">
    <label>User name</label>
    <input type="text" name="usr" placeholder="username"> 
    <label>Password</label>
    <input type="password" name="pword" placeholder="password">
    <input type="submit" value="Login" onSubmit="validateForm();" />
</form>

<script>
    function validateForm() {
        var un = document.loginform.usr.value;
        var pw = document.loginform.pword.value;
        var username = "username"; 
        var password = "password";
        if ((un == username) && (pw == password)) {
            window.location = "main.html";
            return false;
        }
        else {
            alert ("Login was unsuccessful, please check your username and password");
        }
      }
</script>