I am trying to convert Fahrenheit to Celsius but it's not working what am i doing wrong? - Javascript (needs to have a form and a clear button)

<html>
  <head>
    <title>
    Form
     </title>
  </head>
  <body>
      <script type="text/javascript">
      function calculate (form)
  {
   cel = fah * 0.5555 - 32;
   document.getElementById("finish").innerHTML = cel;
  }

</script>

<form name="myform" action="" method="get"> Turn Fahrenheit to Celsius! <br>
<input type="number" name="fah">
<input type="button" name="button" value="calculate" onClick="calculate(this.form)">
<button type="reset" value="Reset">Reset</button>
</form>
<p id="finish">°C</p>
  </body>
</html>

Edit1: Moved the inner.HTML into the Function

So the reset button is the only thing that works. Is it possible to calculate the math this way?

Solution

You asked a question on how to create a pizza form a while ago and you deleted it soon as it was down voted a few times.

The point to note here is, it is okay if a few people dislike your question. You've joined StackExchange not to gain points but to learn a few things. Your question could be helpful to a lot of others out there in this world. So here it is the answer to your pizza question

<html>
<body>

<p>A pizza is 13 dollars with no toppings.</p>

<form action="form_action.asp">
<input type="checkbox" name="pizza" value="Pepperoni" id="pep">Pepperoni + 5$<br>
<input type="checkbox" name="pizza" value="Cheese" id="che">Cheese + 4$<br>
<br>
<input type="button" onclick="myFunction()" value="Send order">
<input type="button" onclick="cost()" value="Get cost">
<br><br>
 <input type="text" id="order" size="50"> 
 </form>

<script>
function myFunction() {
var pizza = document.forms[0];
var txt = "";
var i;
for (i = 0; i < pizza.length; i++) {
    if (pizza[i].checked) {
        txt = txt + pizza[i].value + " ";
    }
}
document.getElementById("order").value = "You ordered a pizza with: " + txt;
}

function cost() {
var pep = 5;
var che = 4;
var pizza = 13;
var total = 0;
if (document.getElementById("pep").checked === true) {
  total += pep;
}

if (document.getElementById("che").checked === true) {
  total += che;
}
  document.getElementById("order").value = "The cost is : " + total;
}

</script>

</body>
</html>

Thanks. I hope this helps you.