Javascript Calculation Onload

I currently have a web page that is building a web form from data contained in a SQL table. I need to do some calculations on the numbers that are populated into the form. I have used the following javascript code to successfully complete my calculations but I can only use this code at the bottom of my page because executing it at the top of my page the form fields are still empty. How would I use or modify the code I have to display the calc at the top of the web form. Example as follows:

<form method="POST" target="_self" name="dialer" enctype="text/plain">

Loan Amount: <input type="text" value="400000" name="loan" />
Interest Rate: <input type="text" value="1.00" name="apr" />
Amortization Term: <input type="text" value="480" name="term" />

</form>

<script language="JavaScript">

var princ = document.dialer.loan.value;
var term = document.dialer.term.value;
var intr = document.dialer.apr.value / 1200;

var output = princ * intr / (1 - (Math.pow(1/(1 + intr), term)));
var calc = (Math.round(output * 100)/100);

</script>