If you're building a calculator that will just display information on the web page rather than storing it in a database or sending it somewhere, JavaScript is more appropriate of a solution.
<form action="javascript:void(0);" onsubmit="calculate('answer', 'form_hourly_rate', 'form_work'); return false;">
Hourly rate: <input type="text" name="form_hourly_rate" id="form_hourly_rate" />
Work: <input type="text" name="form_work" id="form_work" />
</form>
<div id="answer"></div>
<script type="text/javascript">
function calculate(answerID, rateID, workID)
{
document.getElementById(answerID).innerHTML = "Calculated salary = "+(52*document.getElementById(rateID).value*document.getElementById(workID).value/12);
}