Auto Calculation Solution

I have a form where some values are pulled from a database, but I will use a static form for example.

I have a form where I need to choose a value from a dropdown and have calculations performed and populate a hidden field.

<form id="form1 name="form1" method="" action="">
<select name="service">
<input type="text" name="totalHours" />
<input type="hidden" name="totalCost" />

My form is more complex but I should be able to build once I get started.

Example: If I choose 10 from the select menu and then enter 1.5 in the totalHours textbox, I need the page to automatically populate the hidden field with the 10 x 1.5 hours. I am not sure the best way to approach this. My pages are .php.

Any solutions?
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Ahmed MerghaniSoftware EngineerCommented:

you need to do this either throw java script or ajax. and I recommend java script in your case.
And you rather than using input type hidden, I will suggest to use CSS display property. You can hide by default and onchange of totalHours show totalCost.

For you javascript auto calculation....

<input type="text" name="totalHours" onChange="autoSum();'inline';" />
<input type="text" name="totalCost" style="display:none;" />
function autoSum() {
    document.form1.totalCost.value = (document.form1.service.value) * (document.form1.totalHours.value);

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