Name the field that you are entering, i.e.
<form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
<input type="text" name="value" value="">
<input type="submit" value="Submit">
</form>
<?php
$value=$_POST['value'];
if ($value!=""){
//RUN FORMULA HERE:
$total=$value * 1.02;
echo $total;
}
?>
Using this method will allow you to continue entering numbers until you are done.