Display Sum of meta values

I have a meta value for each post, named “revenue”.
When I do a custom query to list all the posts and their revenue it works. However what I’m trying to do is to sum the value of all the “revenue” into one number.

This is my query to list the revenue for each post

<?php
 query_posts('category_name=active-project');
 while (have_posts()) : the_post();
	 $revenue = get_post_meta($post->ID, 'revenue', true);
	echo '<p>'.$revenue.'</p>';
endwhile;
?>

And this is what I get:

<p>10.00</p>
<p>15.00</p>
<p>11.00</p>
<p>15.00</p>

What I’m trying to do is to sum all of this numbers and add the total amount right after the list.